Seasonal Affective Disorder (SAD): Symptoms, Causes, and Treatment

13 min readUpdated 2 June 2026Medically reviewed

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Key takeaways

Seasonal Affective Disorder (SAD) is not a standalone diagnosis in DSM-5 but a "seasonal pattern" specifier applied to Major Depressive Disorder or Bipolar Disorder. The pattern must occur over at least 2 consecutive years with seasonal episodes substantially outnumbering non-seasonal episodes.
Two main patterns exist. Winter-pattern SAD (most common) presents with hypersomnia, increased appetite, carbohydrate cravings, weight gain, low energy, and depressed mood beginning autumn or winter. Summer-pattern SAD (less common) presents with insomnia, weight loss, agitation, anxiety, and depressed mood beginning late spring or summer.
Biological mechanisms include circadian rhythm disruption, melatonin dysregulation, serotonin changes, vitamin D status, and individual variation in retina-brain light processing. SAD prevalence increases with distance from equator; SAD is more common in women than men by approximately 4:1.
Treatment includes light therapy (10,000 lux for 30 minutes morning, 16-24 inches from face, daily through affected season), SSRIs (sertraline, escitalopram, fluoxetine), bupropion XL (FDA-approved for SAD prevention), CBT-SAD (specifically adapted cognitive behavioural therapy), vitamin D supplementation when deficient, and lifestyle measures.
SAD prevalence is lower in tropical and subtropical latitudes including most of India. However, northern India, Himalayan regions, and high-altitude residents can experience SAD. Monsoon-related mood changes are recognised clinically though research on whether these meet seasonal pattern criteria is limited.

Medically reviewed by Dr. Boppana Sridhar (MBBS, MD Psychiatry, Australia-trained), Consultant Psychiatrist with 9+ years of clinical experience in mood disorders including seasonal pattern depression, light therapy management, and complex differential diagnosis. NMC-registered.

Last updated: 2 June 2026 | Last medically reviewed: 2 June 2026

Seasonal Affective Disorder (SAD) is depression that follows a recurring seasonal pattern, most commonly with episodes beginning in late autumn or winter and resolving in spring. Less commonly, summer-pattern SAD presents with episodes in late spring or summer. DSM-5 classifies SAD not as a standalone diagnosis but as a "seasonal pattern" specifier applied to Major Depressive Disorder or Bipolar Disorder; the temporal pattern must persist across at least 2 consecutive years with seasonal episodes substantially outnumbering non-seasonal episodes. This guide covers SAD comprehensively: clinical definition and DSM-5 specifier criteria, the two main patterns with distinctive symptom profiles, biological mechanisms involving circadian rhythm, melatonin, serotonin, and light exposure, treatment including light therapy specifics (10,000 lux for 30 minutes morning), CBT-SAD, antidepressants, lifestyle measures, sub-syndromal seasonal mood changes, and India-specific context where lower latitudes produce lower prevalence but specific regions and populations remain affected.

What SAD is

Seasonal Affective Disorder is depression with a regular seasonal pattern: episodes beginning at a predictable time of year and resolving at a predictable time of year, recurring over years. The condition reflects sensitivity to seasonal changes in light exposure and the biological systems regulated by light.

SAD is not classified as a separate disorder in DSM-5. Instead, "with seasonal pattern" is a specifier applied to Major Depressive Disorder (or sometimes Bipolar Disorder) when the seasonal temporal pattern is present. This framing recognises SAD as a clinical phenomenon while categorising it within the broader mood disorder framework. See our MDD guide for core depression criteria; SAD episodes meet these criteria with additional temporal pattern requirement.

The condition was first formally described by Dr. Norman Rosenthal and colleagues at the National Institute of Mental Health (NIMH) in 1984, with publication of the seasonal pattern observation and light therapy treatment approach. Recognition in DSM-III-R followed in 1987, with continued refinement through DSM-IV and DSM-5.

ICD-10 does not assign a unique code for SAD. Cases are typically coded under F33 (Recurrent depressive disorder) with seasonal pattern noted in clinical documentation, or F31 (Bipolar disorder) for bipolar cases with seasonal pattern. F32 codes apply for single episodes that may evolve into clear seasonal pattern over time.

SAD is highly treatable. Light therapy, antidepressant medication, specific psychotherapy (CBT-SAD), vitamin D supplementation when deficient, and lifestyle measures all have evidence. Many patients achieve complete remission with treatment; others have improvement allowing functional living through affected seasons.

