seasonal affective disorder

Form of depression that occurs in winter and is relieved by the coming of spring. Its incidence decreases closer to the Equator. One type of SAD is associated with increased sleeping and appetite.

SAD is linked to the circadian clock (see circadian rhythm) mechanism, which not only adjusts our physiological rhythm to day length but also measures day length and thereby senses the change of season. Melatonin, a hormone produced by the pineal body in the brain during the normal sleeping hours has also been linked to SAD. Melatonin secretion is inhibited by bright daylight. Thus, bright lights can be used to treat SAD.

Research into winter sleep patterns has shown that people deprived of artificial light sleep for part of the 14-hour winter nights, but also lie awake for as long as 5 hours at a time. During this period of quiet wakefulness their brainwaves resemble those produced during meditation. They feel more rested and energetic after these ‘natural’ winter nights. Hormone production is also affected, with an increase in the secretion of growth hormone, prolactin, and melatonin. Both a shortage of sleep and a disruption in natural hormone production may contribute to SAD.