Seasonal affective disorder (SAD) is characterised by recurrent episodes in autumn and winter of depression, hypersomnia, augmented appetite with carbohydrate craving, and weight gain, and can be successfully treated with bright light. Circadian rhythm hypotheses (summarized in) have stimulated research into the pathophysiology of SAD, postulating that:

1. 1.The illness is a consequence of delayed phase position,

2. 2.It is correlated with diminished circadian amplitude, or

3. 3.It results from changes in the nocturnal duration between dusk and dawn e.g. of low core body temperature or melatonin secretion. Light is considered to act directly on the circadian pacemaker (‘Process C’) and not on sleep dependent processes (‘Process S’). Thus successful treatment of SAD must act via mechanisms within known retinohypothalamic pathways. Conversely, emergence of SAD symptoms may reflect inappropriate neurobiological response to decreasing daylength.

Since the description of SAD the prevalence of this disorder has been of particular interest. The more so, because early studies, indicated the correctness of the on theoretical grounds - expected interaction between latitude and prevalence rate of SAD. More eleborate prevalence studies, mainly in the USA indeed showed a positive correlation between prevalence rate and latitude. In Europe, only few studies have appeared. Although the results of most of these studies are preliminary and research-methods and criteria vary across studies, tentative conclusions are that the prevalence rates in Europe are considerably lower than in the USA, but that there is a similar trend in the direction of an increase of prevalence at higher latitudes.8"10 In the present study, the prevalence of SAD in the Netherlands is investigated.

The common treatment modalities for depression, i.e. pharmacotherapy and psychotherapy have the significant disadvantage of at least a three- to four-week time lag between initiation of treatment and amelioration of mood. Total sleep deprivation (TSD) in contrast, leads to an immediate antidepressant effect in 60% of the patients. However, it has gained only little clinical relevance as usually the improvement is only transient and almost regularly reversed by the next nights of sleep. A procedure preserving the antidepressive effect of sleep deprivation would therefore be of high clinical relevance.