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 during the 1950's, they observed that human sleep follows a rhythmic schedule. They noted that not only was this schedule much the same in healthy persons of the same age with similar habits but, from night to night each individual had an EEG (电脑图) record almost as consistent as a signature. Sleep and wadefulness, once considered to be the light and dard 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 (单一的) state it involves many shades or detrees of detachment from the surrounding world. While sleep may feel like a blander of darkness punctuated by dreams-a time when the mind is asleep nothing could be less true. All night long a person drifts down and up through different levels of consciousness, as if on waves. With laboratory methods, researchers 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 patten. This new pattern, which is known as alpha rhythm, is an even electrical pulsation (震动) of about nine 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 control 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 pleasant and 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 throuhg 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 a sudden spasm that causes his body to jerk. Like the brain waves, this spasm (痉挛) is a sign of neural changes within. Known as the myoclonic jerk (肌痉挛), it is caused by a brief burst of activity in the brain. Although it is related to epileptic (癫痫) seizures, the myoclonic jerk is normal in all human sleep. It is gone in a fraction of a second, after which descent continues. The subject has not felt the peculiar transformation, but now he is said to be truly aleep. This passage states than a person is really asleep only ______.