Sleep cycles between two states: rapid eye movement (REM) and non-rapid eye movement (NREM) sleep. A review of all studies involving normal volunteers has clarified that alcohol shortens the time it takes to fall asleep, increases deep sleep, and reduces REM sleep.
"Certainly a mythology seems to have developed around the impact of alcohol on sleep," added Chris Idzikowski, director of the Edinburgh Sleep Centre. "It is a good time to review the research as the mythology seems to be flourishing more rapidly than the research itself. Also, our understanding of sleep has accelerated in the past 30 years, which has meant that some of the initial interpretations need to be revisited."
At all dosages, alcohol causes a reduction in sleep onset latency, a more consolidated first half sleep, and an increase in sleep disruption in the second half of sleep.
The majority of studies, across alcohol dose, age, and gender, confirm an increase in slow-wave sleep (SWS) in the first half of the night. SWS, often referred to as deep sleep, consists of stages 3 and 4 of NREM. During SWS, the body repairs and regenerates tissues, builds bone and muscle, and appears to strengthen the immune system. Alcohol's impact on SWS in the first half of the night appears to be more robust than its effect on REM sleep.
Alcohol's effects on REM sleep in the first half of sleep appear to be dose related. Low and moderate doses show no clear effects on REM sleep in the first half of the night, whereas at high doses, REM sleep reduction in the first part of sleep is significant. Total night REM sleep percent is decreased in the majority of studies at moderate and high doses.
The onset of the first REM sleep period is significantly delayed at all doses and appears to be the most recognisable effect of alcohol on REM sleep, followed by a reduction in total night REM sleep.