The guide

The two halves of a drinking night

By GyroscopeUpdated November 14, 20267 sources

Alcohol splits your night in two. While your level is high you fall asleep fast, sleep deeply, and your first REM sleep gets pushed back. As it runs out, sleep turns light and broken and you wake more, and the lost REM doesn’t come back by morning. Across 27 studies, even about two drinks cut REM.

The first half is why so many people think a drink helps them sleep. You lie down and you’re out, and the first few hours really are deep. The researchers behind the biggest review on this started from the same point, that alcohol “is commonly consumed prior to bedtime with the belief that it facilitates sleep.” The cost shows up later, at 2am or 3am, when most people are too groggy to connect it to the glass at 10.

Why does a drink feel like it helps?

The first half of the night really does change in the direction people want. A 2013 review of every study of alcohol and healthy sleepers the authors could find saw it at every dose. People fell asleep sooner and slept more solidly in the first half of the night, and most studies found more deep sleep early on too.

It’s like borrowing against your own night. You get a deep first few hours up front and pay for them after 2am.

What happens in the second half?

The same 2013 review found more broken sleep in the second half at all doses. The most recognizable effect on REM sleep was that the first REM period came late, at every dose. Over the whole night, REM was lower in most studies at moderate and high doses.

A 2013 lab study showed both halves. Researchers gave 24 light social drinkers aged 18 to 21, half of them women, a strong dose of alcohol before bed on one night and a placebo on another, and recorded their sleep. With alcohol, the first half of the night had more deep sleep and less REM. The second half had more time awake, less deep sleep and lower sleep efficiency. There was no REM catch-up later in the night, so the REM lost early was gone by morning. In a 2010 sleep lab study of 95 heavy drinkers, alcohol likewise cut sleep efficiency and REM and left people sleepier the next day.

How much does it take?

It takes less than most people think. In 2025, researchers pooled 27 studies on alcohol and the sleep that followed. REM started later and was shorter. That was already true at a low dose, up to 0.5 g of alcohol per kg of body weight, and it got worse as the dose went up. Falling asleep faster, and reaching deep sleep faster, only showed up at 0.85 g per kg or more.

Here’s what those doses are in US drinks.

The same review couldn’t say much about total sleep time, sleep efficiency or time awake across the whole night, because the studies disagreed too much. The REM finding and the dose it starts at are the firm parts.

Why does the second half break?

The broken half lines up with the alcohol running out. Your liver clears a fixed amount an hour, about 15 mg per 100 mL on average, so an evening of drinks is still falling through the first hours of your sleep. As it gets near zero, sleep tends to turn light and you wake more.

On our engine, a 150 lb woman who has two 6 oz glasses of 13.5% red wine with dinner, finished at 8pm and 9pm, is mostly clear at about 2:50am, somewhere between 2:05am and 3:55am depending on her liver. If she goes to bed at 11, that’s about 3.8 hours of her sleep with alcohol in it. The shaded curve in the drawing above is that evening.

Alcohol also changes your breathing. Across 14 trials with 422 people, alcohol added about 2 breathing pauses an hour during sleep, about 4 in people who snore and about 7 in people with sleep apnea, and average blood oxygen fell a little. If you snore or have apnea, a drink near bed costs you more than it costs most people.

Why can I feel fine and still lose sleep?

Feeling and measuring don’t always agree. In a 2025 study, 40 healthy adults wore a smartwatch for 9 days, with 3 dry days, then 3 evenings of drinking, then 3 dry days. Their watches showed no change in sleep stages, but the same people said they slept worse, and their night heart rate went up by 3 beats a minute.

The watch’s sleep stages missed what people felt, and their heart rate caught it, so overnight heart rate is the steadier number to watch. What your watch or ring sees covers it.

What does finishing earlier buy you?

Finishing earlier moves the end of the curve back toward bedtime, and that buys a lot. Take the same woman and the same two glasses.

Nobody has run a trial of a drinking cutoff before bed, so these times come from the clearing math. Tonight’s clear time runs it for your own drinks, and lets you drag the last one earlier to see what it gives back.