Microsleep, when parts of the brain nod off during the day

Christoff Koch in Scientific American:

My last column, “To Sleep with Half a Brain,” highlighted the growing realization of sleep researchers that being awake and asleep are not all-or-none phenomena. Just because you're asleep doesn't necessarily imply that your entire brain is asleep. Conversely, as I will describe now, we have also learned that even when you're awake, your entire brain may not be awake.

A case in point for sleep intruding into wakefulness involves brief episodes of sleep known as microsleep. These intervals can occur during any monotonous task, whether driving long distances across the country, listening to a speaker droning on or attending yet another never-ending departmental meeting. You're drowsy, your eyes get droopy, the eyelids close, your head repeatedly nods up and down and then snaps up: your consciousness lapses.

My last column, “To Sleep with Half a Brain,” highlighted the growing realization of sleep researchers that being awake and asleep are not all-or-none phenomena. Just because you're asleep doesn't necessarily imply that your entire brain is asleep. Conversely, as I will describe now, we have also learned that even when you're awake, your entire brain may not be awake.

A case in point for sleep intruding into wakefulness involves brief episodes of sleep known as microsleep. These intervals can occur during any monotonous task, whether driving long distances across the country, listening to a speaker droning on or attending yet another never-ending departmental meeting. You're drowsy, your eyes get droopy, the eyelids close, your head repeatedly nods up and down and then snaps up: your consciousness lapses. [...]

These results paint a more nuanced view of wakefulness and sleep than the prevailing one, in which both conditions were considered to be global, all-or-none states of consciousness. Instead these data [from electrodes implanted in rats], buttressed by single-neuron recordings from patients with implanted microelectrodes, as used occasionally in epilepsy treatment, suggest that even when the subject is awake, the individual's neurons can become tired and occasionally check out. The heavier the sleep pressure, the more likely this will happen simultaneously at many places in cortex. Conversely, after many hours of restful sleep, some of these neurons become decoupled from these brain-wide oscillations and begin to wake up.