We usually think of sleep as a global phenomenon. Since sleep is defined as a state of unresponsiveness, it seems natural to assume that our brain is either "awake" or "asleep" as a whole. But a new study finds that regions of the human brain typically go silent at different times, giving humans something in common with dolphins, which are known to sleep with one part of their brain while the other part controls swimming to the surface for air.
In a collaborative study carried out jointly by scientists at the University of Wisconsin and the University of California-Los Angeles (UCLA), Dr. Yuval Nir and colleagues examined the sleep of a unique group of 13 epilepsy patients who had electrodes implanted deep into their brains to monitor the sources of their seizures. Usually sleep is studied either in animal models or in sleep labs by recording human brain waves sensed through the surface of the skull via scalp encephalogram (EEG). This study provided a rare opportunity to simultaneously record neuronal activity from a dozen different brain regions in the human brain. Despite their epilepsy, it was found that the sleep in the patients resembled normal sleep in healthy individuals. In addition, bursts of activity associated with epilepsy were removed from the analysis to make sure that results could be generalized to the entire population.
Researchers found that both slow waves and oscillating spindles, which are electrical markers for sleep, were mostly restricted to local regions of the brain. Towards the end of sleep such "local" sleep waves became more and more frequent. Interestingly, even when sleep waves were observed across several brain regions, they did not take place at the exact same time, but tended to propagate along typical paths. Overall, the study shows that electrical activity in sleep is highly complex - both in space and in time, and only some of this action can be picked up by non-invasive measures such as EEG. The findings raise the intriguing possibility that when we are awake but sleep deprived, such local sleep waves may invade our brain activity so that some circuits go offline independently.