Scientifically Reviewed by
Dr. Elaine Blank, PhD
PhD in clinical psychology from the University of Arizona, Clinical internship at the University of Maryland,
Postdoctoral work at the Stanford University School of Medicine
Caffeine and alcohol are the two things most people drink that change how they sleep. Here is what the research says about each, and what to do about timing and amount.
How does caffeine affect my sleep quality?
Caffeine is the most widely used psychoactive substance in the world. In a 2017 systematic review of epidemiological studies and randomized trials, Clark and Landolt found that it lengthens the time it takes to fall asleep, cuts total sleep time and sleep efficiency, and makes sleep feel worse [1].
It also changes the kind of sleep you get. The same review reports less slow-wave sleep (the deep sleep that does most of the physical restoration), more stage 1 sleep (the lightest stage), and more awakenings and arousals across the night [1].
How much caffeine affects you depends on sensitivity, age, sex, weight and genetics [1]. Older adults tend to be more sensitive than younger ones, but the differences between individuals are large at every age.
What's the ideal timing for my last cup of coffee?
When you drink caffeine matters more than most people think. Caffeine close to bedtime does the most damage, and most studies find the biggest effects when it is taken within an hour of your usual bedtime [1]. One study in the review gave people caffeine 0, 3 or 6 hours before bed and found measurable sleep disruption at all three, including the 6-hour dose [1]. Keeping coffee to the first half of the day is the practical conclusion.
Cut the coffee, skipped the wine, still awake?
Rest is a program built to improve your sleep, with daily coaching by text or voice. Among users who completed eight weeks, 73% reached healthy sleep efficiency, up from 12%.
How much caffeine is too much for good sleep?
More caffeine means more disruption [1]. Around 200 to 300 mg, about two cups of brewed coffee, is enough to measurably worsen sleep quality.
Coffee is not the only source. Tea, chocolate, soft drinks, energy drinks and some over-the-counter medications all contain caffeine [2], so reading labels is the only way to know your daily total.
Tolerance varies. Some people stop noticing caffeine's effect on sleep within days of regular use; others stay sensitive [1]. People who feel they have adapted may still have subtler disruption they do not notice, and tolerance can change with age.
Does alcohol help or hurt my sleep?
Many people use alcohol as a sleep aid, and it does work as a sedative at first. The research shows it disrupts sleep quality anyway [3].
The pattern is consistent. Alcohol shortens the time to fall asleep and increases slow-wave sleep in the first part of the night [4]. Once the body has metabolized it, a few hours in, sleep gets lighter, with more stage 1 and REM sleep and more awakenings.
That holds outside the lab too. Lydon and colleagues tracked daily alcohol use and sleep in the same people over time and found that on days with more drinking, sleep quality was lower [3].
How much alcohol is safe before bedtime?
More alcohol means worse sleep and bigger changes to sleep architecture [4]. Even 1 to 3 standard drinks can impair sleep, though the effects at lower doses are smaller and less consistent.
Women reach higher blood alcohol levels than men from the same amount of the same drink, so the same dose can disrupt their sleep more [4].
Timing matters here as well. Most studies look at alcohol within an hour of bedtime, but a smaller number suggest alcohol taken 6 hours before bed can still affect sleep, even once blood alcohol is back to zero [4].
Can I combine caffeine and alcohol - what happens to my sleep?
There is little research on the two together. Studies of each suggest their effects on sleep add up [3]: both raise arousal and both change sleep architecture.
Drinking coffee or energy drinks to stay awake while drinking is the common version of this. The caffeine can hide how sedated you are while keeping you stimulated, which can delay sleep and worsen it once you do fall asleep.
How do these substances fit into overall sleep hygiene?
Limiting caffeine and alcohol is one of the basic sleep hygiene recommendations, and sleep hygiene is part of CBT-I [2]. What matters is not only what you drink but when, how much, and how your body responds.
Tolerance complicates it. Both caffeine and alcohol tolerance can develop within days of regular use, which can hide ongoing sleep disruption [1][4]. People may not realize these substances are affecting their sleep even when objective measures show they are.
Rest is a program built on the principles of Cognitive Behavioral Therapy for Insomnia (CBT-I). Caffeine and alcohol timing are part of its sleep education, alongside the behavioral changes that do most of the work for people with insomnia.
Citations
[1] Clark, I., & Landolt, H. P. (2017). Coffee, caffeine, and sleep: A systematic review of epidemiological studies and randomized controlled trials. Sleep Medicine Reviews, 31, 70-78.
[2] Morin, C.M., & Espie, C.A. (2003). Insomnia: A Clinical Guide to Assessment and Treatment. New York: Springer Publishing.
[3] Lydon, D. M., Ram, N., Conroy, D. E., Piper, M. E., Geier, C. F., & Maggs, J. L. (2016). The within-person association between alcohol use and sleep duration and quality in situ. Addictive Behaviors, 61, 68-73.
[4] Irish, L. A., Kline, C. E., Gunn, H. E., Buysse, D. J., & Hall, M. H. (2015). The role of sleep hygiene in promoting public health: A review of empirical evidence. Sleep Medicine Reviews, 22, 23-36.