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
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