The sensation of mental fatigue we feel, coupled with making more mistakes when we get tired, is because sleep-deprived neurones periodically take an offline "nap" to recover, new research has revealed.
Writing in Nature, University of Wisconsin-Madison scientist Giulio Tononi and his team made the discovery by using microelectrodes to eavesdrop on the neurological chatter taking place in the brains of tired rats.
Normally, sleep is characterised by a stereotypical pattern of "slow wave activity" (SWA) in which cohorts of neurones throughout the brain simultaneously and slowly alternate between being "on" and "off". During the awake state, on the other hand, nerve activity is generally higher and there is much less of this sort of synchronisation.
But when they sleep-deprived a group of rats for four hours by giving them some new toys to play with, the Madison-Wisconsin team found that, as the animals became progressively more sleep pressured, these characteristic sleep patterns of nerve activity began to creep into the electrical recordings they were making. And at the same time, some of the cells they were listening to would periodically switch off.
So, where previously a clutch of 20 neurones might be firing, suddenly there would be just 18 participating in the neurological drumbeat. This also became more prevalent as the animals became more sleep deprived.
Even more interestingly, when the researchers compared the simultanous activity of two different brain regions, the motor and parietal cortices, although both showed these sleep-associated changes, they didn't do so at the same times, suggesting that individual regions of the brain were taking "micronaps".
This intriguing result might shed some light on phenomena such as sleepwalking and other so-called parasomnias, which subjects describe as like a twilight state between wakefulness and sleep.
This sort of local brain sleep may also underlie the ability of some species - like birds that need to remain in flight, fish that have to keep swimming and marine mammals that need to remain afloat to breathe - to rest parts of their nervous system while essential circuits remain active.
The researchers might also have solved another fatigue-related conundrum, why we make more mistakes when we're sleepy: they tested the ability of their rats to retrieve fiddly sugar-pellet rewards with their paws. As the brain cell "off" epsiodes became more frequent, the animals made more mistakes in the task, suggesting that a dwindling ensemble of active neurones can compromise function.