Why Do We Sleep? Human Sleep: Neurobiology and Function

Sleep takes up one third of the human life and has always been a subject of fascination for human beings. In the past century, significant progress has been made in the study of sleep.

Sleep is defined as a reversible state of decreased responsiveness to the environment. It has been historically thought to be a passive state of inactivity; however, electrical recordings of brain waves have shown that the brain remains very active during sleep, but that this activity is distinctively different from the wake state.

Sleep is divided into two main states, non-rapid eye movement (NREM) and rapid eye movement (REM) sleep. The latter is thought to be the state where the most vivid dreams occur. Different brain regions and groups of neurons in the brain are responsible for the generation of each of these two states. Human fetuses have been shown to display sleep brain waves as early as 28 weeks of gestation.

The question of “why do we sleep?” has been occupying scientists for some time and a clear answer has yet to be accepted by all. Many hypotheses exist. The evolutionary adaptation hypothesis claims that sleep evolved to protect the organism by making it quiescent during times where activity would be dangerous (exposure to predators or injury), while also saving energy during those times. More recent evidence suggests that, during sleep, the brain regenerates energy that has been depleted by wakefulness. Other studies also suggest that sleep enhances learning and memory formation.