You may think you're awake but there's a good chance that part of your brain is asleep. And that can cause real problems, especially since you may not even be aware of it. In fact, indivisual neurons and groups of neurons in the cerebral cortex can be independently offline while others are awake. In Scientific American, Christof Koch, president of the Allen Institute for Brain Science, explores the counter-intuitive reality of "Sleeping While 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....
Perniciously, subjects typically believe themselves to be alert all the time during microsleep without recalling any period of unconsciousness. This misapprehension can be perilous to someone in the driver's seat. Microsleep can be fatal when driving or operating machinery such as trains or airplanes, hour after tedious hour. During a microsleep episode, the entire brain briefly falls asleep, raising the question of whether bits and pieces of the brain can go to sleep by themselves, without the entire organ succumbing to slumber.
Indeed, Italian-born neuroscientists Chiara Cirelli and Giulio Tononi, who study sleep and consciousness at the University of Wisconsin–Madison, discovered “sleepy neurons” in experimental animals that showed no behavioral manifestation of sleep...
What this study discovered is the existence of local sleep during sleep deprivation: isolated cortical groups of neurons that briefly go off-line while the animal, to all outward appearances, continues to move about and do what it does. Local shut-eye is more likely to occur if those neurons are actively engaged, as they are when learning to grab a sugar pellet. Neurons, too, become tired and disengaged, a microcosm of what happens to the whole organism.
Extrapolating from these data, it seems plausible that as the pressure for sleep increases, the frequency of these off events and their preponderance in the cortex increase until activity in the entire brain becomes suddenly but briefly synchronized and the brain falls into deep sleep—the eyes close, and the head nods. The subject enters microsleep.
"Sleeping While Awake" (SciAm)