快速动眼期（英语：Rapid Eye Movement，REM）是动物睡眠的一个阶段，又称快速动眼睡眠。在此阶段时眼球会快速移动，同时身体肌肉放松。快速眼动睡眠也被称作异相睡眠（paradoxical sleep, PS）或者去同步睡眠（desynchronized sleep），因为在这个阶段，大脑的神经元的活动与清醒的时候相同，呈现快速、低电压去同步化的脑电波。控制REM睡眠的电化学活动似乎是源于脑干，其特征为大量的神经递质乙酰胆碱，同时伴随着单胺类神经递质，包括组胺、血清素和去甲肾上腺素的几乎完全消失。多数在醒来后能够回忆的栩栩如生的梦都是在REM睡眠发生的。

Rapid eye movement sleep (REM sleep, REMS) is a unique phase of mammalian sleep characterized by random movement of the eyes, low muscle tone throughout the body, and the propensity of the sleeper to dream vividly. This phase is also known as paradoxical sleep (PS) and sometimes desynchronized sleep because of physiological similarities to waking states, including rapid, low-voltage desynchronized brain waves. Electrical and chemical activity regulating this phase seems to originate in the brain stem and is characterized most notably by an abundance of the neurotransmitter acetylcholine, combined with a nearly complete absence of monoamine neurotransmitters histamine, serotonin, and norepinepherine. The cortical and thalamic neurons of the waking or paradoxically sleeping brain are more depolarized—i.e., can "fire" more readily—than in the deeply sleeping brain. The right and left hemispheres of the brain are more coherent in REM sleep, especially during lucid dreams.