http://dailynews.yahoo.com/h/nm/20010809/hl/sleep_1.html

Biology Determines Sleep-Wake Preference

By Melissa Schorr

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 (news -
web sites).

SOURCE: Behavioral Neuroscience 2001;115.