Seasonal affective disorder: symptoms, causes and treatment

Key points

• Seasonal affective disorder is not a separate diagnosis. ICD-11 and DSM-5 record it as a seasonal pattern attached to repeated depressive episodes [3][4].
• Winter episodes carry atypical features: longer sleep, more eating with carbohydrate cravings, weight gain and heavy limbs [4][6][10].
• Around 3% of UK adults are thought to be affected at a clinically significant level [6].
• NG222 treats seasonal episodes as depression, and advises that the evidence for light therapy is uncertain [2].
• Light therapy is widely used and many find it helps, but it is rarely NHS funded and the trials are few and small [1][7].
• Vitamin D supplements have not been shown to treat it [9].

What seasonal affective disorder is

Seasonal affective disorder, or SAD, is depression that arrives and lifts at the same time each year, usually starting in autumn and easing in spring [1]. It is not a separate illness. ICD-11, the World Health Organization’s current classification, and DSM-5, the American manual, record it as a seasonal pattern applied to repeated depressive episodes [3][4].

If you need help now. Samaritans: call 116 123, free, 24/7. Shout: text SHOUT to 85258. NHS 111, then choose the mental health option. Call 999 if anyone is in immediate danger.

The ICD-11 code is 6A80.4, seasonal pattern of mood episode onset [3]. It applies to recurrent depressive disorder or bipolar disorder when most episodes begin and end with the seasons rather than with a yearly stress [3]. DSM-5 asks for the pattern over at least two years [4].

One review put recurrent winter depression at around 3% of UK adults, and a study in North Wales found 2.4% [5][6].

Symptoms

Seasonal episodes carry the usual features of depression plus atypical ones that mark the pattern.

Mood and thinking

• Low mood on most days over weeks, and loss of interest in things normally enjoyed [1]
• Guilt, hopelessness, irritability or poor concentration [1]
• Thoughts of suicide or self-harm [1]

Sleep and energy

• Sleeping longer than usual and finding mornings hard [1][6]
• Daytime sleepiness, often peaking in the late afternoon [6]
• A heavy, leaden feeling in the arms and legs, known in atypical depression as leaden paralysis [10]

Appetite and weight

• Eating more, with cravings for carbohydrate and sweet food, and weight gain over the winter [1][6]

About three-quarters report both longer sleep and increased appetite, the reverse of most non-seasonal episodes [6].

Commonly confused with. Non-seasonal depression that starts in winter, bipolar disorder, an underactive thyroid, and ordinary low winter mood [3][4].

Types

There are no formal subtypes. Three presentations are described, separated by timing and severity.

• Winter pattern. Symptoms begin in autumn or early winter and lift in spring, with the atypical features above [4].
• Summer pattern. Symptoms begin in late spring or summer. It is far rarer, at roughly 0.1% against about 3%, and looks different: trouble sleeping, reduced appetite, weight loss and agitation [4].
• Subsyndromal seasonal affective disorder. Also called the winter blues. A milder seasonal dip that falls short of a depressive episode [6].

Causes and risk factors

No single cause has been established. Reduced winter daylight is the leading explanation, and the mechanisms below are proposed rather than proven [1][4].

• Changes in melatonin, the hormone that regulates sleep timing [1][4]
• Reduced serotonin activity, which affects mood [1][4]
• A shift in the body clock, which sets sleep and wake timing [4]

Risk is higher further from the equator, in women, where there is a family history of mental illness, and where depression or bipolar disorder is present [4].

Low vitamin D is often named as the cause, but supplementing it has not been shown to treat seasonal depression [9]. Nor is it the same as disliking winter: what separates it is an episode’s length, its effect on daily life, and its return each year [3].

How it is diagnosed

There is no blood test or scan. A GP is the usual starting point, and can assess the depression, arrange tests for physical causes and refer on [1]. A diagnosis may also be recorded by a psychiatrist registered with the GMC or a clinical psychologist registered with the HCPC.

Assessment is a structured conversation about current symptoms, how long they have lasted and what they affect. Because seasonal pattern describes timing, the clinician asks when past episodes began and ended, and DSM-5 asks for at least two years of it [4]. Questionnaires such as the PHQ-9 measure severity rather than making the diagnosis [2]. Clinicians also ask about spells of high energy or reduced need for sleep, since seasonal timing is common in bipolar disorder [3][4].

Treatment

NG222, the NICE guideline on depression in adults, sets out no separate pathway for seasonal episodes, so the same options apply [2].

For less severe depression, NG222 lists options in order of preference, beginning with guided self-help, group cognitive behavioural therapy and group behavioural activation, then individual CBT, group exercise, SSRIs and counselling [2]. For more severe depression it begins with combined individual CBT and an antidepressant [2]. Starting, changing or stopping medicine is a decision for a GP or prescriber.

Light therapy. NICE does not recommend it. NG222 recommendation 1.4.39 advises anyone who wishes to try it in preference to medication or psychological treatment “that the evidence for the efficacy of light therapy is uncertain” [2]. The NHS does not usually fund light boxes, because there is not enough evidence, while noting that many people find they help [1].

A Cochrane review of light therapy for prevention found one trial of 46 people, rated very low certainty, with confidence intervals spanning benefit and harm [7]. For an episode already under way, an earlier meta-analysis reported a number needed to treat of about 4 to 5, from a small evidence base [6].

