As the days get shorter, some people notice their mood drop in a way that returns every year. This guide covers what the National Institute of Mental Health (NIMH) and MedlinePlus say about seasonal affective disorder (SAD): the signs, what is and is not known about causes, and the treatments. It is general education, not a diagnosis.
By the Exsura Team · October 25, 2026
NIMH says: “If you or someone you know is struggling or having thoughts of suicide, call or text the 988 Suicide and Crisis Lifeline at 988 or chat at 988lifeline.org . In life-threatening situations, call 911 .” The 988 Lifeline says it is “available 24/7/365” and that conversations are “free and confidential.” Its home page also says text and chat are available in Spanish. For help choosing between 988 and 911, see 988 vs 911: when to call which.
NIMH says: “SAD is a type of depression characterized by a recurrent seasonal pattern, with symptoms lasting about 4−5 months out of the year.” MedlinePlus describes it as a type of depression that comes and goes with the seasons. NIMH says “In most cases, SAD symptoms start in the late fall or early winter and go away during the spring and summer,” known as winter-pattern SAD or winter depression. It adds that summer-pattern SAD is less common.
“The depression associated with SAD is related to changes in daylight hours, not the calendar, so stresses associated with the holidays or predictable seasonal changes in work or school schedules, family visits, and so forth are not the same as SAD.”NIMH, Seasonal Affective Disorder
SAD is a form of depression, so it can include the usual depression symptoms. For winter-pattern SAD, NIMH lists these additional signs:
NIMH says “It is estimated that millions of Americans experience SAD, although many may not know they have this common disorder. In most cases, SAD begins in young adulthood.” It says SAD occurs much more often in women than in men. MedlinePlus says it is more common in women, young people, and those who live far from the equator.
For readers in Texas, NIMH gives this example: “people in Alaska or New England are more likely to develop SAD than people in Texas or Florida.”
NIMH says: “Researchers are still determining what causes SAD.” It describes several areas of research. Studies indicate that people with SAD, especially winter-pattern SAD, have reduced levels of the brain chemical serotonin. Shorter daylight hours may prevent related molecules from functioning properly. Vitamin D deficiency may exacerbate these problems, because vitamin D is believed to promote serotonin activity. NIMH says other studies suggest that both forms of SAD relate to altered levels of melatonin, and that people with winter-pattern SAD produce too much melatonin, which can increase sleepiness and lead to oversleeping.
MedlinePlus says “The exact causes of SAD are unknown.”
NIMH says: “Light therapy and vitamin D are treatments for winter-pattern SAD, whereas psychotherapy and antidepressants are used to treat depression in general, including winter- and summer-pattern SAD.”
NIMH says to talk to a health care provider about any dietary supplements and prescription or over-the-counter medications you are taking, and notes that vitamin D can interact with some medications. MedlinePlus says some people with SAD do not respond to light therapy alone, and that antidepressant medicines and talk therapy can reduce symptoms either alone or combined with light therapy.
NIMH says people with a history of the disorder might benefit from starting treatments before the fall (for winter-pattern SAD) to help prevent or reduce depression symptoms, and notes that “To date, few studies have investigated whether SAD can be prevented.”
NIMH says: “If you or someone you know is showing symptoms of SAD, talk to a health care provider or a mental health specialist about your concerns.” For where to look and how to prepare for a first visit, see how to find a mental health provider, or low-cost mental health care in Dallas County.
NIMH describes SAD as a type of depression with a recurrent seasonal pattern, diagnosed when depressive episodes occur during specific seasons for at least 2 consecutive years. It says holiday stress and predictable seasonal changes in schedules are not the same as SAD.
NIMH says people in Alaska or New England are more likely to develop SAD than people in Texas or Florida.
MedlinePlus says the main treatment is light therapy. NIMH describes light therapy, talk therapy (CBT-SAD), antidepressants and vitamin D as options, and says treatments can be used alone or in combination. A provider can help you choose.
NIMH says studies testing vitamin D as a treatment for SAD have produced mixed results, with some indicating it is as effective as light therapy and others finding no effect. It also says vitamin D can interact with some medications, so talk to a health care provider first.
Sources last checked October 25, 2026. This article is general education, not financial, legal or medical advice. Rules, fees and programs change, so confirm details with the organization before you act.
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