Why Do We Sleep? Human Sleep: Neurobiology and Function

Sleep is defined as a reversible state of reduced responsiveness to environmental stimuli. It roughly occupies one third of a human life. The science of sleep has greatly evolved over the past century. From a state of passive inactivity, sleep has come to be regarded as a complex physiologic state, with characteristic brain activity.

The neurobiology of sleep is complex. Transection studies have led to the description of the ascending reticular activating system as responsible for the awake state, with the main neurotransmitters involved being acetylcholine, norepinephrine, glutamate, serotonin, dopamine, histamine and orexin. Similarly, specific brain regions have been involved in the active generation of sleep. Non rapid eye movement (NREM) sleep is generated in the basal forebrain and anterior hypothalamus, containing the neurotransmitters gamma amino butyric acid (GABA) and galanin. These areas send inhibitory signaling to the excitatory regions of the brain. Rapid eye movement sleep (REM) is generated in the caudal pons and rostral mesencephalon area. It consists of several features, divided into tonic and phasic phases. Each of these is produced by a specific group of neurons. Acetylcholine is one of the major neurotransmitters in REM sleep.

Sleep has been described in the developing human fetus as early as 28 weeks of gestation. At birth, the human infant sleeps up to 16-18 hours a day and spends half of that time in REM sleep. This high fraction of sleep in the human infant has led to the hypothesis that REM is important in brain maturation.