A new study suggests that parts of the brain can actually fall asleep for a fraction of a second when tired, despite the fact that the organ as a whole is awake at the time.
The implications of the study, which was conducted on rats, are far-reaching, particularly for people performing tasks where sleep deprivation could be dangerous, the scientists said.
“Even before you feel fatigued, there are signs in the brain that you should stop certain activities that may require alertness,” said psychiatry professor Chiara Cirelli at University of Wisconsin at Madison.
The researchers found that the more the rats were sleep-deprived, the more some of their neurons took mini-naps, with consequent declines in task performance. Even though the animals were awake and active, brainwave measures revealed that scattered groups of neurons in the thinking part of their brain, or cortex, briefly fell asleep.
“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,” said Dr. Giulio Tononi of the University of Wisconsin at Madison.
“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.”
Previous studies had suggested that such local snoozing with prolonged wakefulness might be occurring, but little was known about how the underlying neuronal activity might be changing.
To further study the issue, Tononi and colleagues tracked electrical activity at multiple sites in the cortex as they kept rats awake for several hours. They put novel objects into their cages ““ colorful balls, boxes, tubes and odorous nesting material from other rats ““ and found that the sleepier the rats became, the more the cortex neurons switched off, seemingly randomly, in various localities.
These tired neurons’ electrical profiles resembled those of neurons throughout the cortex during NREM, or slow wave, sleep. However, the rats’ overall EEG, a measure of brain electrical activity at the scalp, along with the rats’ behavior, confirmed they were indeed awake.
Neuronal tiredness differs from more overt microsleep that is sometimes experienced with prolonged wakefulness. Instead, neuronal tiredness is more analogous to local lapses seen in some forms of epilepsy, the researchers said.
However subtle, having tired neurons did interfere with task performance. If neurons switched off in the motor cortex within a split second before a rat tried to reach for a sugar pellet, it decreased its likelihood of success by 37.5 percent. And the overall number of such misses increased significantly with prolonged wakefulness. This suggests that tired neurons, and the accompanying increases in slow wave activity, might account for the some of the impaired performance of sleep-deprived people who may seem behaviorally and subjectively awake.
Subsets of neurons going offline with longer wakefulness is, in many ways, the mirror image of progressive changes that occur during recovery sleep following a period of sleep deprivation.
Tononi suggests that both serve to maintain equilibrium, and are parts of the compensatory mechanisms that regulate sleep need. Just as sleep deprivation produces a brain-wide state of instability, it may also trigger local instability in the cortex, possibly by depleting levels of brain chemical messengers. So, tired neurons might nod off as part of an energy-saving or restorative process for overloaded neuronal connections.
“Research suggests that sleep deprivation during adolescence may have adverse emotional and cognitive consequences that could affect brain development,” said NIMH Director Dr. Thomas Insel.
“The broader line of studies to which this belongs, are, in part, considering changes in sleep patterns of the developing brain as a potential index to the health of neural connections that can begin to go awry during the critical transition from childhood to the teen years.”
The researchers report their findings online in the April 28, 2011 issue of the journal Nature.
—
On the Net:
Comments