THURSDAY, April 18 (HealthScoutNews) -- If a barking dog or noisy neighbors wake you up in the middle of the night, be sure to vent your anger in the darkness.

New research shows that even short periods of light exposure in the middle of the night -- say more than an hour -- are enough to disrupt your circadian rhythm.

That's important because your circadian rhythm is the natural biological clock that influences a wide number of physiological functions, as well as basic drives like sleep, hunger and sex.

That's the finding of a new study presented today at the American Academy of Neurology annual meeting in Denver. Researchers from Northwestern University Medical School demonstrated that reaction to light is "dose-dependent" -- and even a relatively short exposure can throw off body rhythms, leaving you feeling tired even if you never got out of bed.

"We have long known that light affects the circadian rhythm, the body's natural time clock. But this is one of the only studies to break down our physiological reaction to light by exposure times, and show that length of exposure to that light source makes a difference," says Dr. Carol Rosenbaum, director of the Mount Sinai Sleep Medicine Center in New York City.

An area of the brain called the hypothalamus controls circadian rhythms. They naturally run in 25-hour cycles that begin with waking, progress to sleep time, and end with waking again. However, because there are 24 hours in a day, our body clock must be continually programmed to maintain order within our various physiological systems.

One way to program that clock is with regular sleep schedules. Going to bed and waking up at the same time every day creates a biological time clock that keeps the body functioning normally.

One way to disrupt that body clock, says Rosenbaum, is exposure to light. Not only can light wake you in the morning, it can affect your body clock. When light exposure occurs in the middle of the night, even for just an hour or two, it can "reset" your body clock and significantly alter your circadian rhythm, the new research shows.

The end result: You feel tired, out of sorts, and usually can't function as efficiently. More important, you might not be able to get to sleep when you try, and you could be more susceptible to disease. If you're taking medication, altering your body clock could affect how drugs are metabolized in your body. All of this can continue until you're back on your normal sleep schedule.

The new study looked at 10 men and 22 women whose average age was 29. They all agreed to attend a sleep clinic that lasted four nights and three days.

On the first night, each followed his or her daily routine, maintaining normal bedtimes. They slept for eight hours in the dark. The next day, baseline temperatures and blood levels of melatonin were recorded -- both measurements of a person's circadian rhythm. Melatonin is a hormone produced in the brain that apparently helps our bodies know when it's time to go to sleep and when it's time to wake up.

For the next three nights, however, some in the study group were awakened in the middle of the night and exposed to varying intensities and durations of light, lasting from one to three hours.

The rest were awakened for four hours a night, but not exposed to light.

Again, core body temperature readings as well as blood tests for melatonin were taken for members of both groups. The results were compared to readings taken before the sleep disruptions began.

After four days, the test results were compared. Members of both groups did show some delay in production of melatonin, an indication of change in their body clock. However, those exposed to light saw the most dramatic effects, with significant changes in melatonin levels and body temperature.

"A single pulse of bright light lasting two to three hours is sufficient to delay the phase of circadian rhythms in humans and disruptions in sleep timing can also produce small delays in circadian rhythms," the researchers wrote.

In a second study presented at the conference, another Northwestern researcher found that melatonin supplements were effective in resetting biological clocks and correcting sleep habits.

The small study of nine men and women sought to change "night people" -- those who like to stay awake at night but sleep during the day -- into "morning people" -- those who choose to go to bed at night and function during the day. The study involved taking doses of melatonin ranging from 0.3 milligrams to 3 milligrams three to five hours before bedtime for four weeks, or taking a placebo.

The men and women were also asked to keep sleep diaries and answer extensive questionnaires -- before, during and after the study -- focusing on their sleep habits.

The end result: Five of the six people who took nightly doses of melatonin were able to moderately change their sleep habits compared to the placebo group, which saw no change. One person in the melatonin group experienced no change in sleep habits.

The researcher concludes melatonin supplements are effective in changing sleep patterns and times, improving morning performance and altering circadian rhythms.

What To Do: To learn more about circadian rhythms, visit the National Library of Medicine or the Harvard University Gazette. For a primer on sleep disorders, check The National Sleep Foundation.

From HealthScout.com.