Two main patterns

Winter-pattern SAD

Most common form. Episodes begin in late autumn or early winter and resolve in spring. Characteristic atypical depression features: hypersomnia, increased appetite particularly for carbohydrates, weight gain, low energy, mood depression. Prevalence higher at higher latitudes; women affected approximately 4 times more than men.

Summer-pattern SAD

Less common form. Episodes begin in late spring or summer and resolve in autumn. Symptom profile differs from winter pattern: insomnia, poor appetite, weight loss, agitation, anxiety, possible irritability. Mechanisms may involve heat sensitivity, light overexposure disrupting sleep, or other factors. Less latitude correlation than winter SAD.

Some patients have alternating patterns (winter SAD with summer hypomania in bipolar cases), or "reverse SAD" patterns. Most patients have a single consistent pattern.

DSM-5 seasonal pattern criteria

DSM-5 seasonal pattern specifier requires:

1. Regular temporal relationship

Onset of major depressive episodes (or manic/hypomanic episodes in bipolar) and a particular time of year (autumn or winter for winter-pattern; late spring or summer for summer-pattern).

2. Full remission or shift at characteristic time of year

Complete remission of depressive episodes (or shift to mania/hypomania in bipolar) at a characteristic time of year.

3. Two-year pattern

In the last 2 years, two major depressive episodes occurring in the temporal relationships described above, and no non-seasonal major depressive episodes during this period.

4. Lifetime predominance

Seasonal major depressive episodes substantially outnumber any non-seasonal major depressive episodes that may have occurred over the lifetime.

The 2-year minimum pattern requirement means SAD diagnosis often requires observation over time. A patient with a first winter depressive episode does not yet meet seasonal pattern criteria; subsequent winter recurrence with summer remission establishes the pattern. Treatment can proceed before formal diagnosis if symptoms warrant.

"Seasonal" must be distinguished from psychosocial seasonal patterns (academic year stress, financial year end, holiday-related stress); the specifier requires the seasonal pattern to be associated with the time of year rather than predictable seasonal stressors.

Winter-pattern SAD symptoms

Winter-pattern SAD presents with what are called "atypical" depression features (more common in seasonal depression than in non-seasonal MDD), although the patient still meets full major depressive episode criteria.

Mood and cognitive symptoms:

Persistent low mood beginning in late autumn or winter
Loss of interest in usual activities (anhedonia)
Difficulty concentrating, decreased productivity
Feelings of hopelessness or worthlessness
Social withdrawal, reduced social engagement
Reduced ability to enjoy previously pleasurable activities
In severe cases, thoughts of death or suicide

Physical symptoms (the atypical features):

Hypersomnia: oversleeping, sometimes 2-4 hours more than typical; difficulty waking; feeling unrefreshed despite extended sleep
Increased appetite, particularly for carbohydrate-rich foods (the "winter craving" pattern)
Weight gain during the affected season
Heavy, leaden feeling in arms or legs (leaden paralysis)
Low energy and persistent fatigue not relieved by rest
Slowed movement and thinking (psychomotor retardation)

The "hibernation-like" pattern (oversleeping, overeating, low activity, weight gain) is distinctive of winter SAD and differs from the more common insomnia and weight loss pattern of non-seasonal MDD. The carbohydrate cravings specifically appear linked to serotonin regulation; consuming carbohydrates temporarily improves mood in many SAD patients.

Summer-pattern SAD symptoms

Summer-pattern SAD differs substantially from winter pattern in symptom profile:

Insomnia (difficulty falling asleep, frequent waking, early morning awakening)
Poor appetite, sometimes refusing food
Weight loss during the affected season
Agitation, restlessness, inability to relax
Anxiety, often prominent
Irritability, sometimes substantial
Episodes of impulsive or violent behaviour in some cases (requires careful assessment)
Low mood meeting depression criteria
Difficulty concentrating

Summer-pattern SAD is less common than winter pattern but matters clinically because clinicians may not consider seasonal depression when symptoms occur in summer. Patients may have summer-pattern SAD for years before the seasonal pattern is recognised.

Winter vs summer SAD comparison

Feature	Winter-pattern SAD	Summer-pattern SAD
Onset	Late autumn or early winter	Late spring or summer
Resolution	Spring	Autumn
Sleep pattern	Hypersomnia (oversleeping)	Insomnia
Appetite	Increased, carbohydrate cravings	Poor, decreased
Weight	Weight gain	Weight loss
Activity	Low energy, lethargy	Agitation, restlessness
Anxiety	Variable	Often prominent
Irritability	Less typical	More common
Latitude correlation	Strong (more common at higher latitudes)	Weaker latitude correlation
Prevalence	More common	Less common
Light therapy	First-line treatment	Generally not first-line
Cool environments	Less relevant	Air conditioning may help

Prevalence and demographics

SAD prevalence varies substantially by geographic latitude:

4:1

Women are affected by SAD approximately 4 times more often than men. Female predominance is more pronounced for SAD than for non-seasonal depression. The gender difference suggests hormonal factors in addition to light-related mechanisms.

