Having a drink or two is one way to fall asleep quickly, but how restful do you think an alcohol-induced slumber is?
The latest research, published in Alcoholism: Clinical and Experimental Research, shows that while a nightcap may get you to doze off, you’re more likely to wake up during the night and may not feel as rested following your sleep.
Scientists reviewed 20 studies that included 517 participants who were tested in 38 sleep laboratory experiments. The volunteers drank varying amounts of alcohol, ranging from a low of one to two drinks, a moderate amount of two to four drinks, to a high of four or more drinks. While some experiments examined the results of only one night of drinking, others extended into several consecutive nights. Most of the participants were healthy young adults, and none had drinking problems.
“This review confirms that the immediate and short-term impact of alcohol is to reduce the time it takes to fall asleep,” lead author of the study Irshaad Ebrahim, director of the London Sleep Center, said in a statement. “In addition, the higher the dose, the greater the impact on increasing deep sleep.”
This helps explain why so many people rely on alcohol to fall asleep, despite warnings from experts that it merely postpones and can worsen insomnia. “The effect of consolidating sleep in the first half of the night is offset by having more disrupted sleep in the second half of the night,” Ebrahim said.
That presents a more complicated picture of how alcohol affects sleep, and the trade-off may have implications for understanding how sleep can impact overall health as well. At all doses studied, alcohol increased deep or so-called slow-wave sleep (SWS) during the first part of the night. This type of slumber is associated with healing and regeneration of bones, muscles and other tissues, as well as maintaining a strong immune system.
“SWS or deep sleep generally promotes rest and restoration,” Ebrahim said, cautioning, however, that alcohol increases in this stage can worsen sleep apnea and sleepwalking in people who are prone to those problems.
In contrast, drinking has long been known to reduce REM sleep, the deepest sleep stage in which most dreams occur and during which memories are likely stored and learning occurs. And the current review suggests that it’s the amount of alcohol people drink that may have the biggest effect on their sleep quality. One or two drinks, for example, can increase slow-wave sleep while not affecting deeper REM sleep. But more alcohol can cut into the time spent in the REM stage. So that nightcap may be helpful in getting you to doze off, while a wild night of heavy drinking is likely to make you more restless. Moderation, it seems, is the key to a good night’s sleep.
By Maia Szalavitz
Maia Szalavitz is a neuroscience journalist for TIME.com and co-author of Born for Love: Why Empathy Is Essential — and Endangered.