A nightcap 'works.' But not in the way you want it to.
A drink before bed makes you feel like you're sleeping well. That's not a lie. You really do fall asleep faster. The trouble is that while alcohol accelerates sleep onset, it secretly chips away at your REM sleep.
In 2024, McCullar and colleagues conducted a rather pointed experiment in the journal Sleep. They took 30 healthy adults and had the same individuals spend two different types of nights: one where they drank until an hour before bed (aiming for a breath alcohol concentration of 0.08) and one where they drank a non-alcoholic beverage that looked and tasted identical. They spent three nights in each condition, with their sleep measured via EEG.
The results were clear-cut. On the nights they drank, the accumulation of REM sleep slowed down in the first half, and the total amount of REM sleep for the night was significantly reduced. Even though deep sleep (slow-wave sleep) actually increased in the first half, the total amount remained unchanged.Even though you fall asleep faster, only the REM sleep that processes emotional memories is diminished.Subjective experience and EEG data diverge here. Older reviews (Ebrahim 2013) pointed in the same direction: alcohol temporarily increases deep sleep in the first half of the night and suppresses REM in the second half.
There isn't much you can do.
Stop drinking 1 to 3 hours before bedtime.
The morning after drinking, keep an eye on your resting heart rate and HRV, and dial back your decision-making and training for the day.
Your body will come to collect the debt from the previous night during the day.
To be clear, this was a short-term experiment involving three consecutive nights of drinking at a 0.08 level. Whether someone who has been drinking for a long time can fully recover their REM sleep by quitting is something that varies from person to person. Very small amounts of alcohol, or a drink with plenty of time before bed, are outside the scope of this study.
Still, remember one thing. What a nightcap brings isn't deep sleep.It's sleep onset.It's worth keeping that in mind.