Latitude effects on prevalence:

Equatorial regions (0-10 degrees latitude): very low prevalence, less than 1 percent
Mid-latitudes (30-50 degrees, including parts of India, southern US, southern Europe): low to moderate prevalence, 1-5 percent
High latitudes (50-70 degrees, including northern US, Canada, UK, northern Europe): substantial prevalence, 5-10 percent or higher
Sub-syndromal seasonal mood changes ("winter blues") affect substantially larger populations than full SAD diagnosis

Age and other demographics:

Young adults (20-30 years) have highest first onset rates
SAD can develop at any age including children and elderly
Risk decreases somewhat with age in many studies
Family history of SAD raises risk substantially; genetic component recognised
Family history of any depression increases risk
Pre-existing depression history raises risk for seasonal pattern emergence
Bipolar patients have higher rates of seasonal mood patterns

Causes and biological mechanisms

SAD develops through multiple interacting biological mechanisms sensitive to seasonal light exposure:

Circadian rhythm disruption. The body's internal clock is calibrated by light exposure, particularly morning light. Reduced winter daylight produces phase delays in some patients; the biological clock falls out of sync with social schedules. SAD patients show measurable circadian phase abnormalities; light therapy works partly by phase-shifting the circadian rhythm.

Melatonin dysregulation. Melatonin secretion is suppressed by light and triggered by darkness. Some SAD patients show prolonged or abnormal melatonin secretion during winter months, contributing to lethargy, sleep changes, and mood symptoms. This is one proposed mechanism for hypersomnia in winter SAD.

Serotonin reduction. Reduced light exposure correlates with reduced serotonin activity in research studies. Serotonin influences mood, appetite (particularly carbohydrate craving), sleep, and many other functions. The "carbohydrate craving" of winter SAD may reflect attempts to boost serotonin (carbohydrates indirectly increase brain tryptophan available for serotonin synthesis).

Vitamin D status. Reduced sunlight exposure can reduce vitamin D synthesis. Vitamin D has roles in brain function and mood regulation. The relationship with SAD is complex and probably contributory rather than primary cause. Vitamin D deficiency is common globally including in India despite abundant sunlight (urban indoor lifestyles, sun avoidance, skin pigmentation, dietary factors).

Eye-brain pathway sensitivity. Individual variation in how the retina-brain pathway processes light may explain why some people develop SAD while others do not at the same latitude. Specific receptors (intrinsically photosensitive retinal ganglion cells) communicate light information directly to the brain's circadian centres.

Genetic factors. SAD has familial patterns suggesting genetic vulnerability. Specific genes affecting circadian rhythm, melatonin, serotonin, and light-related neural pathways have been implicated.

Sex hormones. The 4:1 female:male ratio for SAD suggests hormonal involvement. Oestrogen and progesterone interact with serotonin and circadian systems. Premenstrual mood symptoms may worsen during SAD seasons.

Latitude as natural experiment. The strong latitude correlation supports light exposure as central mechanism. Migration studies show patients who move from lower to higher latitudes can develop SAD; those moving from higher to lower latitudes often see SAD resolve.

Sub-syndromal SAD

Sub-syndromal SAD (sometimes called "winter blues" or "summer slump") refers to seasonal mood changes that do not meet full DSM-5 criteria for major depressive episode but cause distress or mild functional impact. Sub-syndromal patterns are substantially more common than full SAD; many people in affected regions report mild seasonal mood variation.

Sub-syndromal SAD differs from clinical SAD in:

Fewer symptoms (typically 2-4 rather than 5+ required for MDD)
Milder symptom severity
Less functional impairment
Does not meet full DSM-5 criteria

Sub-syndromal SAD still warrants attention; light therapy, lifestyle measures, and outdoor exposure can help. Some patients with sub-syndromal patterns develop full SAD over time; recognition allows preventive measures.

