Seasonal Affective Disorder: How the Diagnosis Is Made, What Light Therapy Involves and What to Track Before the Clocks Change

Many people feel slower and flatter when the days get shorter. For some, that seasonal dip is a recognized form of depression called seasonal affective disorder, or SAD. The difference between the two matters, because it changes what gets assessed and what treatment looks like. This article walks through how a clinician decides whether a seasonal pattern is present, what light therapy and other treatments actually involve, and what you can track now, before the clocks change, so the conversation with a care team starts with real information.

How Seasonal Affective Disorder Is Diagnosed

Seasonal affective disorder (SAD) is diagnosed by pattern, not by a single hard winter. According to the National Institute of Mental Health, a person must have depressive episodes that occur during a specific season for at least 2 consecutive years, and those seasonal episodes must be more frequent than depressive episodes at other times of the year. In most cases symptoms start in late fall or early winter, ease in spring and summer, and last about 4 to 5 months of the year.

That two-year requirement is why a diagnosis often cannot be made from how someone feels this month alone. A clinician will usually ask about past years, may use a questionnaire, and will look for the symptoms of depression plus the features that often go with the winter pattern: oversleeping, overeating with carbohydrate cravings and weight gain, and pulling away from people. A less common summer pattern looks different, with trouble sleeping, poor appetite, restlessness and anxiety. The NIMH overview of seasonal affective disorder lays out these criteria in plain language.

It also helps to know what SAD is not. NIMH notes that it is tied to changes in daylight, not the calendar. Stress around the holidays, family visits or a busy season at work can make anyone feel worse, but that is not the same condition.

What Researchers Think Drives the Winter Pattern

Researchers are still working out the causes, and most of the evidence so far is about the winter pattern. The leading explanations, as summarized by NIMH, are:

• Reduced serotonin activity: Shorter daylight hours appear to interfere with molecules that help keep serotonin, a brain chemical involved in mood, at normal levels.
• Too much melatonin: People with winter-pattern SAD may produce excess melatonin, the hormone that regulates sleep, which can add to sleepiness and oversleeping.
• Lower vitamin D: Less sunlight on the skin can mean lower vitamin D, which is believed to support serotonin activity.
• A body clock that cannot adjust: Together, these shifts can leave the daily sleep-wake rhythm out of step with the season, which shows up as changes in sleep, mood and behavior.

NIMH also points out that SAD is more common in women, usually begins in young adulthood, and is more common in people who live farther north. Living in California lowers the odds compared with Alaska or New England, but it does not rule SAD out. The diagnosis rests on the pattern, not the zip code.

What Light Therapy Actually Involves

Light therapy has been a mainstay for winter-pattern SAD since the 1980s. In the form NIMH describes, a person sits in front of a light box rated at 10,000 lux for about 30 to 45 minutes each day, usually first thing in the morning, from fall to spring. These boxes are roughly 20 times brighter than ordinary indoor light and filter out ultraviolet light.

A few practical points tend to get missed:

• It is a daily routine across months, not a one-time fix, so the morning schedule has to make room for it.
• People with certain eye conditions, or who take medications that increase sensitivity to light, may need a different approach or medical supervision. That is a reason to bring a complete medication list to the appointment where light therapy comes up.
• Light therapy is a treatment for the winter pattern. NIMH notes there are no treatments specific to the summer pattern.

Other Treatments and How Long They Take

NIMH groups SAD treatment into four categories that can be used alone or together: light therapy, psychotherapy, antidepressant medication and vitamin D.

Psychotherapy. Cognitive behavioral therapy has been adapted for SAD. The version NIMH describes runs as two weekly group sessions for 6 weeks. It focuses on reworking negative thoughts about the season and on behavioral activation, which means planning engaging activities on purpose to offset the pull toward withdrawal. In head-to-head research summarized by NIMH, it worked about as well as light therapy, and its benefits appeared to last longer.

Medication. Antidepressants can help with SAD, alone or alongside therapy. NIMH notes they usually take 4 to 8 weeks to work, and that sleep, appetite and concentration often improve before mood does. Which medication, if any, is a decision for a prescriber who knows the full history. Do not start, stop or change a medication on your own.

Vitamin D. Studies of vitamin D for SAD have produced mixed results. Ask a clinician before relying on it.

The Two-Winter Check You Can Do This Week

Because the diagnosis depends on at least two years of history, the most useful thing you can bring to an appointment is a rough timeline. In California, clocks fall back at 2 a.m. on Sunday, November 1, 2026, and do not spring forward until Sunday, March 14, 2027. That is 19 weeks of earlier evenings. Before then, take 20 minutes and do this:

1. Open your phone calendar, photos or messages for October through March of the last two winters.
2. Note roughly when you started sleeping more, canceling plans, eating differently or missing work, and when that lifted.
3. Write down anything you tried and whether it helped.
4. Ask one person who sees you often whether they noticed the same timing.
5. Bring the page to your next appointment and ask directly: “Does this look like a seasonal pattern, and would that change what you recommend?”

Families often see the pattern before the person living it does. The same partner or parent who notices withdrawal every January can add dates that someone in the middle of a depressive episode may not remember clearly.

Why Seasonal Depression and Bipolar Disorder Are Checked Together

NIMH notes that SAD is more common in people with bipolar disorder, especially bipolar II, which involves depressive episodes alternating with hypomanic episodes. In practice, that means a clinician who hears “I crash every winter” will usually also ask what happens in spring. A lift that feels like more than relief, such as needing much less sleep, racing thoughts or unusual spending, is important to mention. It can change the treatment plan, so share it even if it felt good at the time.

When Seasonal Depression Needs More Than Outpatient Care

Most people with SAD are treated in outpatient settings. A higher level of care may be worth discussing when depression has stopped someone from working or caring for themselves, when outpatient treatment has not helped, or when SAD sits alongside another condition such as an anxiety disorder, panic disorder or bipolar disorder. A residential mental health program provides daily structure, therapy and psychiatric care in one place, and a seasonal history gathered in advance helps the team plan from day one. You can compare levels of care on our treatment programs page or verify your insurance before you call. To talk through options, call 877-883-0780.

If Things Feel Urgent

Thoughts of death or suicide can be part of any depression, including seasonal depression. If you or someone you love is having thoughts of suicide or is in emotional crisis, call or text the 988 Suicide and Crisis Lifeline at 988, available 24 hours a day. SAMHSA explains how the 988 Lifeline works. If anyone is in immediate danger, call 911 or go to the nearest emergency room. Residential treatment programs are not emergency services.

Getting Started

A seasonal pattern is a real, treatable form of depression, and it is easier to treat when the history is clear. If you are noticing the same slide each fall and want to talk through what kind of care fits, call 877-883-0780. For a broader look at depression symptoms and treatment, see NIMH’s depression overview.

This article is for educational purposes only and is not medical advice, a diagnosis or a substitute for evaluation by a qualified health care professional. Decisions about diagnosis, light therapy and medication should be made with a licensed clinician. Do not start, stop or change any medication without talking to your prescriber.