REM (Dream Sleep)
REM sleep accounts for 20-25% of total sleep time. In its first occurrence, it lasts only a few minutes but increases in duration as sleep continues. A dramatic decrease in muscle tone and essential paralysis characterize this stage of sleep. Other characteristics are irregular breathing, increased heart rate, and rapid eye movements. The soft tissues of the upper airway relax, the brain's oxygen consumption increases, and temperature regulatory mechanisms are absent. In males, penile erections are associated with REM sleep. In this stage, people experience vivid, active dreams with complex psychological rest and long-term emotional well-being. It may also bolster memory. REM sleep accounts for approximately 20% of adult sleep.