"Why Sleep?"

We need to sleep every day. Why do we spend a third of our lives in a dormant state? Sleep deprivation leads to loss of judgment, failure of health, and eventually to death. The cycle of sleep and alertness is controlled by circadian rhythms, which also affect body temperature, digestion and other regulatory systems. Despite the importance of sleep its purpose is a mystery.

The brain remains highly active during sleep, so the simple explanation that we sleep in order to rest cannot be the whole story. Activity in the sleeping brain is largely hidden from us because very little that occurs during sleep directly enters consciousness. However, electrical recordings and more recently brain imaging experiments during slow-wave sleep have revealed highly ordered patterns of activity that are much more spatially and temporally coherent than brain activity during states of alertness. Slow-wave sleep alternates during the night with rapid eye sleep movement (REM) sleep, during which dreams occur and muscles are paralyzed. For the last 10 years my colleagues and I have been building computer models of interacting neurons that can account for rhythmic brain activity during sleep.

Computer models of the sleeping brain and recent experimental evidence point toward slow-wave sleep as a time during which brain cells undergo extensive structural reorganization. It takes many hours for the information acquired during the day to be integrated into long-term memory through biochemical reactions. Could it be that we go to sleep every night in order to remember better and think more clearly?

Introspection is misleading in trying to understand the brain in part because much of the processing that takes place to support seeing, hearing and decision-making is subconscious. In studying the brain during sleep when we are aware of almost nothing, we may get a better understanding of the brain’s secret life and uncover some of the elusive principles that makes the mind so illusive.

Terrence Sejnowski, a computational neurobiologist and Professor at the Salk Institute for Biological Studies, is a coauthor of Thalamocortical Assemblies: How Ion Channels, Single Neurons and Large-Scale Networks Organize Sleep Oscillations.