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The True Nature of Sleep I
August 28, 2000

Each year in my undergraduate course at Stanford, "Sleep and Dreams," the teaching assistants (TA's) and I survey the several hundred students who have registered. Many of the questions in the survey are very straightforward mainly asking about the students' sleep habits and what they believe about sleep. Others are a little far out, such as "Do you believe that dreams foretell the future?" One survey result that endlessly intrigues me is the very high percentage (usually greater than 90) of students who check 'yes' to the statement "I love to sleep." I am intrigued because a fundamental property of sleep is that the sleeper is unaware of being asleep. Moreover, there is no evidence that we can recall when we wake up how we felt while we were sleeping. The lone exception to the foregoing is lucid dreaming; that is, knowing we are dreaming while we are dreaming, and if we know we are dreaming, by definition we know we are asleep. However, experiencing the dream world cannot be equated with experiencing sleep because we are awake when we are in the dream world (another conundrum).

I mention the students' apparent love affair with something they do not and cannot experience in order to emphasize that the most fundamental characteristic of sleep is the absence of waking consciousness and in particular, the absence of one of the most important properties of waking consciousness, the ability to reflect upon what we are experiencing. I repeat, the most fundamental characteristic of sleep is the absence or shutdown of waking consciousness. This does not mean that our brains or our minds actually shut down or turn off. Indeed, it is a well-established scientific fact that the brain keeps working pretty much at the same level as when we are awake.

In my recently published book, The Promise of Sleep, I describe an experiment designed by my colleague, Dr. Christian Guilleminault, which we carried out in a large number of volunteers:

"The subject, a young man in this instance, is lying on a bed, eyes wide open, staring straight up. As part of our experiment, he is allowed only four hours of sleep on the previous night. Much of the time he is sleepy but unambiguously awake. We have positioned his head so that he is looking directly into a very bright strobe light--essentially like a camera flash, except his nose is only six inches away. We have asked him to stare at the light and to press a tiny switch taped to his index finger whenever he sees the flash. To make sure he cannot possibly miss the flash during a blink, we have taped his eyelids open. To some it looks uncomfortable, but it doesn't hurt.
The light goes off straight into his eyes-flash!-like a photographer ambushing a celebrity. Our volunteer presses the switch. A few seconds later another flash, another press of the response key. The strobe light is programmed to go off irregularly, on average every six seconds. For a few minutes the sleepy volunteer taps the switch after each flash. Then the bright flash surges through his pupils, and the switch is not pressed.
"Why didn't you press the switch just now?" we ask.
"Because there was no flash," the young man replies.
But there was a flash. I saw it. The three other people in the room saw it. And this unblinking volunteer didn't see it. Looking straight into the strobe light six inches away from his nose, he could not possibly have missed it. His retina was flooded with light many times brighter than the brightest bulb in his home, and yet he is absolutely sure that there was no flash. How is this possible?
The answer to the puzzle was revealed by the machines we use to monitor brain activity. They showed us that something had happened to his brain at the very moment the light flashed. The young man actually had fallen asleep for two seconds, so briefly that he wasn't even aware he had slept. And yet, in that moment of sleep, the door of perception between the brain and the outside world had slammed shut, barring entry to even a bright light."

In summary, the crucial event that occurs as we fall asleep is an actively initiated shut down of our ability to perceive the world around us. At one moment, we are awake and can see, and a fraction of a second later, we are asleep and completely blind. Another way of saying this is that sleep is a behavioral state of complete perceptual disengagement from the environment. Thus, at the moment we fall asleep and throughout sleep, our brain abolishes waking consciousness by erecting a barrier to our awareness of the world around us and in particular, the sights, sounds, and touch of our immediate environment. Sleep is an active process that includes the active blocking or modifying of sensory stimulation in some way such that we are no longer conscious of the world around us.

The best short answer to the question "What is sleep?" is that sleep is a state in which we are not conscious of the world around us. In spite of the fact that the Guilleminault experiment was done over a quarter of a century ago, if one asks any group including physicians and even some of my sleep research colleagues to define sleep, there is a tendency for them to remain silent.

Of course there is much more to say about the true nature of sleep about which most people are unaware. Stay tuned.

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