Seasonal affective disorder (SAD) is a form of depression that the NHS estimates to affect approximately one in 15 people between September and April. It can be particularly severe during December, January and February. For some people, SAD is so disabling that they cannot function in winter without continuous treatment. Others may experience a milder version called sub-syndromal SAD or ‘winter blues’.

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It occurs throughout the northern and southern hemispheres but is extremely rare in people living within 30 degrees of the Equator, where daylight hours are long, constant and extremely bright.

What are the symptoms of SAD?

SAD may begin at any age, but it most commonly starts between 18 and 30. Symptoms generally appear between September and November and continue until March or April, when there may be a sudden burst of energy and activity accompanying the longer, brighter spring and summer days. A diagnosis is usually made after you’ve experienced two or more consecutive winters of symptoms.

Common symptoms include:

• sleep problems – usually oversleeping and difficulty staying awake but in some cases disturbed sleep and early morning waking
• lethargy – lacking in energy and unable to carry out normal routine due to fatigue. Heaviness in the arms and legs overeating – craving for carbohydrates and sweet foods, which usually leads to weight gain
• depression – feeling sad, low and weepy, a failure, sometimes hopeless and despairing
• apathy – loss of motivation and ability to concentrate
• social problems – irritability and withdrawal from social situations, not wanting to see friends
• anxiety – feeling tense and unable to cope with stress
• loss of interest in normally pleasurable activities
• loss of libido – decreased interest in sex and physical contact
• weakened immune system – vulnerability to catching winter colds and flu
• mood changes – for some people bursts of over-activity and cheerfulness (known as hypo-mania) in spring and autumn.