Differential diagnosis

Condition	Distinguishing features
Non-seasonal Major Depressive Disorder	No regular temporal pattern; episodes at varying times of year; less atypical features in winter
Bipolar Disorder with seasonal pattern	Manic or hypomanic episodes also follow seasonal pattern; different treatment
Persistent Depressive Disorder (PDD)	Chronic continuous symptoms 2+ years without clear seasonal pattern
Adjustment disorder with seasonal stressor	Tied to specific seasonal life events (academic, financial, family) rather than seasonal light
Anxiety disorders	Anxiety prominent; depression secondary; less clear seasonal pattern
Hypothyroidism	Can mimic winter SAD symptoms (low energy, weight gain, mood); thyroid testing rules out
Vitamin D deficiency	Common; can contribute to SAD or mimic it; testing and supplementation appropriate
Sleep disorders	Primary sleep disorders (obstructive sleep apnea, restless legs) can produce SAD-like fatigue
Premenstrual Dysphoric Disorder (PMDD)	Cyclical mood pattern but tied to menstrual cycle, not season
Substance use	Alcohol increases in winter for some patients; can contribute or mimic

Thyroid function testing (TSH, T3, T4), vitamin D level, and full blood count are typical workup for new presentations with SAD-like symptoms. Sleep assessment if sleep complaints prominent. Mood tracking across at least 2 years to confirm seasonal pattern.

Light therapy specifics

Light therapy (bright light therapy, BLT) is the most distinctive SAD-specific treatment with strong evidence for winter-pattern SAD.

Standard protocol:

Light intensity: 10,000 lux full-spectrum white light box (not blue-spectrum or low-lux devices)
Duration: 30 minutes daily; some patients benefit from longer or shorter durations
Timing: in the morning, shortly after waking (typically before 8 AM); evening exposure may worsen sleep and is generally avoided
Distance: 16-24 inches (40-60 cm) from the face
Position: light box positioned to illuminate eyes without direct staring; eyes open, can read, eat, work, exercise during exposure
Duration of treatment: daily throughout the affected season; many patients continue until natural light increases in spring
Onset of effect: response typically begins within 3-7 days; full effect within 2-4 weeks

Side effects (usually mild):

Eye strain or temporary visual disturbance
Headache (often resolves with continued use or reduced exposure time)
Agitation or restlessness
Insomnia if exposure too late in day
Nausea (uncommon)
In patients with bipolar disorder, occasional manic switches; warrants monitoring

Equipment considerations:

10,000 lux full-spectrum light boxes available commercially in many countries
UV-filtered units important; some commercial products lack proper filtering
Light therapy devices may not be readily available in all Indian cities; online ordering is possible
Avoid tanning beds (UV exposure damages skin without mood benefit)
Standard indoor lighting is far less intense (typically 300-500 lux) and insufficient for therapeutic effect

Who should not use light therapy without consultation:

Patients with bipolar disorder (risk of triggering mania)
Patients with eye conditions (macular degeneration, retinitis pigmentosa)
Patients taking photosensitising medications (some psychiatric medications, some antibiotics)
Patients with photosensitivity disorders

Other treatments

Antidepressant medication. SSRIs (sertraline, escitalopram, fluoxetine) are commonly used and effective for SAD. Bupropion XL is specifically FDA-approved for SAD prevention when started in autumn before symptom onset; this is unique among antidepressants for prevention indication. Many patients respond to either light therapy or medication; some benefit from combination. Antidepressant treatment typically continues through affected season; some patients use continuous treatment.

Cognitive Behavioural Therapy for SAD (CBT-SAD). Specifically adapted CBT for seasonal depression includes: behavioural activation strategies for winter months (scheduled outdoor activity, social engagement); cognitive work on seasonal mood beliefs ("winter is awful", "I cannot function in dark months"); relapse prevention planning for upcoming seasons; identification of personal seasonal mood patterns. Some research suggests CBT-SAD produces more enduring benefit than light therapy alone, with reduced recurrence in subsequent seasons.

Vitamin D supplementation. When vitamin D deficiency identified (serum 25-OH vitamin D below 30 ng/mL), supplementation is appropriate. Typical doses range from 1,000-4,000 IU daily depending on deficiency degree and clinician guidance. Vitamin D alone is not primary SAD treatment but may help as adjunct. Indian patients often have vitamin D deficiency despite abundant sunlight (urban indoor lifestyles, sun avoidance, skin pigmentation, dietary factors).

Dawn simulation. Lights gradually brighten over 30-90 minutes before scheduled waking, simulating natural sunrise. Some patients respond well; an alternative for those who cannot maintain 30-minute morning light therapy sessions.

