Being woken up and exposed to bright light at night can throw off a person's biological clock for the next few days. What's more, the researchers found that being woken up at night at all -- even in a dark room -- also disrupts the body's timing, although to a lesser degree.

The wakening seem to introduce a "lag" into the body clock, pushing back the release of hormones and other body processes by as much as an hour and a half.

Dr. Samir Bangalore from Northwestern University Medical School in Chicago, Illinois found that while such sleep interruptions don't harm health, a person experiencing such a delay in the body clock "would feel tired in the morning and feel more aroused in the early evening.

The findings also offer clues to treating seasonal depression and other conditions marked by biological clock abnormalities.

Humans -- and many other creatures -- have roughly 24-hour body clocks that help regulate sleep patterns and energy levels, and also govern when hormones are secreted and other biological processes occur. These daily patterns are called circadian rhythms.

Dr. Bangalore and his colleagues tested the effects of awakening and nighttime bright light exposure on the circadian rhythms of 32 healthy volunteers. The study participants spent one night sleeping in the dark for 8 hours at the time that was normal for them. The next night, some patients were woken up and exposed to 1, 2 or 3 hours of bright light. As a "control," some patients were kept awake for varying amounts of time but not exposed to light.

They gauged the state of participants' biological clocks by measuring their secretion of melatonin. Release of this hormone, which peaks at night, is partially regulated by the biological clock.

Light pulses of 1, 2, or 3 hours all led to significant delays in the circadian rhythm of the melatonin profile by 35 to 75 minutes. Patients who were kept awake for 4 hours but not exposed to light also had a half-hour delay in melatonin secretion, while small delays were also seen in people who were kept awake in the dark for shorter amounts of time.

Such delays would persist for a few days. For example, a person would feel the effects of an hour's delay in melatonin secretion for 3 or 4 days. The findings help clarify the relationship between the duration of light exposure and the response of the biological clock.

This is important because circadian rhythm disorders have been linked to many health problems. For example, elderly people often have advanced biological rhythms, meaning they fall asleep and wake up early, while adolescents have delayed ones. Both states can lead to severe sleep deprivation.

People suffering from seasonal affective disorder (SAD) often have disordered circadian rhythms, and some researchers believe light exposure helps SAD patients because it normalizes these rhythms.

American Academy of Neurology's annual meeting in Denver, Colorado April 18, 2002