People with seasonal affective disorder (SAD) instinctively shift their melatonin levels with the seasons, paralleling the hibernation patterns of mammals, according to researchers at the National Institute of Mental Health in Bethesda, Maryland.

This does not appear to be true for those who don't react to shorter days and longer nights with deepening depression, their study findings indicate.

Seasonal affective disorder, or "winter depression," is a psychiatric disorder that strikes during winter months, when daylight hours are naturally shortened. The disorder in humans seems to mimic the behavior exhibited by hibernating animals, such as increased sleep and decreased activity.

Melatonin, a hormone produced by the pineal gland in response to the darkness of nighttime, promotes sleep and helps regulate the body clock. Hibernation in mammals is triggered when the brain responds to the body's increase in melatonin production.

Prior research suggests that people with seasonal affective disorder are unable to use artificial light to readjust their body clock, but remain susceptible to the seasonal rhythms of shorter daylight hours during wintertime.

The new report has been published in the December issue of the Archives of General Psychiatry.

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Khorsheed.com - Jan 2002