91. Read the following passage, then answer the question below.

For much of the twentieth century, sleep was regarded as a period of rest during which the brain simply switched off. Research since the 1950s has overturned that view. When people fall asleep, the brain cycles through distinct stages, including slow-wave sleep, in which neurons fire in synchronized waves, and rapid eye movement, or REM, sleep, in which brain activity resembles that of waking. Each stage seems to serve different functions.

The link between sleep and memory has attracted particular attention. In a typical experiment, volunteers learn a list of word pairs or a finger-tapping sequence, and half are allowed to sleep afterward while the others stay awake for the same interval. When tested later, the sleepers usually perform better, and the difference cannot be explained by tiredness alone, since tests given after a night of normal sleep show the same advantage. One influential theory holds that during slow-wave sleep the hippocampus, which temporarily stores new experiences, replays them to the cortex, where they are gradually integrated with older knowledge. The replay seems to strengthen important connections between neurons while allowing weaker ones to fade.

Not all researchers are convinced that sleep is essential for memory. Some point out that the improvement in the experiments is modest, and that people who lose small amounts of sleep still learn effectively. Others suggest that the benefit may arise from the simple fact that sleeping prevents new information from interfering with what has just been learned. Both explanations may be partly true. What is clear is that deliberately cutting sleep to study longer is likely to backfire, since the hours given up are also the hours in which the brain does some of its most valuable processing of the day's lessons.

What is the main idea of the passage?