NEW YORK (Reuters Health) - Whether you are a night owl or a morning lark may be predetermined by how long your body keeps a day's cycle, according to researchers.

``It's not just social factors or that you're lazy,'' co-author Dr. Jeanne F. Duffy, an associate neuroscientist at Brigham and Women's Hospital in Boston, Massachusetts, told Reuters Health.

To find out which factors influence the daily sleep-wake cycle, also known as daily circadian rhythm, researchers tracked the natural sleep cycles of 17 healthy, young adult males who lived in a sleep clinic for up to 1 month. Each was questioned whether they considered themselves morning-risers or late-night types.

In this study, the 17 men had an average daily cycle of 24.2 hours. ``We had a few morning types and a few evening types,'' Duffy said, ``but most people were neither, most were in between.''

However, according to the report published in the August issue of Behavioral Neuroscience, the investigators found that those who naturally fell into a shorter daily cycle of less than 24 hours tended to be morning risers, while those who naturally had a longer cycle tended to stay up later.

Duffy said that early risers and night owls are, for the time being, stuck with their lot.

``We know from animal studies the period length is determined genetically,'' Duffy said. ``We don't know any way of changing the cycle length yet.''

The study was funded by the National Institute on Aging, the Air Force Office of Scientific Research and the National Institutes of Health.

SOURCE: Behavioral Neuroscience 2001;115.

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