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lag upsets body clock in neural centres to disrupt sleep Washington:
Scientists at the University of Washington have moved a step closer to developing
more effective treatments for jet lag, by finding out that this problem disrupts
sleep by upsetting internal clocks in two separate but linked groups of neurons
in a structure in the brain called the suprachiasmatic nucleus. The researchers
have revealed that this structure lies below the hypothalamus at the base of the
brain. According to them, one group is synchronized with deep sleep that results
from physical fatigue, and the other controls the dream state of rapid eye movement
(REM) sleep. The bottom neurons receive light information directly from the eyes
and govern rhythms in tune with periods of light and dark, while the top neurons
do not receive direct light information and so govern rhythms as a more independent
internal biological clock. oracio de la Iglesia, a UW associate professor of biology,
points out that some of the body's rhythms seem to be "more loyal" to the bottom
neurons, and others are much more in tune with the top neurons. Normally the two
neuron groups are synchronized with each other, but disruptions like jet travel
across time zones or shift work can throw the cycles out of kilter.
Deep sleep
is most closely tied to light-dark cycles, and typically
adjusts to a new schedule in a couple of days. However,
REM sleep is more tied to the light-insensitive dorsal
neurons, and can be out of sync for a week or more.
"When we impose a 22-hour light-dark cycle on animals,
the ventral center can catch up but the dorsal doesn't
adapt and defaults to its own inner cycle," de la
Iglesia said. In the rats he tests while conducting
lab experiments, that normal cycle is 25 hours. Upon
imposing the artificial 22-hour light-dark schedule,
the researcher observed that the rats' deep sleep
quickly adapted to the 22-hour cycle, but their REM
sleep continued to follow a 25-hour cycle. The researcher
said that REM sleep, consequently, did not occur in
a normal progression following deep sleep. "We found
that after exposing rats to a shift of the light-dark
timing that simulates a trip from Paris to New York,
REM sleep needed 6 to 8 days to catch up with non-REM,
or deep, sleep, the sleep you usually experience in
the first part of the night," de la Iglesia said.
The study showed that the two types of sleep overlap
immediately after the simulated jet lag occurs, and
that there is a greater likelihood of the animals
entering REM sleep earlier than they should. According
to de la Iglesia, this may help understand why travellers
and shift workers may take several days to adapt to
their new schedules. "It also could explain why jet
lag is associated with lower learning performance.
We think the disruption of the normal circadian sequence
of sleep states is very detrimental to learning,"
he said. "One of the problems is that you are doing
things at times that your body isn't prepared to do
them. One group of neurons tells your body it is Paris
time and another says that it is New York time. You
are internally desynchronized," he added. The researcher
believes that this study may be useful in fine-tuning
pharmaceutical and other therapies. "We can go back
to the treatments that are believed to be effective
and see where they might be acting in the circuitry
of these neuron centres, then refine them to be more
effective," he said. A researcher article on the study
has been published online in the journal Current Biology.
-Apr
23, 2009
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