Seasonal Affective Disorder: Symptoms, Causes and Light Therapy Explained

Seasonal affective disorder (SAD) is a type of depression that comes and goes with the seasons, most often starting in late autumn and lifting in spring. As the clocks go back, on 25 October in the UK and on 1 November in the US and Canada this year, darker evenings arrive and many people notice lower mood and energy. For some it is a mild “winter blues”; for others it is a clinical depression that affects work, relationships and daily life. This guide explains the symptoms, causes and evidence-based treatments, including how light therapy works and when to see a doctor.

What Is Seasonal Affective Disorder?

According to the US National Institute of Mental Health (NIMH), SAD is a type of depression defined by a recurring seasonal pattern, with symptoms lasting about four to five months a year. Psychiatrists diagnose it as major depressive disorder (or bipolar disorder) “with seasonal pattern”, which means the episodes have started and ended at around the same times of year for at least two consecutive years.

Most people with SAD have winter-pattern SAD, which begins in autumn or early winter. A less common summer-pattern SAD starts in late spring or summer. The American Psychiatric Association estimates that around 5% of adults in the US experience SAD, with symptoms often lasting about 40% of the year.

What Are the Symptoms of SAD?

SAD shares the core symptoms of depression:

• Feeling sad, low or hopeless most of the day, nearly every day
• Losing interest in activities you used to enjoy
• Low energy and feeling sluggish
• Difficulty concentrating
• Feeling worthless or guilty
• Thoughts of death or suicide

Winter-pattern SAD also tends to bring a distinctive cluster of symptoms, according to the NIMH:

• Oversleeping (hypersomnia) and difficulty getting up
• Overeating, particularly craving carbohydrates
• Weight gain
• Social withdrawal, which some people describe as “hibernating”

Summer-pattern SAD more often involves trouble sleeping, poor appetite, weight loss, restlessness and anxiety.

Is It SAD or Just the Winter Blues?

Many people feel a little flatter in winter, especially when it is dark on the way to and from work. The difference is severity and impact. The winter blues are mild and do not stop you functioning. SAD is a form of depression: symptoms last most of the day, most days, for weeks, and interfere with work, study or relationships. If that sounds familiar, it is worth talking to a doctor rather than waiting for spring.

What Causes Seasonal Affective Disorder?

Scientists do not fully understand SAD, but the leading explanations relate to reduced daylight:

• Disrupted body clock: shorter days may throw the circadian rhythm out of step with your sleep-wake schedule
• Serotonin: research suggests people with SAD may have reduced activity of serotonin, a brain chemical that helps regulate mood
• Melatonin: darkness increases melatonin, the hormone that makes you sleepy, and people with SAD may produce too much of it in winter
• Vitamin D: less sunlight means less vitamin D, which may play a role in serotonin activity, although the evidence for supplements treating SAD is mixed

Who Is Most at Risk?

• People living further from the equator, where winter days are shortest; the NIMH notes SAD is more common in northern states such as Alaska and New England than in Florida
• Women, who are diagnosed more often than men
• Younger adults, as SAD usually begins between the ages of 18 and 30
• People with depression or bipolar disorder, or a family history of them

How Is SAD Treated?

The good news is that SAD is treatable. The NIMH describes four main evidence-based approaches, which can be used alone or together:

Light Therapy

Light therapy is often the first-line treatment for winter-pattern SAD. It involves sitting near a special light box that gives off bright light, typically 10,000 lux, which is around 20 times brighter than ordinary indoor lighting, for about 30 to 45 minutes each day, usually first thing in the morning, from autumn to spring. You do not stare into the light; you can read or eat breakfast with it to one side.

Choose a box designed for SAD that filters out ultraviolet light. Side effects are usually mild, such as eye strain or headache. People with bipolar disorder, eye conditions or who take medicines that increase light sensitivity should speak to a doctor first, because bright light can trigger mania in some people.

Talking Therapy

Cognitive behavioural therapy adapted for SAD (CBT-SAD) helps people challenge negative thoughts about winter and plan enjoyable activities, a technique called behavioural activation. One trial found CBT-SAD was as effective as light therapy in the first winter and had longer-lasting benefits in later winters.

Medication

Antidepressants, especially selective serotonin reuptake inhibitors (SSRIs), can help. In the US, the Food and Drug Administration has approved extended-release bupropion for preventing winter depression in people with a history of SAD, started in early autumn before symptoms appear.

Vitamin D

Because many people are low in vitamin D in winter, doctors may check levels and recommend supplements. However, studies of vitamin D as a treatment for SAD have produced mixed results. Our guide to how much vitamin D you need per day explains safe amounts.

What Can You Do Yourself to Feel Better in Winter?

• Get outside in daylight, ideally in the morning, even when it is cloudy
• Keep moving: regular exercise helps mood; a brisk daily walk outdoors combines activity with light, and our guide to how many steps a day explains what to aim for
• Keep a regular sleep schedule, waking at the same time every day, even at weekends
• Sit near windows at home and work
• Stay connected: plan regular contact with friends and family, even if you do not feel like it
• Plan ahead: if SAD returns every year, start treatment in early autumn, before symptoms build

Persistent exhaustion can also be a sign of work-related stress, so it can be helpful to compare symptoms with our guide to burnout signs and recovery.

When Should You See a Doctor?

See a doctor or mental health professional if low mood lasts more than two weeks, affects your daily life, or you find yourself using alcohol to cope. They can rule out other causes, such as an underactive thyroid, and recommend treatment. If you have thoughts of harming yourself, seek help immediately: in the US, call or text 988 for the Suicide and Crisis Lifeline; in Canada, call or text 9-8-8; and in the UK, call Samaritans on 116 123.

Key Takeaways

SAD is a recognised form of depression linked to the seasons, usually starting in autumn and easing in spring. It is more common further from the equator, in women and in young adults. Light therapy, CBT, antidepressants and good daily habits all help, and planning ahead can prevent the worst of it. If winter regularly knocks you down, you do not have to wait for spring; talk to a doctor. This article is general information, not medical advice.

Featured image: Seasonal patterns affect Airmen, combated by 28th MDOS 141126-F-MU239-017, by Senior Airman Hailey Staker, US Air Force / Wikimedia Commons, public domain. Cropped, colour-graded and combined with MediaPavilion branding.

Sources

• National Institute of Mental Health — Seasonal affective disorder (accessed 28 October 2026).
• American Psychiatric Association — Seasonal affective disorder (SAD) (accessed 28 October 2026).
• NHS — Seasonal affective disorder (SAD) (accessed 28 October 2026).
• Rohan KJ et al., American Journal of Psychiatry — Outcomes one and two winters following cognitive-behavioral therapy or light therapy for seasonal affective disorder (2016).
• 988 Suicide and Crisis Lifeline — Get help (accessed 28 October 2026).