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Seasonal Affective Disorder

Written by Ada’s Medical Knowledge Team

Updated on

What is seasonal affective disorder?

Seasonal affective disorder, also known as SAD or seasonal depression, is a type of depression that occurs at the same time every year, usually beginning in fall and ending in spring. The condition is considered to be a subset of depression with a seasonal pattern, rather than a separate disorder. 1 2

Around 5 percent of the US population will experience seasonal depression each year. 2 However, statistics vary according to location. For example, just 1 percent of the population of Florida experience the condition, whereas 9 percent of people living in Alaska experience seasonal affective disorder. 3

In around 10 percent of people, seasonal affective disorder occurs during the summer months. Some people with bipolar disorder may also experience seasonal patterns to their condition. 4

Treatment can improve symptoms but many people will continue to experience the condition every year. Left untreated, symptoms can be debilitating. However, due to its seasonal nature, the onset of depression can be anticipated and preventive action taken. 5 4

If you think you or someone you know may be experiencing symptoms of seasonal affective disorder, try the Ada app for a free assessment.

Symptoms of seasonal affective disorder

Symptoms are broadly the same as for major depression, though specific to one season, including:

  • Low mood most of the day, throughout the season
  • Feeling hopeless
  • Lacking in energy
  • Changes in appetite and weight
  • Difficulty concentrating
  • Feeling hopeless, worthless or guilty
  • Thoughts of suicide or death

Read more about Signs of Depression »

Symptoms specific to winter seasonal depression include: 1 6 7

  • Sadness
  • Crying
  • Hypersomnia, i.e.sleeping too much
  • Lethargy
  • Social withdrawal
  • Tiredness
  • Restlessness
  • Cravings for carbohydrates
  • Changing appetite
  • Change in weight

Symptoms of a rare form of SAD known as summer seasonal depression include: 1 6

  • Insomnia, i.e. not sleeping enough
  • Agitation
  • Restlessness
  • Poor appetite
  • Weight loss
  • Anxiety

If you think you or someone you know may be experiencing symptoms of seasonal affective disorder, try the Ada app for a free assessment.

Seasonal affective disorder causes

It is not entirely clear why some people experience depression that is connected to seasons, though there are some associated factors.

Brain chemistry

It is thought that there are associations between seasonal affective disorder and levels of certain subtances that affect the brain:

  • Serotonin
  • Melatonin
  • Vitamin D


People with seasonal affective disorder have problems regulating levels of serotonin in the brain, which is thought to be responsible for maintaining mood levels. A protein called SERT cancels the action of serotonin by transporting it out of the place of action quickly, and people with seasonal depression have more SERT in the winter months than during summer. Higher SERT levels are associated with lower serotonin levels, which is believed to cause depression. 3


People with seasonal affective disorder may produce too much melatonin, which is a hormone that regulates sleep. The body produces more melatonin in the dark, so winter causes increased production, particularly in countries where there is little sunlight in winter. Too much melatonin in the body, combined with lower serotonin levels, can cause people to feel sleepier and can affect the body’s internal biological clock (circadian rhythms). Circadian rhythms are responsible for regulating people’s 24-hour cycles of waking and sleeping, and can also be influenced by temperature variations during seasons. 1 2

Read more about Melatonin

Vitamin D

People with seasonal affective disorder have been found to have lower-than-normal levels of vitamin D in their bodies. Low levels of vitamin D can be associated with not getting enough sunlight, though poor diet can also be a factor. There is no firm evidence that a lack of vitamin D causes seasonal depression, but there appears to be an associated link. 1 3

Read more about Vitamin D Deficiency ».

Causes of summer seasonal affective disorder

Approximately one in 10 people with seasonal affective disorder experience symptoms of depression in summer. The reasons are unclear, but some psychiatrists believe that there are links to: 8 9

  • Disruption to everyday routines caused by summer vacations
  • Extended daylight causing disruption to sleep schedules
  • Summer heat, which some people can find oppressive
  • Increase in socialising and more opportunities to consume alcohol
  • Body image issues that may be more evident in hot weather because of more revealing clothing
  • Financial concerns caused by social pressures to travel or socialize

Risks factors for seasonal affective disorder

Some people are at a higher risk of developing seasonal affective disorder. These people include: 1 3

  • Women are four times as likely as men to develop SAD
  • People with a family history of depression or who have depression or bipolar disorder themselves are more likely to be diagnosed with SAD
  • SAD can affect people of any age, though it is more common in younger adults, aged 20-30 years old
  • The farther away from the equator a person lives, the more likely they are to develop SAD


Seasonal affective disorder is no longer considered a disorder of its own by the DSM-5 10, but as a type of depression with a seasonal pattern.

