Day 78
Passage 78
Cycles of Sleep
1
Our bodies require sleep in order to maintain health. In fact, we are programmed to sleep each night as a means of restoring our bodies and minds to their proper functions. Prior to the era of modern sleep research in the early 1920s, scientists regarded sleep as an inactive brain state. It was generally accepted that as night fell and sensory inputs from the environment diminished, so did brain function. In essence, scientists used to think that the brain simply shut down during sleep, only to restart again when morning came.
2
In 1929, however, an invention that enabled scientists to record brain activity challenged this assumption. From recordings known as electroencephalograms (EEGs), researchers could see that sleep was a dynamic behavior, in which the brain was not turned off at all but was highly active at times. [1] Over time, sleep studies using EEGs and other instruments that measured eye movements and muscle activity revealed two main types of sleep. These were defined by characteristic electrical patterns in a sleeping person's brain as well as the presence or absence of eye movements.
3
The two main types of sleep are rapid-eye-movement (REM) sleep and non-rapid-eye-movement (NREM) sleep. In healthy adults, sleep typically begins with NREM sleep, which can be broken down into three distinct stages: N1, N2, and N3. In the progression from stage N1 to N3, brain waves become slower and more synchronized, and the eyes remain still. To begin with, the pattern of clear rhythmic alpha activity associated with wakefulness gives way to N1 which is defined by a low-voltage, mixed-frequency pattern. The transition from wakefulness to N1 occurs seconds to minutes after the start of the slow eye movements seen when a person first begins to nod off, and the first period of N1 typically lasts just one to seven minutes. The second stage, or N2, which is signaled by sleep spindles and/or K complexes in the EEG recording, comes next and generally lasts 10 to 25 minutes. As N2 sleep progresses, there is a gradual appearance of the high-amplitude, low-frequency waves and spindles which are characteristic of N3, the third stage of NREM sleep. This stage, which generally lasts 20 to 40 minutes, is referred to as "slow-wave", "delta", or "deep" sleep. As NREM sleep progresses, the brain becomes less responsive to external stimuli, and it becomes increasingly difficult to awaken an individual from sleep.
4
Following the N3 stage of sleep, a series of body movements usually signals an "ascent" to lighter NREM sleep stages. Typically, a 5- to 10-minute period of N2 precedes the initial episode of REM sleep, which is often called "active sleep", and is identifiable by its characteristic low-amplitude, high-frequency waves, and the alpha rhythm on an EEG, as well as the rapid eye movements for which it is named. Many experts in sleep behavior think that these eye movements are, in some way, related to dreams. Typically, when people are awakened from REM sleep, they report that they had been dreaming often extremely vivid and sometimes bizarre dreams. In contrast, people report dreaming far less frequently when awakened from NREM sleep. Interestingly, during REM sleep muscles in the arms and legs are temporarily paralyzed. This is thought to be a neurological barrier that prevents us from "acting out" our dreams.
5
NREM sleep and REM sleep continue to alternate in a cyclical fashion through the night. Most slow-wave NREM sleep occurs in the first part of the night; REM sleep episodes, the first of which may last only 1 to 5 minutes, generally become longer through the night. During a typical night, N3 sleep occupies less time in the second cycle than the first and may disappear altogether in later cycles. The average length of the first NREM-REM sleep cycle is between 70 and 100 minutes; the average length of the second and later cycles is about 90 to 120 minutes. The reason for such a specific cycling pattern of NREM and REM sleep across the night is unknown. Some scientists speculate that specific sequences of NREM and REM sleep optimize both physical and mental recuperation as well as some aspects of memory consolidation that occur during sleep, but this has not been confirmed.
——2011年05月13日北美考试机经
The word "dynamic" in Paragraph 2 is closest in meaning to____.
A. diminished
B. active
C. complicated
D. sluggish
核心词汇：
词汇练习：
阅读下列句子，用所给单词（或词组）的正确形式填空：
frequency reveal maintain progression sensory
consolidation diminish sequence transition responsive
1. These animals are at the mercy of the climate to _____ a livable body temperature. (TPO-8: Extinction of the Dinosaurs)
2. Furthermore, one _____ ability may back up another. (TPO-11: Orientation and Navigation)
3. Village handicrafts _____ or died out completely as people bought imported consumer goods, like cloth and tools, instead of making them themselves. (TPO-32: Siam, 1851—1910)
4. Different ratios in the shells preserved in various layers of sediment ____ the temperature changes in the oceans during the Pleistocene epoch. (TPO-19: Discovering the Ice Ages)
5. Fossil formations like the Burgess Shale show that evolution cannot always be thought of as a slow ____. (TPO-5: The Cambrian Explosion)
6. Precise measurements can be made of the length and _____ of the infant's attention between two stimuli. (TPO-13: Methods of Studying Infant Perception)
7. The _____ from forest to treeless tundra on a mountain slope is often a dramatic one. (TPO-1: Timberline Vegetation on Mountains)
8. Also, during sleep the automatic metabolic system is less _____ to carbon dioxide levels and oxygen levels in the blood. (TPO-24: Breathing During Sleep)
9. But as more and more accumulations of strata were cataloged in more and more places, it became clear that the _____ of rocks sometimes differed from region to region and that no rock type was ever going to become a reliable time marker throughout the world. (TPO-6: William Smith)
10. This is because the gaps among the original grains are often not totally plugged with cementing chemicals; also, parts of the original grains may become dissolved by percolating groundwater, either while ____ is taking place or at any time afterwards. (TPO-1: Groundwater)
参考答案：
1. maintain 2. sensory 3. diminished 4. reveal 5. progression
6. frequency 7. transition 8. responsive 9. sequences 10. consolidation
注释
[1]与今日百句译（Sentence 76）相同的句型。