Some animals - such as dolphins and whales - are able to "sleep with half their brain". One side of the brain goes into sleep-mode activity while the other remains awake.
But a remarkable new study has revealed that something similar may happen in humans as well - every night.
The research used a combination of scalp EEG, and electrodes planted inside the brain, to record brain activity from 5 people undergoing surgery to help cure severe epilepsy. The subjects were then allowed to go to sleep for the night, while recording took place.
As expected, after falling asleep, the EEG showed delta wave activity - strong, slow waves of electrical activity (0.5 to 4 Hz) which are typical of deep, dreamless "slow wave sleep".
However, the electrodes inside the brain told a different story. While they recorded delta waves most of the time, they also showed that there were episodes, lasting from a few seconds to up to 2 minutes, in which the motor cortex suddenly went into "waking mode". Delta waves disappeared, and were replaced with fast, unpredictable activity.
This image shows one episode, lasting just 5 seconds. The hotter the color, the more activity in a particular frequency. The higher the band, the higher the frequency. This shows a clear burst of high frequency activity in the motor cortex. The other parts of the brain showed the opposite effect - even stronger slow wave activity - at the same time.
Another area, the dorsolateral prefrontal cortex, also showed this phenomenon occasionally, but it was much less common than in the motor cortex.
There's a few caveats. These patients had severe epilepsy, and they were taking anti-convulsant drugs. This wouldn't obviously create the effects seen here, but we can't rule it out. Still, these results are intriguing.
They challenge the view of slow wave sleep as a "whole brain" phenomenon. We've known for a while that this isn't true of animals, and in people with certain sleep disorders, but this is first demonstration in healthy humans.
It may help to explain the mysterious fact that, although slow wave sleep is often referred to as "dreamless", there are consistent reports that people woken up from this phase of sleep do report dreaming (or at least thinking) about things.
While episodic arousal of the motor cortex probably wouldn't explain this per se, if the same thing happens in the visual cortex or other sensory areas, it might create dreams.
Nobili L, Ferrara M, Moroni F, De Gennaro L, Russo GL, Campus C, Cardinale F, & De Carli F (2011). Dissociated wake-like and sleep-like electro-cortical activity during sleep. NeuroImage PMID: 21718789