By Melissa Schorr

NEW YORK, Dec 19 (Reuters Health) - People with seasonal affective disorder (SAD) instinctively shift their melatonin levels with the seasons, paralleling the hibernation patterns of mammals, according to US researchers.

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.

"In patients who had SAD, the duration of melatonin secretion became longer in winter and shorter in summer, just as it occurs in other mammals," lead author Dr. Thomas A. Wehr, a research psychiatrist at the National Institute of Mental Health in Bethesda, Maryland, told Reuters Health. "That could be controlling this panoply of changes that occurs in people when they get depressed in the winter."

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 researchers compared melatonin levels in 55 SAD patients with levels in a "control group" of 55 healthy volunteers. All of the study participants were placed in a dimly lit room for 24 hours and the amount of melatonin in their blood was measured every half hour, once during the winter and again during the summertime.

As expected, Wehr's team found the healthy individuals were immune to shifts in the natural daylight, with their melatonin levels remaining stable throughout the seasons. However, those with winter depression had a moderate decrease in the length of time they produced melatonin during the summertime.

The duration of active melatonin secretion was about 9 hours in the control group, whether in winter or summer. In the SAD group, active melatonin secretion was 9 hours in winter and 8.4 hours in summer, according to the report in the December issue of the Archives of General Psychiatry.

"We're seeing a remnant of our own evolutionary heritage," Wehr said. "The fact that the symptoms resemble changes that animals experience speaks to a root of this problem in evolutionary biology."

These findings also suggest that medications that curtail the longer duration of melatonin production may be an effective treatment for these patients. "Anything you can do to manipulate melatonin signals might be useful as a treatment for seasonal depression," Wehr said. "If you could trim off the last hour or two melatonin is produced, you would convert a winter signal to a summer."

SOURCE: Archives of General Psychiatry 2001;58:1108-1114.

http://www.reutershealth.com/archiv...219elin013.html