To be diagnosed with seasonal depression, a person must: 6

Meet the criteria for being diagnosed with depression. For more information, see the resource on signs of depression.

Have depressive episodes during particular seasons.

Not experience depressive episodes during other seasons, or experience depression primarily during one season.

Experience the pattern of seasonal depression for at least two years.

Have experienced more seasonal depressive episodes than non-seasonal depressive episodes over the course of a lifetime.

There is no medical or blood test to diagnose seasonal affective disorder. A doctor or other mental health practitioner will make a diagnosis based on dialogue, and interaction with the person affected and by listening to their primary symptoms, according to DSM-5 criteria.

Other medical tests and examinations might be performed to rule out other possible causes, including 7

Seasonal Pattern Assessment Questionnaire

One diagnostic test to identify whether a person has seasonal affective disorder is the Seasonal Pattern Assessment Questionnaire (SPAQ). This is a quiz developed by a researcher working under the National Institute of Mental Health. The test is freely available to download here and does not require training to use.

The SPAQ asks the person taking the test to rank their mood levels and activities, such as eating, sleeping and socializing, over the course of a year. 3

The Seasonal Pattern Assessment Questionnaire has been shown to measure results consistently from person to person and has been appropriately designed to measure symptoms of seasonal affective disorder. However, there is some doubt about the reliability of the SPAQ for the following reasons: 3 6 11

The test is taken retrospectively and people may not accurately recall their moods over the course of a year

The questions may lead people to score themselves higher for seasonal depression than their symptoms justify

There is doubt about the SPAQ’s test-retest reliability, which is how reliable the test is at providing consistent results over a period of time

Symptom measurement is based on observation and is subjective, so may not be adequately captured by a scoring system

For these reasons, the Seasonal Pattern Assessment Questionnaire should not be the only tool used to predict how a person’s condition will progress over the years.

The Ada app can also give a free symptom assessment.


Once a person becomes aware that they experience seasonal depression, it may be possible to plan ahead of time to prevent or lessen the severity of symptoms. This can include: 12 6 13

Starting medication, such as bupropion in early fall, or early spring for summer seasonal affective disorder.

Starting light therapy in early fall by using a light therapy box. Read more about the treatment of seasonal affective disorder »

Using behaviors learned in previous psychotherapy sessions, such as stress-relieving practices or reducing intake of carbohydrates.

Taking exercise, particularly walking outside.


There are four main methods of treating seasonal affective disorder, which can be used alone or in combination.

Light box therapy

Light therapy, also known as phototherapy, is based on the idea that seasonal affective disorder is caused by lack of light during winter. People sit in front of a bright lamp or light box each day from early fall to late spring. This is thought to counteract the effects of lost daylight and reduce melatonin production.

The lamps or light boxes used to treat seasonal depression contain light bulbs that emit very bright white fluorescent light, with no ultraviolet light. The light bulbs emit around 10,000 lux, 14 which is a similar level of illumination to daylight and around 20 times brighter than normal indoor lighting. ref1 12

A person with seasonal affective disorder should sit 12 to 18 inches away from the light box for 20 to 60 minutes each morning. The person must have their eyes open but is not required to look at the lamp, making it possible to do other activities at the same time, such as have breakfast or read.

The clinical effectiveness of light box therapy for seasonal affective disorder is not yet established. 15 However, many people report benefits, and the treatment has relatively few side effects.

