Caffeine, alcohol and your deep sleep

An afternoon dose of caffeine can still cut your sleep at night. The cutoff for a cup of coffee is about 9 hours before bed, which is earlier than most people guess.¹

A 2023 review pooled 24 studies of caffeine and sleep.¹ On average, caffeine cut total sleep by 45 minutes. It added 9 minutes to the time it took to fall asleep and 12 minutes of lying awake during the night.

Timing mattered more than most people expect. In a 2013 study, people took 400 mg of caffeine at bedtime, 3 hours before bed, or 6 hours before bed.³ They slept at home wearing a portable sleep monitor. All three times disturbed sleep compared with a placebo, including the dose taken 6 hours early.

The 2023 review worked out how long to wait. A standard cup of coffee, which it counted as 107 mg, should come at least 8.8 hours before bed. A typical serving of pre-workout powder, about 217 mg, needs at least 13.2 hours.¹

Bottom line

If you go to bed at 10:30 p.m., your last coffee belongs before about 1:45 p.m. A pre-workout drink belongs before about 9:15 a.m.

Late caffeine doesn't only shorten the night. It also changes its makeup, so the sleep you do get is a little lighter.

In the same review, caffeine added about 6 minutes of the lightest stage of sleep.¹ It removed about 11 minutes of deep sleep, the slowest and heaviest stage.

Eleven minutes sounds small, and on one night it is. Over a week of late lattes, though, that would add up to more than an hour of deep sleep you didn't get.

Bottom line

Late caffeine costs you minutes of deep sleep and gives you lighter sleep in their place.

Drinking before bed can help you fall asleep. It also disturbs the second half of the night and cuts the dreaming stage of sleep, called REM.

A 2013 review of every known study in healthy volunteers found the same pattern at all doses.⁴ People fell asleep faster and slept more soundly in the first half of the night, then woke more often in the second half. Deep sleep rose early in the night, and REM sleep was delayed and, at moderate and high doses, reduced.

The larger 2025 review, of 27 studies, looked closely at dose.² REM sleep dropped after a low dose, about two standard drinks, and the loss grew with every extra drink.

On one point the two reviews disagree. The 2025 review found faster sleep onset only at high doses, about five drinks,² while the 2013 review saw it at all doses.⁴ Either way, the quick drop-off comes with a worse second half.

A standard drink in the US holds about 14 grams of alcohol.⁵ That's a 12-ounce beer at 5 percent alcohol, a 5-ounce glass of wine at 12 percent, or a 1.5-ounce shot of spirits at 40 percent.

Bottom line

Alcohol trades a faster start for a rougher finish, and even two drinks reduce your dreaming sleep.

Most sleep trackers read your heart rate and your movement. The clearest sign of an evening drink is a higher heart rate and a lower recovery score.

A 2018 study followed 4,098 Finnish employees who wore heart rate monitors through ordinary days, some with alcohol and some without.⁶ Recovery scores for the first 3 hours of sleep fell by about 9 points after a low dose. They fell by 24 points after a moderate dose and 39 points after a high one.

The low dose was 0.25 grams per kilogram of body weight or less.⁶ For a 70 kg (154 lb) person, that's a little more than one standard drink.⁵ The drop held for active people as well as inactive ones,⁶ so fitness didn't protect anybody.

Treat the deep sleep and REM minutes on your tracker with more caution. A 2021 lab study tested seven consumer sleep devices on 34 adults against full sleep lab recordings.⁷ Most devices detected sleep well, but their sleep stage estimates were inconsistent, and they did worse on disrupted nights.

Bottom line

Trust your tracker's total sleep and heart rate more than its stage minutes, especially after a drink.

The fix for caffeine is a cutoff time. The fix for alcohol is a smaller amount, because every review above found that more alcohol did more damage.

Set your caffeine cutoff about 9 hours before your usual bedtime, and move any pre-workout drink to the morning.¹ If you train in the evening and want a boost, a caffeine-free warm-up is kinder to your night.

For alcohol, the evidence doesn't offer a safe amount for REM sleep, since even a low dose reduced it.² One drink costs less than two, and two cost less than four.

Bottom line

Caffeine needs a cutoff time. Alcohol needs a smaller pour.

1. 01

   Caffeine cut sleep by about 45 minutes on average and swapped some deep sleep for light sleep.

2. 02

   Your last coffee should come about 9 hours before bed, and pre-workout powder about 13 hours before.

3. 03

   Alcohol helps you fall asleep, then disturbs the second half of the night and cuts REM sleep.

4. 04

   Even about two drinks reduced REM sleep, and the damage grew with each drink.

5. 05

   Trackers show a drink clearly in heart rate and recovery, and less reliably in sleep stages.