Seasonal Affective Disorder

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Seasonal affective disorder (SAD) is a type of depression which occurs when you develop symptoms of depression during the darker winter months each year. Winter blues or sub-syndromal SAD (S-SAD) is a less severe form of the condition.

In the UK and other countries north of the equator, the symptoms usually develop at some time between September and November and continue until March or April. Symptoms tend to be worse in December, January and February.

What is seasonal affective disorder?

Prof Swaran Singh

People with SAD start to develop symptoms, like difficulty in waking up and eating more starchy foods, in September. These symptoms become worse as the hours of daylight become shorter. The most severe symptoms, such as not wanting to see family and friends and depression, tend to occur between November and January.

  • Difficulty in waking up in the morning
  • Decreased energy/lethargy
  • Craving for sweet and starchy foods (carbohydrates)
  • Increased appetite
  • Increased sleep
  • Weight gain
  • Difficulty concentrating
  • Decreased interest in sex (loss of libido)
  • Withdrawal from family and friends
  • Depression/anxiety/irritability

SAD can markedly impair quality of life in winter. Symptoms tend to improve and go fairly quickly in the spring, over a week or so. Some people develop great bursts of energy and creativity in the spring. In a small number of cases, as spring arrives the mood changes from depression into an abnormally high and elated mood (mania or hypomania).

Some people who work in buildings without windows may have SAD symptoms throughout the year. On the other hand, if you have SAD whilst living in the UK, the symptoms may not occur if you move to a country nearer the equator, such as southern Spain.

Winter blues/sub-syndromal seasonal affective disorder (S-SAD)

In the winter many people feel more tired, sleep more, put on some weight, and feel a little low. However, they do not develop the full features of depression to be classed as having SAD.

Symptoms of depression

When symptoms develop in the winter, they are similar to those that occur in the non-seasonal ordinary type of depression. The following is a list of common symptoms of depression. You may not have them all; however, several usually develop:

  • Core (key) symptoms:
    • Persistent sadness or low mood. This may be with or without weepiness.
    • Marked loss of interest or pleasure in activities, even for activities that you normally enjoy.
  • Other common symptoms:
    • Disturbed sleep compared with your usual pattern. This may be difficulty in getting off to sleep, or waking early and being unable to get back to sleep. Sometimes it is sleeping too much (particularly in SAD - see below).
    • Change in appetite. This is often a poor appetite and weight loss. Sometimes the reverse happens with comfort eating and weight gain. (People with SAD often put on weight - see below.)
    • Read more about the common symptoms of depression.

Between 3 and 6 people in 100 in the UK are thought to experience SAD. Many more, perhaps as many as 12-13 in 100, have winter blues. SAD is less common in countries near to the equator where the hours of sunlight are more constant and bright throughout the year. SAD usually first begins between the ages of 20 to 30 but it can develop at any age. It affects four times as many women as men.

The exact cause is not clear. The amount of sunlight affects the number of nerve messages which you send from the eyes to certain parts of the brain. The activity of nerve messages caused by sunlight affects the level of certain brain chemicals (such as serotonin) and hormones (such as melatonin). These chemicals and hormones affect your mood. With less sunlight during the winter months, changes in the balance of these chemicals and hormones may trigger depression.

Some people seem to inherit a tendency to develop SAD. About 1 in 7 first-degree relatives (mother, father, child, brother, sister) of people with SAD are also affected.

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The diagnosis of SAD is based on you having episodes of depression which have occurred at least two years running during the winter months and with no symptoms during the spring. Also, if you have SAD your symptoms are more likely to include the atypical features of depression (craving sweet things, increased appetite, weight gain, increased sleepiness).

You and your doctor may not realise that you have SAD for several years. This is because recurring depression is quite common. You may have been treated for depression several times over the years before it emerges that you have the seasonal pattern of SAD. It is sometimes useful to record your symptoms in calendars or to use a special questionnaire called the Seasonal Pattern Assessment Questionnaire (SPAQ), to help to decide whether you may have SAD. To view the SPAQ, see references below.

Seasonal affective disorder

Throughout the winter, we are less exposed to the sun because the days are shorter. When it's dark at night, your eyes send a signal to the brain that it's time to feel tired. If by 5pm the sun begins to set, it's bound to interfere with the body's alert levels.

— Dr Francisco Cruz, Is Seasonal Affective Disorder real?


These self-help measures will usually be enough to help mild S-SAD but should also be used at the same time as treatment for more severe SAD.