Lifestyle measures. Morning outdoor exposure (even on cloudy days; outdoor light is 100-1,000+ times brighter than indoor light); regular sleep-wake schedule; physical activity, particularly outdoor exercise in winter; maintaining social connections through affected season; balanced diet limiting excessive carbohydrate dependence; alcohol moderation (winter increases drinking for some patients).

Treatment selection considerations. First-line for winter SAD is often light therapy (good evidence, fewer side effects, fast onset). SSRIs or bupropion XL appropriate alone or combined. CBT-SAD for patients preferring psychotherapy or wanting more enduring benefit. Combination treatment for severe or treatment-resistant cases. Summer SAD treatment relies more on cooling environments, sleep optimisation, and antidepressants since light therapy is not appropriate.

Self-management strategies

Several self-management approaches support treatment or help with sub-syndromal seasonal mood patterns:

Morning outdoor exposure within 30-60 minutes of waking, even briefly (5-15 minutes), throughout affected season
Maximising indoor light during day; positioning workspace near windows; keeping curtains open during daylight
Regular sleep-wake schedule with consistent bedtime and wake time
Limiting screen exposure in evening (blue light can disrupt circadian rhythm and sleep)
Regular physical activity, ideally outdoor or near windows
Maintaining social connections through affected season; isolation worsens SAD
Balanced diet limiting excessive carbohydrate dependence; understanding carbohydrate cravings as serotonin-related
Stress management through affected season; chronic stress worsens depression
Planning meaningful activities through affected season; behavioural activation principles
Tracking mood across years to recognise patterns and trigger early treatment
Vitamin D supplementation if deficient
Alcohol moderation; winter drinking patterns can worsen SAD
For severe cases, planning travel to lower latitudes during affected season if feasible

India context

SAD in Indian context has distinct considerations from high-latitude countries:

Latitude and prevalence. India spans 8 to 37 degrees north latitude; even northernmost areas (Ladakh, Kashmir) are at latitudes where day length variation is modest compared to Scandinavia, Canada, or UK. Most Indian patients live at lower latitudes (Hyderabad at 17 degrees, Mumbai at 19 degrees, Delhi at 28 degrees, Bengaluru at 12 degrees) where seasonal light variation is mild. Overall SAD prevalence is substantially lower than in higher-latitude countries.

Regions where SAD does occur in India. Northern India (Delhi, Punjab, Uttar Pradesh, Bihar) experiences more pronounced winter daylight reduction than southern India. Himalayan regions (Ladakh, Kashmir, Himachal Pradesh, Uttarakhand, Sikkim, parts of Arunachal Pradesh) experience SAD comparable to northern European latitudes for some residents. Hill stations and high-altitude residents may be affected. Indian psychiatrists in these regions report SAD cases regularly.

Indoor lifestyle considerations. Urban workers spending most daylight hours indoors (offices, shops, factories) may experience SAD-like mood patterns regardless of latitude. India's growing urban indoor population may show patterns resembling SAD even at lower latitudes. Workers in shift patterns, particularly night shifts, may experience similar circadian-related mood patterns.

Monsoon-related mood changes. Indian clinical experience includes patients reporting distinct mood changes during monsoon season (June-September in most of India). Symptoms may include low mood, fatigue, hypersomnia, social withdrawal during prolonged rainy periods. Whether this represents true seasonal pattern (meeting DSM-5 criteria) or response to weather-related lifestyle disruption (reduced outdoor activity, limited social engagement, increased humidity, mould exposure, infection rates) is unclear. Some patients may have a monsoon-pattern presentation distinct from classical winter SAD; research is limited. Clinical recognition of monsoon-related mood changes can guide intervention regardless of formal diagnostic categorisation.

Vitamin D considerations. Indian patients frequently have vitamin D deficiency despite abundant sunlight, due to urban indoor lifestyles, skin pigmentation reducing synthesis efficiency, sun avoidance (particularly women), traditional clothing covering skin, and dietary factors. Vitamin D deficiency contributes to fatigue, low mood, and bone health concerns. Testing and supplementation appropriate when symptoms suggest SAD-like patterns. Vitamin D alone is not primary SAD treatment but may help as adjunct.

Migration considerations. Indian migrants to higher-latitude countries (UK, Canada, northern US, northern Europe) sometimes develop SAD after migration. Recognition matters because migrants may attribute mood changes to migration stress rather than seasonal light effects; clinical assessment can distinguish.

Access to light therapy. Light therapy devices are not widely available in Indian retail markets. Online ordering through e-commerce platforms is possible; some specialty medical equipment suppliers stock light boxes. Patient cost for quality 10,000 lux device ranges 8,000-25,000 INR. Government psychiatric services may not commonly stock light therapy equipment; private psychiatric care more likely to recommend it.

