There is consensus that sleeping mostly does something good for the brain. Decades of research have given scientists a plethora of details about the events that occur during sleep. Scientists have measured the physiological changes that hearts, lungs, and other organs undergo during sleep, and psychiatrists have picked apart dreams. Much of the genetic basis for circadian rhythms has been worked out, and recent studies indicate that melatonin can set the biological clock right and compensate for the jet lag that accompanies travel across time zones. Scientists can even measure the activity of individual neurons in sleeping animals.
Researchers know that mental acuity declines with sleep deprivation.
There is also evidence that a little sleep deprivation can relieve the symptoms of severe depression. Despite eating considerably more than rats allowed to sleep normally, the sleep-deprived animals lost weight. They also could not maintain their body temperatures, and they developed unusual skin disorders. The specific cause of death has never been determined. But despite having tantalizing clues about what occurs during sleep, and about the need for shuteye, modern researchers are, at the most fundamental level, as confounded by the purpose and ultimate control of sleep as were Hippocrates and Aristotle more than 2500 years ago.