Neurons of sleep-deprived rats can briefly go ‘offline’ even while they seem wide awake.
As we've experienced, staying up too long causes lapses in attention, poor performance, and even impaired judgment. Now researchers show that sleep deprivation affects brain and cognitive function because localized areas of the brain fall asleep.
Sleep is thought of as an all-encompassing state profoundly different from, well, being awake. But as it turns out, sleep doesn’t have to involve the whole brain. There are episodes of “local sleep” in the brains of rats forced to stay up way past their bedtimes – while their brains were wired to an EEG machine with ultra-fine probes.
"The rats were awake, but awake with a nice sprinkling of localized sleep in the cortex," says study author Giulio Tononi of the University of Wisconsin–Madison.
Normally "rats take lots of siestas," he adds, but these were kept up for hours with new toys to play with. While their behavior and EEG recordings indicate that they’re awake, some of their neurons fell into an OFF state while other neurons remained in an ON state.
OFF states occurred randomly throughout the cortex of these rats. "If we could watch the whole brain, it would be like watching boiling water – when you are awake, just before boiling, all the neurons are ON. As the animal gets tired, the OFF periods would then be the bubbles; where they appear is impossible to predict," Tononi says.
The number of OFF periods increases the longer they're kept up. When local sleep is observed in the brain area required for a task – like reaching for a sugar pellet through a slot in their plexiglass cage – the rat's performance is impaired. And it gets worse with prolonged deprivation.
The task involves some coordinated moves, such as rotating the wrist, that aren’t part of a rat’s normal repertoire, so if the animals’ brains aren’t firing on all cylinders, the grab could fail. When brain cells in the motor cortex – a part of the brain that controls movement – fell asleep, rats failed in attempts to grab the sugar cubes for several hundred milliseconds afterward. But sleeping neurons in the parietal cortex, which is not involved in the task, did not lead to mistakes.
In humans, this could explain why sleep deprivation can lead to driving mistakes, indecision making, or errors in judgment.
The most-used neurons are likely the ones that turn off. “Neurons that you use the most during the waking day are the ones that need to go to sleep the most when you're tried,” says study coauthor Chiara Cirelli. “At a certain point, and we don't know exactly why, they start saying 'I've had enough. I'm going offline.'"
This work may pin down the function of sleep, Nature News reports. Sleep seems to be a time of recalibrating synaptic activity in the brain, so Tononi’s group wants to determine whether napping neurons in an awake brain are protective, restorative, or just plain harmful.
"When you're starting to nod off, it's too late,” Cirelli says. “Even before that, there may be impairment. Respect your need for sleep."
The study was published in Nature last week.
Image: Janet Fang