NIMHANS, AIIMS, and specialist resources. NIMHANS Bengaluru and AIIMS Delhi see SAD cases through their mood disorder clinics. Tele-MANAS (14416) can guide patients to appropriate assessment. Private psychiatric consultation in major cities typically familiar with SAD diagnosis and treatment.

When to seek help

Professional consultation is appropriate when:

You experience depressive symptoms at the same time of year for 2 or more consecutive years
Symptoms substantially impair work, relationships, or self-care during affected seasons
Self-management measures have not adequately improved symptoms
You experience thoughts of self-harm or suicide (immediate help)
Substantial weight, appetite, or sleep changes during specific seasons
Family or friends have noticed seasonal mood patterns
You suspect SAD based on symptom recognition
Considering light therapy and want medical guidance on protocol
Existing mental health conditions are seasonally exacerbated
You have moved to higher latitude and developed new seasonal mood patterns
You are pregnant or planning pregnancy during likely affected season; treatment review needed

First contact can be a GP, psychiatrist, or clinical psychologist. In India, Tele-MANAS (14416) provides 24x7 guidance. Mood tracking before consultation (when symptoms occur, severity, duration, what helps) supports diagnostic assessment.

A note from Dr. Boppana Sridhar

SAD presents differently in Indian clinical practice than in high-latitude countries. In Hyderabad and most of India, classical winter SAD is uncommon; we see it more in patients who have lived in higher latitudes or who experience monsoon-related mood patterns. The clinical observation of monsoon-related mood changes is genuine in many patients I see; whether it represents true seasonal pattern in DSM-5 terms or response to weather-related lifestyle disruption is an open question. Either way, recognition allows intervention. For patients with classical winter SAD patterns including those at higher altitudes or who have lived abroad, light therapy can be transformative; I have seen patients respond within days to a 10,000 lux light box used 30 minutes each morning. For patients suspecting they have seasonal mood patterns, the key clinical question is whether the timing is consistent across multiple years; one bad winter does not constitute SAD, but pattern across 2 or more years does. Mood tracking even as simple as marking calendar entries or smartphone notes can help establish pattern over time. The good news with SAD is that it is highly treatable; many patients have lived with seasonal patterns for years assuming nothing could change, and discover effective treatment exists. If you suspect you have seasonal mood patterns, consider consultation; clarity itself is valuable and treatment is available.

Frequently asked questions

What is Seasonal Affective Disorder (SAD)?

Seasonal Affective Disorder (SAD) is depression that follows a recurring seasonal pattern, with episodes occurring at the same time each year and full remission between episodes. Per DSM-5, SAD is not a standalone diagnosis but a 'seasonal pattern' specifier applied to Major Depressive Disorder or Bipolar Disorder. The most common form is winter-pattern SAD, with depressive episodes beginning in late autumn or early winter and resolving in spring. Less commonly, summer-pattern SAD presents with depressive episodes in late spring or summer. To meet DSM-5 seasonal pattern criteria, the relationship between season and mood episodes must be present for at least 2 consecutive years without non-seasonal episodes during this period, and seasonal episodes must substantially outnumber any non-seasonal episodes across the lifetime course. SAD reflects sensitivity to seasonal changes in light exposure, circadian rhythm, and related biological systems. It is highly treatable with light therapy, psychotherapy, medication, and lifestyle measures. ICD-10 does not have a unique code for SAD; cases are typically coded under F33 (recurrent depressive disorder) with seasonal pattern noted.

What are the symptoms of seasonal depression?

Symptoms of seasonal depression differ between winter and summer patterns. Winter-pattern SAD symptoms include: persistent low mood beginning in late autumn or winter; loss of interest in usual activities (anhedonia); low energy and persistent fatigue; hypersomnia (oversleeping, sometimes 2-4 hours more than usual); increased appetite particularly for carbohydrate-rich foods; weight gain during the affected season; difficulty concentrating; social withdrawal; feelings of hopelessness or worthlessness; in severe cases, thoughts of death or suicide. Summer-pattern SAD symptoms differ and include: insomnia (difficulty sleeping); poor appetite and weight loss; agitation and restlessness; anxiety; irritability; episodes of violent or impulsive behaviour in some cases. Both patterns meet DSM-5 criteria for major depressive episode (5 of 9 symptoms, 2 weeks or longer, functional impairment). The seasonal pattern specifier requires the temporal pattern (regular onset at particular season, regular remission at particular season) over at least 2 consecutive years. Sub-syndromal seasonal mood changes (sometimes called 'winter blues' or 'summer slump') do not meet full depression criteria but can still affect quality of life and warrant attention when persistent.