  • Natural sunlight. Aim to get as much natural daylight as possible, especially at midday and on bright days. For example, if possible, go for a walk outside every day for 1-2 hours during the daytime, as this may well improve symptoms. If you work indoors with artificial lighting, try to go out at lunchtime, even if it's raining. Perhaps go for a walk or eat lunch on a park bench if one is available.
  • Winter holiday. For people who can afford it, a winter holiday to a sunny country will usually improve symptoms - but only for the duration of the time spent in the sunny country.
  • Regular exercise. It is best to do this outside since this gives you daylight as well.
  • Tell your family and friends. This is so that they can understand what is happening and be more supportive.
  • Make plans for springtime. This is the time of year when the days will become longer.

Light therapy

Many people find that bright light therapy helps to improve their symptoms of SAD. During research studies, it is difficult to measure the real effect of improving symptoms with light versus the placebo effect. It is generally agreed by doctors that there is a good chance that light therapy can improve symptoms if you have SAD. However, light treatment takes time and commitment. See below for details.

Usual treatments for depression

It is important that the depression symptoms of SAD should be treated in the same way as any kind of depression. These include antidepressant medicines and various forms of talking (psychological) treatments such as cognitive behavioural therapy (CBT). See separate leaflet called Depression for more details.

What is light therapy?

This treatment consists of sitting in front of a special bright light for a session each day and/or using a dawn simulator. Light intensity is measured in lux. Ordinary light bulbs are not strong enough, as they only give out 200-500 lux. To treat SAD you need a light source of at least 2500 lux (about ten times that of ordinary light bulbs).

What does light treatment involve?

Special light boxes are made for the purpose of treating SAD. There are various shapes and sizes. Perhaps the most commonly used one is a box about the size of a sheet of A4 paper that stands on a desk or table. Follow the instructions that come with the box. This may be something like:

  • You start treatment in the autumn, as soon as symptoms begin. (Ideally, you start treatment even before symptoms begin.)
  • You sit 2-3 feet away from the light box.
  • You face the bright light but you do not have to look directly into it.
  • The length of light therapy needed each day varies. If the light source is very powerful (10,000 lux) then 30-45 minutes per day is usually sufficient. With less powerful light boxes, 2-3 hours per day are needed.
  • You can do things such as eating, desk work, reading, knitting, etc, whilst sitting in front of the light box.
  • Some studies suggest that treatment early in the morning works best; however, other studies do not confirm this. Therefore, it is often recommended that light therapy be carried out as early as possible in the day.
  • Some people have their light therapy session whilst having their breakfast and reading the morning paper.

Some people use a dawn simulator instead of, or in addition to, a light box. Dawn simulators are devices that slowly increase the room light. They gradually come on in the early morning over a period of around 60-90 minutes at the time just prior to when you normally wake up.

How does light therapy work?

The logic is that it replaces the bright sunlight which you normally see in the summer. However, it is not clear exactly how it works. It is not simply extending the length of the daylight hours. Bright light affects the back of the eye (the retina) which sends nerve signals to parts of the brain. This is thought to affect the level of certain chemicals and hormones which you make in parts of the brain which affect mood.

How quickly does light therapy work?

Many people notice an improvement in symptoms within 3-4 days. If symptoms improve, they tend to stay improved so long as you keep on with treatment every day until spring. In some cases it takes up to 4-6 weeks of treatment for symptoms to improve. It does not work in every case but it is thought that around 8 in 10 people with SAD experience improvement with light therapy. See your doctor for other treatment options if you do not notice an improvement after three weeks. 

Is light therapy safe?

There is a theoretical risk of damaging the retina. However, there do not seem to be any reports of harm with the specially designed light boxes. The light boxes used to treat SAD do not emit much ultraviolet (UV) light (the main damaging part of sunlight) to the skin and eyes. Side-effects occur in some people and include headaches, difficulty sleeping after an evening session of light therapy, irritability and tiredness.

Who should not use light therapy?

You should speak with your doctor before using light therapy if you have:

  • Retinal disease.
  • Macular degeneration.
  • Medication you take which increases your sensitivity to light (for example, some blood pressure medications, antibiotic medicines or cancer treatments).

Note: you should not use suntan machines as a source of bright light. The light from suntan machines gives off a lot of UV rays, which can harm your eyes. It is best to use only the light boxes which are made especially to treat SAD.

How can I obtain a light box or dawn simulator?

Light therapy with a light box that meets evidence-based guidelines from a reputable vendor is recommended for first-line treatment of SAD. However, you cannot obtain a light box or dawn simulator on prescription from the NHS but various companies make and sell them. Some companies will allow you to try before you buy, to see if it works for you before you commit to buying a light box.

There is some evidence to suggest that a course of CBT or antidepressants taken before the winter arrives can prevent some cases of SAD. Further research is needed to confirm the place of these preventative treatments.

Around 6 in 10 people with SAD continue to have depressive symptoms each year in the long term. However, as discussed above, you have a good chance of improving symptoms with treatment when symptoms develop. In about 2 in 10 people with SAD, the condition goes away completely after a few years and treatment is then no longer needed.

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Further reading and references