One fact was clearly demonstrated by the early sleep researchers: one part of the night is not just like another. As scientists began to compare the records of volunteers, they observed that human sleep follows a rhythmic schedule; they noted that not only was this schedule much the same in healthy person of the same age with similar habits but, from night to night, each individual had an EEG record almost as consistent as a signature.
Sleep and wakefulness, once considered to be the light and dark of consciousness, no longer seem to differ so sharply. To sleep does not mean to drown in an ocean of darkness. Actually, sleep is not a unitary state; it involves many shades or degrees of detachment from the surrounding world. Sleep is made up of separate stages of light and deeper slumber. Typically, only during the first hours of sleep does a person reach the stage of the deepest slumber, the rest of them is spent drifting between the other lighter stages in specific patterns, all night long a person drifts down and up through different levels of consciousness, as if on waves. While sleep may feel like a blanket of darkness punctuated by dreams — a time when the mind is asleep — nothing could be less true. With laboratory methods, researches have been able to chart the typical stages of the journey into sleep.
The journey starts while the subject is still awake but beginning to relax; his brain waves, which have been low, rapid, and irregular, begin to show a new pattern; this new pattern, which is known as alpha rhythm, is an even electrical pulsation of about 9 to 12 cycles per second. Most people do not know what the alpha state feels like, but during the last few years researchers have been able to teach subjects how to recognize and learn to sustain their alpha rhythm.
When their EEG shows an alpha rhythm, the subjects are notified, either by a sound or by the appearance of a color on a screen. Because the alpha state tends to be relaxed, the ability to sustain it can help tense people ease their passage into sleep. A moment of tension, a loud noise, an attempt to solve a problem, however, and the alpha rhythm may vanish.
As the subject passes through the gates of the unconscious, his alpha waves grow smaller, and his eyes roll very slowly. For a moment, he may wake up during this early part of the descent, alerted by sudden spasm that causes his body to jerk. Like the brain waves, this spasm is a sign of neural changes within. Known as the myoclonic jerk, it is cause by a brief burst of activity in the brain. The myoclonic jerk is normal in all human sleep, it is gone in a fraction of a second, after which the descent continues, the subject has not felt this peculiar transformation, but now he is said to be truly asleep.
1. The sentence “each individual had an EEG record almost consistent as a signature” means （ ）.
2. In the second paragraph, the author describes that （ ）.
3. The word “unitary” in the second paragraph means （ ）.
4. When can we say that a person is asleep?
5. We can infer from the passage that the “alpha rhythm” is（ ）.