Is SAD a separate condition from depression?

SAD is not a separate diagnostic category in DSM-5; it is a 'seasonal pattern' specifier applied to Major Depressive Disorder or Bipolar Disorder. The clinical reality is that patients with SAD have episodes meeting full criteria for major depressive episode (or sometimes manic or hypomanic episodes in bipolar cases) that follow a regular seasonal pattern. The DSM-5 approach recognises SAD as a clinical phenomenon while categorising it within the broader mood disorder framework. The seasonal pattern specifier requires: regular temporal relationship between onset of mood episodes and a particular time of year; full remissions or shift from depression to mania/hypomania at a characteristic time of year; in last 2 years, 2 major depressive episodes occurring in the temporal relationships above, and no non-seasonal episodes in this period; seasonal episodes substantially outnumber non-seasonal episodes across lifetime. Bipolar disorder can also follow seasonal pattern (depressive episodes one season, manic/hypomanic episodes another), with implications for treatment. ICD-10 does not have a unique code for SAD; cases are typically coded under F33 (recurrent depressive disorder) or F31 (bipolar disorder) with seasonal pattern noted in clinical documentation.

What causes Seasonal Affective Disorder?

SAD causes involve multiple interacting biological systems sensitive to seasonal changes in light exposure. Circadian rhythm disruption: reduced daylight in winter affects the body's internal clock, leading to phase delays and mood symptoms. Melatonin dysregulation: melatonin production is regulated by light exposure; SAD patients may produce more melatonin during winter months, contributing to lethargy and sleep changes. Serotonin reduction: reduced light exposure correlates with reduced serotonin activity in some studies; serotonin influences mood, appetite, and sleep. Vitamin D deficiency: reduced sunlight exposure can reduce vitamin D synthesis; vitamin D has roles in mood regulation though the direct causal link with SAD remains under research. Latitude correlation: SAD prevalence increases with distance from equator, supporting the light exposure hypothesis. Genetic predisposition: SAD has familial patterns suggesting genetic vulnerability. Sex hormones: SAD is more common in women than men by approximately 4:1 ratio, suggesting hormonal involvement. Eye-brain pathway sensitivity: individual variation in how the retina-brain pathway processes light may explain why some people develop SAD while others do not in the same latitude. The biological complexity is why multiple treatment approaches (light therapy, antidepressants, vitamin D supplementation, psychotherapy) can each help different patients.

How is SAD treated? Does light therapy work?

SAD is highly treatable with several evidence-based approaches. Light therapy (bright light therapy, BLT) is the most distinctive SAD-specific treatment. Standard protocol: 10,000 lux full-spectrum light box, 30 minutes daily, in the morning shortly after waking, at 16-24 inches from the face with eyes open but not staring directly at the light. Response typically begins within 1-2 weeks. Continued throughout the affected season; some patients use throughout winter. Side effects mild and uncommon (eye strain, headache, agitation, occasional manic switches in bipolar patients). Antidepressant medication: SSRIs (sertraline, escitalopram, fluoxetine) are first-line; bupropion XL is specifically FDA-approved for SAD prevention started in autumn. Many patients respond similarly to light therapy or antidepressants; combination treatment for treatment-resistant cases. Cognitive Behavioural Therapy for SAD (CBT-SAD): adapted CBT specifically for seasonal depression includes behavioural activation in winter months, cognitive work on seasonal mood beliefs, and relapse prevention. Strong evidence; some research suggests CBT-SAD has more enduring benefit than light therapy alone. Vitamin D: supplementation considered when vitamin D deficiency identified; not a primary treatment but adjunct. Lifestyle measures: morning outdoor exposure even on cloudy days, regular sleep schedule, physical activity, social connection. Dawn simulation: gradual lights brightening before waking; alternative for some patients. Treatment is individualised; some patients respond best to one approach, others need combination.

Can you have SAD in tropical countries like India?