Some side effects of using light boxes can include: 3

  • Eye strain
  • Headaches
  • Irritability
  • Difficulty sleeping after exposure to light, which is why use is recommended for mornings

Rarer side effects can include:

  • Hypomania
  • Suicidal thoughts
  • Nausea
  • Irritability or agitation
  • Hyperactivity
  • Could trigger migraines

Light boxes should not be used by people who: 3

  • Take medications which increase sensitivity to light, such as lithium, melatonin, phenothiazine antipsychotics and some antibiotics
  • Have retinal disease
  • Are at risk of age-related macular degeneration
  • Have a condition that makes skin especially sensitive to light, such as systemic lupus erythematosus
  • Suffer sensitivity to light in general

Improvements in people undergoing light box therapy treatment are generally observed within one or two weeks. Treatment should be continued until spring to avoid relapse. Some people may find it difficult to dedicate the time needed to commit to light box therapy over the course of the changing seasons. 12 4

Light boxes and lamps can be expensive to buy, but some insurance policies recognize the benefits of light box therapy for seasonal affective disorder. Light box therapy is a medical procedure and should only be used with the guidance of a health professional. 6

Dawn simulators

Some people find a dawn simulator preferable to light box therapy to treat seasonal affective disorder. Dawn simulators are lamps, often attached to alarm clocks, which gradually increase in brightness to simulate the effect of a sunrise.

A small study found dawn simulators to be as effective as light box therapy in reducing symptoms of depression. Some people may find dawn simulators a more convenient light therapy option when compared to light boxes, as treatment is mainly administered while asleep. 16

Medication for seasonal affective disorder

Seasonal affective disorder can be treated with antidepressants, particularly selective serotonin reuptake inhibitors (SSRIs) such as fluoxetine. SSRIs have been found to be as effective in treating seasonal depression as light box therapy. 3

The US Food and Drug Administration (FDA) has approved the use of bupropion to treat seasonal affective disorder. Bupropion is another type of SSRI antidepressant.

It may take four to five weeks to feel the effects of SSRIs. If medication is effective, it should be taken continuously until the season changes. 4

People taking fluoxetine and bupropion can experience side effects. A doctor will be able to advise on how to manage them.


Forms of psychotherapy and counselling can be helpful to people with seasonal affective disorder whose symptoms are not severe.

Cognitive behavioral therapy has been found to be effective by helping people think differently about their seasonal depression. New ways of approaching the condition include: 3 4

  • Eating more healthily and eating fewer carbohydrates
  • Engaging in an exercise routine
  • Practising good sleep hygiene, such as sleeping in a cool, dark room and not being disturbed by electronic equipment
  • Practising yoga, mindfulness and other stress management techniques
  • Spending time with friends and family
  • Doing activities the person finds pleasurable
  • Spending time outdoors

Counseling can produce long-term benefits from one course of therapy, which is generally one or two sessions a week for six weeks. This compares favourably to treatments such as medication and light box therapy, which need to be repeated each season. 17

Vitamin D

Low levels of vitamin D are associated with seasonal affective disorder. It is thought that vitamin D supplements may help relieve symptoms of the condition. 1

Evidence for effectiveness is currently mixed, but there is the suggestion that taking vitamin D supplements before the onset of winter could help prevent, or lessen, the severity of seasonal depression symptoms. 3

Treatment for summer seasonal affective disorder

Because the causes of summer seasonal affective disorder are thought to be different to those of people affected in winter, treatments such as light box therapy and vitamin D are unlikely to be effective. However, effective treatment may include some of the following: 9 4

However, medication with antidepressants is thought to alleviate symptoms.

Psychotherapy such as cognitive behavioral therapy can help to build awareness of trigger points and develop coping strategies, such as getting enough sleep.

Limiting the amount of daylight a person receives and staying cool is also thought to help reduce the severity of symptoms.


Q: Is seasonal affective disorder real?
A: Seasonal affective disorder is real. There are people whose depression recurs in connection with a particular season. The reason is thought to be linked to fluctuations in serotonin and melatonin levels over the year. Read more about the causes of seasonal affective disorder »

Q: Is seasonal affective disorder a bipolar variant?
A: Seasonal affective disorder is not a variant of bipolar disorder. When a person has bipolar disorder, their moods swing from highs of mania and hypomania to lows of depression. Sometimes those episodes may occur at specific times of the year. People with seasonal affective disorder experience only depression at a specific time of year.

Q: Do SAD light boxes work?
A: Light boxes to treat seasonal affective disorder work by using bulbs which emit a very bright light; the person sits close to the box for 20 to 60 minutes each morning. Some people report that their depression symptoms are improved by using a light box, and light therapy is a recognized treatment for SAD. However, the scientific evidence for clinical effectiveness is not yet established.

Other names for seasonal affective disorder

  • Seasonal depression
  • SAD
  • Winter blues