Does alcohol ruin deep sleep?
Not the way most people think. Alcohol tends to add deep sleep in the first half of the night, then breaks up the second half. The stage it reliably cuts is REM, which starts later and runs shorter after drinking.
People see a low deep sleep number on their watch after a night out and blame the drinks. Others see a high one and decide the wine helped. Both are looking at the wrong stage. Deep sleep is the one alcohol pushes up early, and the losses are in REM and in how broken the second half gets.
What we’d do
After drinking, watch your REM, your wake-ups and your overnight heart rate, and give your deep sleep number less weight. Extra deep sleep in the first hours doesn’t make up for a broken second half. To get the whole night back, finish earlier so the drinks are mostly gone by bedtime.
What happens to each stage
Alcohol moves your sleep around, like furniture shoved to one side of a room. The first half gets crowded with deep sleep, and the second half is left bare and easy to wake from.
| Stage | First half of the night | Second half | Studies |
|---|---|---|---|
| Deep sleep | Up in most studies | Down | 2013 review, 2013 lab study |
| REM | Starts later, less of it | No catch-up | 2013 review, 2013 lab study, 2025 analysis |
| Time awake | Less | More | 2013 lab study |
A 2013 review of every study its authors could find in healthy people concluded that most studies, across doses, ages and both sexes, show more deep sleep in the first half than on dry nights. At high doses, total deep sleep for the night went up too. The most reliable change to REM was that the first REM period came later at every dose.
A lab study the same year, with 24 light drinkers aged 18 to 21, found the other side. Alcohol raised deep sleep and cut REM in the first half, and in the second half it cut deep sleep, raised time awake and lowered sleep efficiency, with no REM rebound.
How much does it take?
A 2025 analysis of 27 studies found REM loss after a low dose, up to 0.5 g of alcohol per kg of body weight, about 2.4 US drinks for a 150 lb woman or 3.1 for a 190 lb man, and it got worse with more. Reaching deep sleep faster only showed up at 0.85 g per kg or more, about 4 drinks for the same woman or 5 for the same man. The same analysis couldn’t settle what alcohol does to total sleep or time awake, because results varied too much.
Why your watch might not show it
In a 2025 study, 40 healthy adults wore a smartwatch for 9 days: 3 dry, then 3 evenings of drinking, then 3 dry. On the drinking nights, the watch’s sleep stages didn’t change. Their average night heart rate went from 63.6 to 66.6 bpm, and they rated their sleep worse.
In that study the stage chart missed what drinking did, and overnight heart rate caught it. That’s the number to compare on your own device.
The two halves of a drinking night
A dry night
After two glasses of wine at 8pm and 9pm, mostly cleared about 2:50am
4 sources
- Ebrahim IO, Shapiro CM, Williams AJ, Fenwick PB, Alcohol and sleep I: effects on normal sleep, Alcoholism: Clinical and Experimental Research, 2013pubmed.ncbi.nlm.nih.gov
- Chan JKM and others, The acute effects of alcohol on sleep architecture in late adolescence, Alcoholism: Clinical and Experimental Research, 2013pubmed.ncbi.nlm.nih.gov
- Gardiner C and others, The effect of alcohol on subsequent sleep in healthy adults: a systematic review and meta-analysis, Sleep Medicine Reviews, 2025pubmed.ncbi.nlm.nih.gov
- Strüven A and others, The Impact of Alcohol on Sleep Physiology: A Prospective Observational Study on Nocturnal Resting Heart Rate Using Smartwatch Technology, Nutrients, 2025pmc.ncbi.nlm.nih.gov
