Do Flies and Worms Sleep?
Penn Researchers
Look for an Answer
Can you tell if a fly is asleep? This is precisely the question asked seven years ago by Dr. Joan Hendricks, who is now the Dean of the School of Veterinary Medicine at the University of Pennsylvania. In collaboration with Dr. Amita Sehgal and Dr. Allan Pack of the Penn Center for Sleep and Respiratory Neurobiology (CSRN), Dr. Hendricks wanted to determine if the fruit fly Drosophila melanogaster, an animal used since the early 1900s for genetic studies in biology, has a sleeplike state much as we do. But unlike human studies, where one can measure brain electrical activity to distinguish sleep from the waking state, one cannot easily measure fly brain waves. Instead, Dr. Hendricks used behavioral measures.
It was already known that Drosophila are not
active all day, but rather have rest and activity
patterns that follow an approximately 24-hour,
or circadian, rhythm. They are active during
the day and inactive at night, just like us.
To determine if the inactive period corresponded
to fly sleep, or simply to restful wakefulness,
Dr. Hendricks first asked if the flies were less
responsive during this period, much as we are
less likely to respond to stimuli when asleep.
She found that indeed, the flies were less likely
to move in response to a gentle tap during these
nocturnal quiescent periods than during the day.
In
addition, just as we become increasingly sleepy
with sleep deprivation and will sleep during
the day if awake all night, the flies too fell
asleep during a time when they are normally active
after staying awake all night. Caffeine, a chemical
we consume in soft drinks and coffee to keep
us awake, also keeps the fly awake, indicating
that the underlying sleep regulatory mechanisms
are the same in flies as in humans.
We spend one third of our lives
asleep and yet the purpose of sleep
remains unknown.
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Why does it matter that fruit flies sleep?
It means that sleep is so fundamental to animal
life that it is likely to be found in all animals. David
Raizen, MD, PhD, of the Department of Neurology
is a CSRN clinician scientist who is extending
the findings in fruit flies to an even simpler
animal known as Caenorhabditis elegans, a soildwelling
round worm. He has identified a quiescent behavioral
state that like sleep in humans and mice is controlled
by the activity of circadian genes and that cannot
be bypassed, just as sleep cannot be bypassed
in humans.
We spend one third of our lives asleep and
yet the purpose of sleep remains unknown. If
we can understand the purpose of sleep in the
lowly fly or worm, perhaps we can gain insight
as to why we need sleep. In addition, genes that
control sleep can be easily identified and studied
in fruit flies and round worms; these genes are
likely to also have relevance to human sleep.
If
we knew which genes function to control sleep
and wakefulness, then we could study how these
genes malfunction in human sleep disorders and
develop better treatments. In fact, this approach
has already proven itself with the finding that
the genes which control circadian rhythms in
fruit flies, also control the timing of human
sleep and can malfunction in certain human circadian
sleep disorders. Researchers at Penn are actively
searching for additional genes that control sleep
and wakefulness in fruit flies and round worms.
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