Neurons in some areas of a rat brain’s cortex briefly go “offline”when sleep-deprived, researchers at the University of Wisconsin-Madison have demonstrated.

The researchers tracked electrical activity at multiple sites in the cortex as they kept rats awake for several hours. They put novel objects into the rats’ cages, including colorful balls, boxes, tubes, and odorous nesting material from other rats.

As the rats got sleepy, subsets of cortex neurons switched off, seemingly randomly, in various localities. The electrical profiles of these “tired” neurons resembled those of neurons throughout the cortex during NREM (non-dreaming slow-wave sleep), but measuring overall EEG and observing behavior, the researchers confirmed that the rats were actually awake.

Neuronal tiredness differs from more obvious nodding-off microsleep (3–15 second lapses with eyes closing and sleep-like EEG) that is sometimes experienced with prolonged wakefulness.

The researchers showed that tired neurons interfered 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. The overall number of such misses increased significantly with prolonged wakefulness. Imagine driving under those conditions.

Tired neurons, and accompanying increases in slow wave activity, might help to account for the impaired performance of sleep-deprived people who may seem behaviorally and subjectively awake, the researchers said.

“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 Giulio Tononi, M.D., Ph.D.

The researchers suggested that sleep deprivation produces a brain-wide state of instability and may also trigger local instability in the cortex, possibly by depleting levels of brain chemical messengers. Tired neurons might switch off as part of an energy-saving or restorative process for overloaded neuronal connections.

Ref.: Chiara Cirelli & Giulio Tononi et al., Local sleep in awake rats, Nature, April 27, 2011