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 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. Conversely, as I will describe now, we have also learned that even when you're awake, your entire brain may not be awake.Ī case in point for sleep intruding into wakefulness involves brief episodes of sleep known as microsleep. Just because you're asleep doesn't necessarily imply that your entire brain is asleep.
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. Such recurring silent periods, synchronized across large parts of the cortex, are the cellular hallmark of deep sleep. Tuning into the discharge of individual neurons during deep sleep reveals discrete off periods, when nerve cells cease generating any electrical activity for 300 to 400 milliseconds. These voltage oscillations, referred to as delta waves, can be as slow as once every four seconds and as fast as four times a second (that is, in the 0.25- to four-hertz frequency range). Indeed, the deeper and more restful the sleep, the slower and larger the waves that reflect the brain's idling, restorative activity. Rapid eye movement (REM) sleep is characterized by low-voltage, choppy, swiftly changing brain waves (paradoxically, also typical of relaxed wakefulness), whereas non-REM sleep is marked by slowly rising and falling waves of larger amplitude. Like the surface of the sea, the electrical brain is ceaselessly in commotion, reflecting the unseen, tiny tremors in the cerebral cortex underneath the skull that are picked up by the EEG electrodes. In my last Consciousness Redux column, I described how clinicians define sleep by recording brain waves from a net of electroencephalogram (EEG) sensors placed on the scalp of the sleeper. If deprived of sleep, humans experience an ultimately irresistible need to seek repose-they, in fact, become “sleep drunk.” An older, 19th-century term, closer to the truth, is “cerebral exhaustion,” the brain demanding its rest. Sleep is homeostatically regulated with exquisite precision: pressure to go to sleep builds up during the day until we feel sleepy in the evening, yawn continuously and nod off. Humans share this need for daily sleep with all multicellular creatures, as anybody growing up with dogs, cats or other pets knows.Īn understanding of sleep's importance can be observed by contemplating the biological process itself. Ah, I know you're thinking, Wouldn't it be great if we cut down on this “wasted” time to be able to do more! When I was younger, I, too, lived by the motto “You can sleep when you're dead.” But I've woken up to the fact that for optimal, long-term physical and mental health, we need sleep. Cumulatively, this amounts to several decades' worth of sleep over the lifetime of an average person. We spend about one third of our lives in a state of repose, defined by relative behavioral immobility and reduced responsiveness to external stimuli. Fortunately, fatigue is reversible and disappears after a night or two of solid sleep. Attention wanders, your reaction time slows, you have less cognitive-emotional control. The reasons for sleeplessness may be many, but the consequences are always the same: You are fatigued the following day, you feel sleepy, you nap.
You toss and turn but can't find the blessed relief of sleep. But sometimes your inner world does not turn off-your mind remains hypervigilant.
A timeless interval later, you wake up, refreshed and ready to face the challenges of a new day (note how you can never catch yourself in the act of losing consciousness!). You go to bed, close your eyes, blanket your mind and wait for consciousness to fade. A strong wind would make me think my body was about to be blown to the end of the earth, to some land I had never seen or heard of, where my mind and body would separate forever.” My very existence, my life in the world, seemed like a hallucination. My body had no more feeling than a drowned corpse. “It was literally true: I was going through life asleep.
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