May 2, 2011

Sleep-Deprived Neurons Caught Nodding Off

Photo of a woman nodding off at her desk

A new study sheds light on how sleep deprivation might affect daily function. When researchers kept rats awake, they caught neurons in the thinking part of the animals’ brains taking catnaps. The finding gives insight into the roots of sleepiness.

It’s clear that people and other animals need sleep, but why is largely unknown. When you stay awake too long — even if you don’t feel sleepy — you can have attention lapses, show poor judgment and make frequent mistakes in cognitive tasks. Studies of brain activity in sleepy people suggest some similarities with that of people who are asleep. However, little has been known about the underlying neuronal activity at work.

A team led by Dr. Giulio Tononi of the University of Wisconsin-Madison set out to learn more about the sleepy brain. They kept rats awake for several hours by putting novel objects into their cages — colorful balls, boxes, tubes and nesting material from other rats. They used electroencephalography to track electrical activity at multiple sites in the brain’s cortex, which is important for thinking and cognition. The study was funded in part by NIH’s National Institute of Mental Health (NIMH) and National Institute of Neurological Disorders and Stroke (NINDS). The results appeared in the April 28, 2011, issue of Nature.

As the rats grew sleepy, subsets of cortex neurons switched off, seemingly at random, in various locations. The electrical profiles of these tired neurons showed “slow wave” activity, resembling neurons throughout the cortex during nonrapid-eye-movement (NREM) sleep, which makes up about 80% of all sleep. But the overall electrical activity in the rats’ brains confirmed that they were awake, as did their behavior.

To test if this neuronal “tiredness” affects performance, the researchers trained the rats during the sleep deprivation period to perform a task that involves reaching for a sugar pellet. A rat’s likelihood of success dropped by nearly 38% when neurons anywhere in the motor cortex turned off within a split second before a rat tried to reach for a sugar pellet. The overall number of such misses, as expected, increased significantly with prolonged wakefulness.

These results suggest that tired neurons might help to account for the impaired performance of sleep-deprived people. “Such tired neurons in an awake brain may be responsible for the attention lapses, poor judgment, mistake-proneness and irritability that we experience when we haven’t had enough sleep, yet don’t feel particularly sleepy,” explains Tononi. “Strikingly, in the sleep-deprived brain, subsets of neurons go offline in one cortex area but not in another — or even in one part of an area and not in another.”

Similar local neuron lapses, the researchers note, have been observed in epilepsy. That suggests this finding could hold significance for understanding not only sleepiness but also certain brain disorders.

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