WHY SLEEP?
some basis, as the author of the review observes, in the fact that it isn't necessary to make up for lost sleep minute-for-minute. The person who has stayed "awake" (more or less, because usually "microsleeps" intrude on wakefulness) for weeks doesn't need to sleep for weeks or even days thereafter to be restored. Twenty four hours of solid sleep appears to suffice. However, the nature of the sleep that finally ensues after lengthy deprivation is different from normal sleep, comprised mostly of the deeper, more restorative stages that normally occupy only a fraction of the night.
It is here that the author takes a step towards what may prove to be the beginning of an understanding of the function of sleep. He notes that, while similar processes of "rebound" occur after deprivation of both deep, "slow-wave," sleep and Rapid Eye Movement (REM, "dreaming"), these processes are more "robust" for the non-REM (NREM) sleep component. That is to say, after sleep deprivation, the body first restores the missing component of deeper, non-REM sleep; likewise, the body suffers more from the deprivation of this type of sleep. In contrast, even during the usual cycle of nightly sleep, REM sleep only starts to emerge to a considerable degree after the body has achieved most of its nightly quota of NREM sleep. Moreover, in some cases of depression, selective deprivation of REM sleep actually has a therapeutic, rather than harmful effect. Still further, many medications suppress REM sleep to a greater or lesser degree--in come instances, almost completely--without obvious harm.
These kind of observations lead the reviewer away from some of the longstanding theories about the function of sleep, which emphasize the function of REM sleep in particular, for example, with respect to the process of learning. They also lead him away from a theory that trivializes sleep as a primitive way for the body to maintain inactivity--perhaps safer and less wasteful of resources--when activity isn't needed, as at night. They lead him towards a specific focus on the need for NREM sleep.
Beyond this, they do not take him much further than speculation about molecular and neuronal mechanisms underlying the need for NREM sleep. Presuming that the average reader will find this less understandable and informative, I will not attempt to follow the author into these far reaches.
For now, and for practical relevance, it seems enough to recognize, along with the author, that a person needs sleep--perhaps having
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