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.

Many of the dramatic seasonal changes that occur in animals are triggered by changes in the length of night. Changes in the duration of melatonin secretion, which usually occurs exclusively at night and therefore lasts longer in winter than in summer, often act as a chemical transducer of the effects of seasonal changes in night-length on animals' behavior and physiology. Cells in sites that regulate seasonal behaviors, such as those associated with breeding, appear to possess melatonin receptors and interval timing mechanisms that enable them to detect melatonin and measure the duration of the nightly interval when it is present. These sites then trigger changes that are appropriate for the season that is indexed by the measured length of the interval.

The present review deals with preventive and treatment strategies for sleep and wake disorders, and the other problems of shift workers. Management must take into account the direction, speed and other parameters of the rotation schedule. Good sleep-wake hygiene may alleviate the problems. Further, additional napping may compensate – at least partially – for sleep deprivation and fatigue. Hypnotics must be avoided as much as possible. Finally, new techniques for manipulation of circadian rhythms are discussed: light, melatonin and benzodiazepines.

Melatonin is a hormone secreted by the pineal gland mainly during the night. The discovery that this melatonin secretion decreases under the influence of bright light, gave rise to the use of light therapy in some affective disorders. The literature on the relationship between melatonin secretion and mood is reviewed concerning seasonal affective disorder, non-seasonal affective disorder and premenstrual syndrome. Light therapy could reduce an abnormal high melatonin secretion back to normal proportions. None of the affective disorders, however, is accompanied by an unusual high melatonin level. Nevertheless, light therapy as well as other therapies that suppress melatonin have a therapeutic effect. This is not the case with the administration of melatonin. Mood is not affected by extra melatonin in seasonal affective disorder but it is in both other affective disorders. Melatonin plays a part in the pathogenesis of the affective disorders but it is not yet clear which one.