Lamps in the literature are rated at 10,000 lux, with ultraviolet filtered out and a CE or UKCA mark, used for about 30 minutes each morning at roughly 18 to 21 inches from the eyes [1][6]. Lamps are not advised after about 5pm, because of the risk to sleep, and a response usually appears within the first week [6]. Headache and blurred vision are reported by up to 20% of users, and light therapy can occasionally trigger hypomania in bipolar disorder [6]. Speak to a GP first if you have an eye condition or take medicines that raise light sensitivity [1].

CBT adapted for seasonal depression. In a trial of 177 people, CBT-SAD and light therapy did equally well in the first winter, but by the second the CBT group had fewer recurrences, 27.3% against 45.6% [8].

Supplements. Vitamin D on its own is not considered an effective treatment, and the few melatonin trials suggest no role either [9].

Living with seasonal affective disorder

Timing is the one predictable thing about this condition, so planning around it helps.

• Get outside in daylight, especially in the morning, even when it is overcast [1]
• Keep wake times steady, since a drifting body clock worsens sleep and mood [1]
• Keep exercise going through winter, which NICE lists among the options for depression [2]
• Note when symptoms began in previous years, and see a GP before that point comes round

You do not have to tell an employer or university your diagnosis. Occupational health or a student support service can arrange adjustments, such as a desk nearer a window or a later winter start, without passing on clinical detail. Any private therapist should be registered with the HCPC, BPS, BACP, UKCP or GMC.

When to speak to a professional

Speak to a GP if:

• Low mood has lasted most of the day, most days, for two weeks or more [1]
• Sleep, appetite or weight have changed noticeably over autumn or winter [1]
• The same pattern has returned in more than one year, or treatment is not helping [1]

A GP can treat you directly or refer you to talking therapies or a community mental health team [1]. Call NHS 111 and choose the mental health option if you cannot manage daily tasks.

If you are having thoughts of suicide or self-harm, or cannot keep yourself safe, call Samaritans on 116 123, text SHOUT to 85258, or call 999 if anyone is in immediate danger.

Common questions

Is seasonal affective disorder a real diagnosis?

The pattern is recognised, but it is not a standalone diagnosis. ICD-11 and DSM-5 record it as a seasonal pattern added to recurrent depressive disorder or bipolar disorder [3][4].

Do SAD lamps work?

Some people find them helpful, but the evidence is small and uneven [6][7]. NICE does not recommend light therapy and advises that its efficacy is uncertain, and lamps are rarely NHS funded [1][2].

Does vitamin D help with seasonal affective disorder?

Trial results are mixed, and vitamin D alone is not considered an effective treatment for it [9]. UK advice on winter supplementation is a separate question, so ask a GP.

Clinical review pending

References

1. NHS. Seasonal affective disorder (SAD). NHS, 2025. https://www.nhs.uk/mental-health/conditions/seasonal-affective-disorder-sad/
2. National Institute for Health and Care Excellence. Depression in adults: treatment and management. NICE guideline NG222, 2022. https://www.nice.org.uk/guidance/ng222/resources/depression-in-adults-treatment-and-management-pdf-66143832307909
3. ICD-11 MMS entry 6A80.4 Seasonal pattern of mood episode onset, as reproduced by Find-A-Code. https://www.findacode.com/icd-11/code-822487798.html
4. Munir S, Gunturu S, Abbas M. Seasonal Affective Disorder. StatPearls, NCBI Bookshelf, 2024. https://www.ncbi.nlm.nih.gov/books/NBK568745/
5. Michalak EE, Wilkinson C, Dowrick C, Wilkinson G. Seasonal affective disorder: prevalence, detection and current treatment in North Wales. The British Journal of Psychiatry, 2001. https://www.cambridge.org/core/journals/the-british-journal-of-psychiatry/article/seasonal-affective-disorder-prevalence-detection-and-current-treatment-in-north-wales/D1372C721195A805D50135CCA5ED3087
6. Eagles JM. Light therapy and the management of winter depression. Advances in Psychiatric Treatment, 10(3), 2004. https://www.cambridge.org/core/journals/advances-in-psychiatric-treatment/article/light-therapy-and-the-management-of-winter-depression/40650835E113944CA60DA46546A1AA76
7. Nussbaumer-Streit B, Forneris CA, Morgan LC, Van Noord MG, Gaynes BN, Greenblatt A, et al. Light therapy for preventing seasonal affective disorder. Cochrane Database of Systematic Reviews, 2019. https://www.cochrane.org/evidence/CD011269_light-therapy-prevention-winter-depression
8. Rohan KJ, Meyerhoff J, Ho SY, Evans M, Postolache TT, Vacek PM. Outcomes one and two winters following cognitive-behavioral therapy or light therapy for seasonal affective disorder. American Journal of Psychiatry, 2016;173(3):244 to 251. https://psychiatryonline.org/doi/10.1176/appi.ajp.2015.15060773
9. National Center for Complementary and Integrative Health. Seasonal Affective Disorder and Complementary Health Approaches: What the Science Says. NCCIH. https://www.nccih.nih.gov/health/providers/digest/seasonal-affective-disorder-and-complementary-health-approaches-science
10. Lojko D, Rybakowski JK. Atypical depression: current perspectives. Neuropsychiatric Disease and Treatment, 2017. https://www.dovepress.com/atypical-depression-current-perspectives-peer-reviewed-fulltext-article-NDT