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

Many people feel a little slower when the days get shorter and colder. For some, though, the change is much more significant: a recurring pattern of depression that starts in late fall or winter and lifts in spring. This is known as seasonal affective disorder (SAD), now often described clinically as major depressive disorder with a seasonal pattern. It is common, real, and very treatable.

What is SAD?

SAD is a form of depression in which symptoms begin and end at about the same times each year. Most people experience the winter pattern, with symptoms typically starting in October or November and easing by March or April. A less common summer pattern also exists. SAD is more common in people living farther from the equator, where winter days are shortest, and it is diagnosed more often in women and young adults. Many more people experience milder "winter blues" that do not meet the criteria for SAD but still affect wellbeing.

Common symptoms

SAD shares the core features of depression — persistent low mood, loss of interest, feelings of hopelessness or worthlessness, low energy and difficulty concentrating. The winter pattern is also often characterized by:

• Sleeping much more than usual (hypersomnia) and difficulty waking up
• Increased appetite, particularly cravings for carbohydrates
• Weight gain
• Feeling heavy, sluggish or "hibernating"
• Social withdrawal

The summer pattern, by contrast, more often involves poor sleep, reduced appetite, restlessness and anxiety.

What causes SAD?

The exact causes are still being studied, but several factors seem to play a role:

• Reduced daylight can disrupt the body's internal clock (circadian rhythm), which regulates sleep, hormones and mood.
• Serotonin, a brain chemical involved in mood, may function differently in people with SAD during winter months.
• Melatonin, a hormone that promotes sleep, is produced in response to darkness; longer nights may lead to overproduction, contributing to sleepiness and low energy.
• Vitamin D levels fall with less sunlight; its exact role in SAD is uncertain, but low levels are worth checking.
• Genetics and personal history: SAD is more common in people with a family history of depression, or who have depression or bipolar disorder themselves.

How SAD is diagnosed

A doctor or mental health professional will ask about your symptoms and whether they have followed a seasonal pattern for at least two consecutive years. They may check for other causes of fatigue and low mood, such as thyroid problems or anemia, and screen for bipolar disorder, since seasonal mood changes can be part of that condition and treatment differs.

Effective treatments

Light therapy

Bright light therapy is one of the main treatments for winter-pattern SAD. It involves sitting near a special light box that emits bright light (typically 10,000 lux, with UV filtered out) for about 20–30 minutes each morning, usually soon after waking. Many people notice improvement within one to two weeks. Light boxes are generally safe, but side effects such as headache, eye strain or feeling "wired" can occur. People with eye conditions, those taking light-sensitizing medications, and people with bipolar disorder should consult a doctor before starting.

Cognitive behavioral therapy (CBT)

CBT adapted for SAD (CBT-SAD) helps people identify unhelpful thoughts about winter and plan enjoyable, energizing activities to counter the urge to hibernate. Research suggests its benefits may last longer into future winters than light therapy alone.

Medication

Antidepressants such as SSRIs can be effective for SAD. Extended-release bupropion is approved in the US to help prevent seasonal depressive episodes when started before symptoms typically begin. Decisions about medication should be made with a prescriber.

Vitamin D

If blood tests show low vitamin D, a doctor may recommend supplementation. Evidence that vitamin D alone treats SAD is mixed, so it is best considered as part of a broader plan.

Self-care strategies

• Get outside in daylight, especially in the morning — even on cloudy days.
• Let light in at home and work: open blinds and sit near windows.
• Keep a regular sleep schedule, resisting the temptation to oversleep.
• Stay active. Regular exercise, ideally outdoors, can lift mood and energy.
• Plan things to look forward to during winter months, and stay connected with others.
• Prepare early. If you know winter is hard for you, start healthy routines (or treatment) in early fall.

When to seek help

It is normal to have some days when you feel down. But if you feel low for days at a time, lose interest in activities, notice significant changes in sleep or appetite, or feel hopeless, talk with a health professional. If you have thoughts of suicide, call or text 988 in the US right away. With the right support, the darker months can feel much more manageable.

Medical disclaimer: This article is for general educational purposes and is not a substitute for professional medical advice, diagnosis or treatment. Always consult a qualified health professional about your situation. See our full disclaimer.