SAD prevalence is substantially lower in tropical and subtropical latitudes including most of India compared to high-latitude countries, but it does occur. The latitude correlation with SAD is well established: prevalence increases with distance from equator. In equatorial regions, where day length varies minimally across seasons, SAD prevalence is very low. India's geographic position (8 to 37 degrees north latitude) means seasonal light variation is modest compared to northern Europe or Canada, leading to lower SAD prevalence overall. However, some considerations apply. Northern India and higher altitudes (Himalayan regions including Ladakh, Kashmir, Himachal Pradesh, Sikkim, parts of Uttarakhand) experience more pronounced seasonal light variation and colder winters, with SAD reported in clinical practice. Hill stations and high-altitude residents may experience SAD-pattern symptoms. Indoor workers with limited daylight exposure in any region can experience mood patterns similar to SAD even at lower latitudes. Monsoon season in India produces distinct mood phenomena in some patients (variously described as monsoon blues or monsoon-related depression); whether this represents true seasonal pattern or response to weather-related lifestyle disruption requires more research. Migrants from India to higher-latitude countries (UK, Canada, northern Europe) sometimes develop SAD after migration. Indian psychiatrists should be alert to seasonal mood patterns particularly in northern India patients and high-altitude residents.

What is summer-pattern SAD?

Summer-pattern SAD is the less common form of SAD with depressive episodes occurring in late spring or summer rather than winter. Symptoms differ from winter SAD in distinctive ways. Sleep: insomnia (difficulty falling or staying asleep) rather than winter's hypersomnia. Appetite: poor appetite and weight loss rather than winter's increased appetite and weight gain. Activity: agitation, restlessness, anxiety rather than winter's low energy and lethargy. Behaviour: episodes of violent or impulsive behaviour in some cases; this presentation requires careful assessment. Mood: low mood meeting depression criteria during the affected season. Triggers may differ from winter SAD. Proposed mechanisms include: heat sensitivity in some patients; increased daylight disrupting sleep; humidity effects on mood; social changes during summer months (reduced structure, missed routines, holidays). Latitude correlation is weaker for summer-pattern SAD than winter-pattern; prevalence in India and tropical countries for summer-pattern SAD may be relatively higher proportionally though absolute prevalence remains low. Treatment differs from winter SAD: light therapy is not the foundation; instead air-conditioned environments, cool showers, sleep optimisation, structured routine maintenance, antidepressants (SSRIs), and psychotherapy. Recognition matters because summer-pattern SAD can be missed when clinicians associate seasonal depression only with winter.

How do you tell SAD from regular depression?

Distinguishing SAD from non-seasonal Major Depressive Disorder requires careful temporal pattern documentation across multiple years. Key distinguishing features. Temporal pattern: SAD episodes begin at predictable time each year (typically autumn/winter for winter-SAD, late spring/summer for summer-SAD) and resolve at predictable time each year. Non-seasonal MDD has no such temporal regularity. DSM-5 requires this pattern for at least 2 consecutive years with no non-seasonal episodes in this period, and seasonal episodes substantially outnumbering any non-seasonal episodes lifetime. Symptom profile: winter SAD often features atypical features (hypersomnia, increased appetite, weight gain, carbohydrate cravings) less common in non-seasonal MDD where insomnia and weight loss are more typical. Full remission: SAD episodes typically resolve completely during the off-season; non-seasonal MDD episodes may have less complete remission patterns. Family history: SAD has stronger familial patterns than non-seasonal MDD. Latitude or environment: SAD onset is more common in higher latitudes or in patients who have moved to higher latitudes. Response to light: SAD patients often respond to light therapy alone or as adjunct; non-seasonal MDD typically does not. The distinction matters because SAD-specific treatment (light therapy, seasonal-pattern medication strategies) is available; treatment plans benefit from accurate identification. Patients may need multiple years of mood tracking before clear seasonal pattern emerges; treatment can proceed before final diagnostic clarity.

Medical disclaimer: This article provides general health education and does not replace personalised consultation with a qualified mental health professional. SAD requires individual clinical assessment for accurate diagnosis and appropriate treatment planning. Light therapy is not appropriate for all patients (particularly those with bipolar disorder or certain eye conditions); medical consultation before starting is recommended. If you are experiencing severe symptoms or thoughts of self-harm, contact a crisis helpline or emergency services immediately.

About the author

247healthcare.blog editorial team writes general health and preventive medicine content reviewed by qualified doctors. Every article is fact-checked against current guidance from DSM-5, ICD-10/11, NICE NG222, APA, WHO, NMHS NIMHANS, Cochrane reviews, and peer-reviewed mood disorders literature before publication.

About the medical reviewer

Dr. Boppana Sridhar (MBBS, MD Psychiatry, Australia-trained) is the Consultant Psychiatrist and department lead for Psychiatry and Psychology at Vivekananda Hospital, Begumpet, Hyderabad. He has 9+ years of clinical experience in mood disorders including seasonal pattern depression, light therapy management, monsoon-related mood patterns, and complex differential diagnosis. NMC-registered.