Researchers know that sleep deprivation makes people and animals less functional. Now a team of researchers in Wisconsin and Italy has found that in rats kept awake past their bed times, the brains begin to turn themselves off, neuron by neuron, though the rat is still awake.
Not only that, but the neurons that we use the most during the day are the ones that appear most likely to go offline.
"It's very worrisome. It means that even before we have obvious global signs of sleepiness, there are more local signs of tiredness and they have consequences on performance," says Chiara Cirelli, a professor of psychiatry at the University of Wisconsin-Madison? and one of the researchers.
"It's the first time anyone has thought about sleep at the single cell level," says Christopher Colwell, a professor in the Laboratory of Sleep and Circadian Medicine at the University of California-Los Angeles School of Medicine. "This study was really clever in suggesting a whole new way of thinking about sleep."
Sleep is crucial to mental and physical health in all animals and it's clearly visible in the brain. When we sleep, slow-wave activity appears in our brains as the neurons in the cortex switch themselves off and go electrically silent. Wakefulness is when our neurons are on and generating spikes of electrical activity.
The researchers, at the University of Wisconsin-Madison? and the Perceptual Robotics Laboratory in Pisa, Italy, kept rats up four hours past their bedtimes and measured their brain waves using an EEG, or electroencephalogram. They found that although the rats were awake, individual neurons in their brains turned themselves off in a random pattern. The shut-down of those neurons, the researchers believe, is why the rats did progressively less well on a task in which they had to find a sugar pellet. Their research is published in this week's edition of the journal Nature.
There's no reason to think this doesn't also happen in the human brain, and Cirelli's group is beginning to do similar studies in people getting ready for brain surgery whose neural pathways are being mapped.
She thinks that it's also likely that the most-used neurons are the ones that turn off. "We have done several studies showing that the 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. At a certain point, and we don't know exactly why, they start saying 'I've had enough. I'm going off-line.'"
While much research has been done on how the brain functions when asleep and when awake, looking at brain function in the sleep-deprived brain at this level is new, says Colwell, who wrote a viewpoint piece about their the paper that is also in Nature this week.
"What they're suggesting is that in the morning, maybe 90% of yours cells are in the wake state and you perform very well," Colwell says. "But as you stay awake longer, your cells start to drift off into sleep and your performance starts to go down, which could provide an explanation as to why we function less well the longer we've been awake."
The state they describe is different than another reported effect of severe sleep deprivation, called 'microsleeps,' when an individuals suddenly closes their eyes, stops responding to stimuli and shows sleep-like brain wave patterns. These episodes last only three to 15 seconds and appear when subjects are extremely sleep deprived. In this research, the rats were only kept up for four hours beyond their normal bedtime so they weren't severely sleep deprived, only tired. To keep them awake, the researchers continually supplied them with new objects to play with. They videotaped the rats to insure that they were actually awake at all times.
Such testing at the single neuron level requires inserting electrodes into the brain, so they're unlikely to be done on human subjects. However as imaging technology gets better, it may soon be possible to see if the same thing is happening in people.
The research could mean that the 35% of Americans who told the Centers for Disease Control and Prevention that they routinely sleep less than seven hours a night are also having portions of their brains go off-line even though they're still awake.
"The message is you need to take sleep very seriously," says Cirelli. "When you're starting to nod off, it's too late. Even before that, there may be impairment. Respect your need for sleep."