Seasonal affective disorder (SAD), also known as winter depression or winter blues, is a subtype of mood disorder consisting of recurrent major depressive episodes of varying severity, which occur with a seasonal pattern. The most common type of SAD is winter depression with symptoms beginning in autumn and winter. This is followed by full remission or hypomanic states (mild state of mania) during the following spring and summer. A rare form of SAD occurs during the summer. SAD is generally characterised by four central features,
1. Recurrent major depressive episodes, which start around the same time each year, for example September to October, and end around the same time each year, for example March to April,
2. Full remission of symptoms during the unaffected period of the year, for example May to August,
3. Relatively more seasonal depressive episodes than non-seasonal episodes, over the lifetime course of the illness,
4. Seasonal depressive episodes occur in at least 2 consecutive years.
Subsyndromal SAD is a disorder with similar but milder symptoms to SAD, which do not affect the patient's ability to function. It has been reported that there is a paucity of information concerning the incidence and detection of SAD in UK populations. Prevalence rates suggested by studies carried out have ranged from 1 to 12% depending on the diagnostic criteria used. The overall lifetime incidence of SAD is said to range from 0 to 9.7%, depending on the specific population studied and how it is diagnosed. SAD prevalence may be higher in northern latitudes than southern latitudes and may vary within ethnic groups at the same latitude. It has been reported that the risk of SAD probably increases if people move to live further away from the equator. Almost all the studies of the incidence of SAD report that women are more likely to suffer from SAD. The average ratio of women to men across all studies is 1.8 to 1.2 During the reproductive years, female sufferers predominate; however a reduction in incidence and a narrowing of gender differences is seen in old age. With respect to age, studies report that the life time incidence of SAD increases with age until around age 60. After the age of 50-54, it is reported that the incidence declines dramatically and as such, over the age of 65 the incidence of SAD is very low. Regardless of this however, patients over 65 may still present to hospitals for treatment. It has been suggested that the response of patients over 65 to treatment does not differ from that of younger patients with SAD. It should be noted that the low incidence of SAD in older individuals may be as a result of misdiagnosis as physicians may consider symptoms presented by such individuals as being down to old age and not SAD. Children also suffer from SAD, although this is quite rare as children have more opportunity than adults to play and interact outdoors thereby exposing them to more sunlight and as such suppressing any possible triggers of SAD; however, incidence rates rise at puberty. Severity of SAD can be mild, moderate or severe.