Rapid eye movement sleep (REM sleep or REMS) is a unique phase of sleep in mammals and birds, distinguishable by random/rapid movement of the eyes, accompanied with low muscle tone throughout the body, and the propensity of the sleeper to dream vividly. Scientists agree that sleep is essential to health, and while stages 1 to 4 and REM sleep are all important, deep sleep is the most essential of all for feeling rested and staying healthy. The average healthy adult gets roughly 1 to 2 hours of deep sleep per 8 hours of nightly sleep. Patients with sleep apnea, which may be associated with a complete loss of REM sleep, are at increased risk for cardiovascular disease, diabetes, obesity, and depression.