Think a nightcap may help you get a better night's sleep? Think again.

A new review of 27 studies shows that alcohol does not improve sleep quality. According to the findings, alcohol does allow healthy people to fall asleep quicker and sleep more deeply for a while, but it reduces rapid eye movement (REM) sleep.

And the more you drink before bed, the more pronounced these effects. REM sleep happens about 90 minutes after we fall asleep. It's when people dream, and it's thought to be restorative. Disruptions in REM sleep may cause daytime drowsiness, poor concentration, and rob you of needed ZZZs.

"Alcohol may seem to be helping you to sleep, as it helps induce sleep, but overall it is more disruptive to sleep, particularly in the second half of the night," says researcher Irshaad Ebrahim. He is the medical director at The London Sleep Centre in the U.K. "Alcohol also suppresses breathing and can precipitate sleep apnea," or pauses in breathing that happen throughout the night.

The more a person drinks before bed, the stronger the disruption. One to two standard drinks seem to have minimal effects on sleep, Ebrahim says.

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