Seasonal Affective Disorder Has Biological Roots

It turns out that we may be similar to certain animals when the seasons change.

Patients with seasonal affective disorder generate a biological signal that's similar to the signal used to regulate changes in animal's seasonal behavior, according to an article in the December issue of the Archives of General Psychiatry.

Seasonal affective disorder is a common disorder in which sufferers have feelings of depression during the winter season. Symptoms include weight gain, increased sleep, decreased activity and loss of interest in sex. Such changes in mammals have been shown to occur in response to seasonal changes in sunlight.

Melatonin is a naturally occurring hormone secreted by the pineal gland in the brain that's involved in regulating the body's internal clock. Dr. Thomas A. Wehr from the National Institute of Mental Health in Rockville, Md., and colleagues measured the levels of melatonin secretion in constant dim light during both the summer and winter. The subjects included 55 patients with seasonal affective disorder and 55 healthy volunteers.

The researchers found that the patients and volunteers responded differently to seasonal change. The patients with the disorder produced melatonin for an average of nine hours a night during the winter and for an average of 8.4 hours during the summer. The healthy volunteers had no seasonal change in melatonin production.

Many mammals can detect changes in day length and use this information to control the timing of seasonal behavior. In animals, an internal clock regulates seasonal changes in behavior, resulting in more melatonin production in the winter than in the summer.

The authors suggest that humans may have the same type of internal clock that animals have to mediate seasonal behavior changes.