SAD is more than just longing for some sunshine, and has effects similar to depression. As the nights draw in, sufferers take no pleasure in everyday activities, feel irritable and lethargic, sleep more, and find it harder to get up, as well as craving sugary carbohydrates and piling on weight. Unlike standard depression though, it usually lifts in the spring and summer.
Psychiatrists say the causes of SAD are still not fully understood, but it's likely that lower levels of exposure to daylight in winter play an important role in affecting the brain.
Some studies have found that low levels of vitamin D - also known as ‘the sunshine vitamin’ - are associated with depression, and may be a trigger for SAD, although more research is needed to establish any causal link.
Vitamin D is made by the body on exposure to sunlight, meaning those who live far from the equator where the UV index falls below 3 (the minimum required for vitamin D synthesis) in the winter months, don’t get enough to maintain levels.
Another theory is that people with SAD have a problem regulating the feel-good brain chemical, serotonin – also known as the ‘happy hormone’ - believed to be responsible for balancing mood, and instead overproduce the ‘sleep hormone’ melatonin, which is produced in response to darkness to induce sleepiness.
SAD symptoms typically start when the days become shorter in the late autumn (late October in the UK) and become more intense in December, January and February until they ease off in the Spring.
The symptoms are like that of depression but, as stated above, they have a seasonal pattern. They include: feelings of sadness; crying; low mood; lack of interest in normal activities; withdrawal from social activity; difficulties getting up in the morning; craving sugary carbohydrates; overeating; weight gain; and loss of sex drive.
Eight per cent of the population are estimated to suffer SAD symptoms.
Although only identified as a medical condition in the 1980s, SAD is now internationally recognised as a depressive illness, and can be diagnosed using a questionnaire called the Seasonal Pattern Assessment Questionnaire for those who have suffered from symptoms for several years.
Who gets SAD?
SAD is three times as common in women as men and, as stated, those living furthest from the equator are most at risk of developing SAD. For example, in the US, SAD affects one per cent of people in Florida but nine per cent in Alaska.
Shift workers, such as nurses, may also have an increased risk.
In the winter months, SAD can be treated with selective serotonin reuptake inhibitors (SSRIs) like fluoxetine (Prozac). However, a Cochrane review of high quality studies found there wasn't enough evidence for SSRI use in SAD, and pointed out that 27 per cent of participants in studies withdrew because of drug side effects.
Using a lightbox with 10,000 lux of cool white fluorescent light for 20 to 60 minutes daily during autumn and winter (20 times the intensity of ordinary indoor lighting) has also been shown to be an effective treatment for SAD. Daylight alarm clocks which wake you slowly with low-level light, simulating sunrise, may also help you get up in the morning.
Because low vitamin D levels are associated with an increased risk of depression, taking vitamin D supplements in the autumn and winter months may be beneficial for preventing SAD symptoms.
Cognitive behavioural therapy (CBT), a talking therapy, has also been shown to be effective for treating SAD. In one study, six weeks of CBT, consisting of two 90 minute group sessions a week, were as effective as 30 minutes of daily light box therapy.