Those nightcaps might affect your sleep more than you thought -- and not in a good way.
A new review of studies, published in the journal Alcoholism: Clinical & Experimental Research, shows that alcohol changes the normal rhythms of sleep by increasing the amount of time people spend in "deep sleep," but also decreasing the amount of time people spend in REM sleep by disrupting sleep during this stage, which is necessary for memory, concentration and motor skills.
"In sum, alcohol on the whole is not useful for improving a whole night's sleep," study researcher Chris Idzikowski, who is the director of the Edinburgh Sleep Centre, said in a statement. "Sleep may be deeper to start with, but then becomes disrupted. Additionally, that deeper sleep will probably promote snoring and poorer breathing. So, one shouldn't expect better sleep with alcohol."
Sleep has two parts that cycle back and forth: non-REM sleep, and REM sleep. The cycle takes 90 minutes, and continues on through the night -- it starts with non-REM sleep, and then goes on to REM sleep, and then back to non-REM sleep, and so on and so forth.
But alcohol increases the amount of time a person spends in the later stages of non-REM sleep, called "slow-wave sleep," or "deep sleep." While it may seem good that alcohol increases deep sleep -- which is important for the body to repair itself, as well as amp up the immune system -- it could actually have a negative effect among people who are predisposed to sleep apnea or sleep walking, by making them more vulnerable to these conditions.
"This effect on the first half of sleep may be partly the reason some people with insomnia use alcohol as a sleep aid," study researcher Irshaad Ebrahim, the medical director at the London Sleep Centre, said in a statement. "However, 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."
Researchers also found that alcohol seems to delay the onset of the first cycle of REM sleep, a consequence of which "would be less restful sleep," Idzikowski said in the statement.
In 2011, a study in the same journal also showed that alcohol could disrupt sleep, especially in women. That study, conducted by researchers at the University of Michigan, showed an association between alcohol consumption and sleepiness and disrupted sleep in women.