人为什么要睡眠?

Why do we sleep?

睡眠是指机体失去对周围环境知觉和反应的一种可逆性行为.根据脑电变化,睡眠分为非快速眼动(non-rapid eye movements,NREM)睡眠和快速眼动睡眠(rapid eye movements,REM)两种状态,前者又包括了由浅入深的3个睡眠时相.睡眠是一个动态变化的主动过程,是大脑的重要功能之一.觉醒的维持及入睡均具有明确的神经解剖学和神经生物学基础,受内源性昼夜节律、内环境稳态调节过程及社会和环境因素的交互作用的影响.作为最基本的生命活动之一,睡眠的功能尚未完全明确,可能与进化与适应、组织的修复、免疫功能的增强、能量的保持与恢复、学习与记忆的巩固均有关系.新兴的睡眠医学为从睡眠疾病的角度认识睡眠的本质提供了机遇.

According to a simple behavioral definition, sleep is a reversible behavioral state of perceptual disengagement from and unresponsiveness to the environment. Sleep is dividing into two states： Non-rapid eye movement （NREM） sleep which is subdivided into three stages （N1, N2, N3）; and rapid eye movement （REM） sleep characterized by rapid eye movements, muscle atonia and desynchronized EEG. Mammalian sleep and wake states are regulated by multiple neuronal systems located in the brainstem, diencephalon, and telencephalon. Wakefulness is maintained by activation of the ascending re- ticular activating system involving several neurotransmitters including glutamate, acetylcholine and the monoamines. NREM sleep onset is associated with a reduction in activation of the ascending reticular activating system and an in- crease in neural activity within the ventrolateral preoptic area, anterior hypothalamus and basal forebrain. REM sleep is triggered by activation of cholinergic neurons in the laterodorsal and pedunculopontine tegmental nuclei. The suppres- sion of motor activity in REM sleep is generated by glutamate-mediated activation of descending medullary reticular formation. The two-process model postulates that a homeostatic process rises during waking and declines during sleep and interacts with a circadian process that is not directly dependent of sleep and waking. Sleep function remains controversial. Sleep is thought to be restorative, conservative, adaptive, thermoregulatory and memory consolidative functions. Sleep also serves host-defense mechanisms and conserves caloric expenditures. Emerg- ing evidences indicated that sleep replenishes brain energy stores and that sleep serves a glymphatic function by remov- ing toxic byproducts of waking activity. Finally, there is experimental evidence that connectivity changes with sleep, sleep loss, and with changing afferent input, and that those changes are linked to sleep regulatory mechanisms. Sleep medicine as a medical specialty has existed for more than 30 years. The study on over 90 kinds of sleep disor- ders has shed light on the understanding to the mechanisms of sleep. For example, the finding of the role of hypocretin/orexin in the patho-physiology of narcolepsy led to the reveal of hypocretin/orexin as a major wakefulness promoting substance. The evolution of the field will help us recognize the impact of sleep disorders on society as well as the nature of sleep itself.