Many people enjoy an alcoholic drink in the evening, and it’s a common belief that a ‘nightcap’ can improve sleep. But is this true? Discover why the answer is ‘yes and no’. Find out what the scientific evidence says about alcohol and sleep, uncover the surprising effects of popular drinks from a pint of beer to a glass of wine, and learn how to avoid a vicious cycle of sleep deprivation.
What I Would Rather Not Know About Sleep Science
I like drinking alcohol toward the end of the day. I look forward to an after-work beer, a glass of wine in the evening, or a whisky and a long conversation with a friend. While I’ve known about the effects of alcohol on sleep for some time, and even add control questions in my research to account for this, I haven’t read much about this subject of late; perhaps because I’m worried that I will not like the answer. Nonetheless, I feel a responsibility to make an informed decision, so I recently re-read some of the scientific literature. In this article, I’ll share some of my findings with you.
First, here’s a recap of what is happening in our brain. During sleep, brain activity cycles through two states:
- Non-rapid eye movement (NREM) sleep, sometimes called ‘deep sleep’, or slow-wave sleep (SWS).
- Rapid eye movement (REM) sleep
During NREM sleep, tissues regenerate, the body repairs itself, our immune systems are strengthened, and the brain appears to carry out a sort of ‘self-cleaning’ process1. During REM sleep, our brain is more active, we dream, and this phase of sleep may be associated with the formation of memories. Restricting REM sleep is also associated with worse performance in a range of cognitive tasks2.
Typically, we begin a sleep cycle with NREM, followed by a brief period of REM sleep, then cycle back into NREM. This cycle takes approximately 90 to 120 minutes, resulting in four to five such cycles in a standard 8-hour sleep period3. Each cycle features varying proportions of NREM and REM.
Alcohol Reduces The Time It Takes To Get To Sleep
In 2013, a review evaluated every study which had investigated the impact of drinking on sleep. The analysis confirmed that alcohol reduces the time it takes to fall asleep, and even increased NREM deep sleep, initially4. The review also determined that the higher the dose of alcohol, the greater the impact on increasing deep sleep during the first part of the night. This possibly reinforces the idea that alcohol can aid sleep, as we remember falling to sleep quickly and deeply after drinking in an evening. However, this is not the full story.
But Alcohol Impairs Our Brain’s Recovery
Unfortunately, for those of us who enjoy a nightcap, after examining all of the available English language medical databases, from 1960 to 2013, the authors concluded that the effect of improving sleep in the first half of the night is offset in the second half.
While sleep arrived more quickly and deeply after drinking alcohol, this appears to come at the cost of reducing the amount of REM sleep – the sleep that may be biased toward memory formation and cognitive performance – during the whole night. At all doses of alcohol, REM sleep seems to be delayed, and the total amount of REM sleep is reduced (4).
The Vicious Cycle
It’s also possible that our evening drinking habits can increase the risk of us being caught in a vicious cycle. Sleep deprivation appears to encourage more alcohol consumption3. In turn, higher alcohol consumption makes sleep even worse, further increasing sleep deprivation, resulting in a downward spiral. Even relatively low doses of alcohol, equivalent to ≤0.25 g/kg of body mass, appear to disrupt sleep and impair recovery. A recent study, including 4000 participants, suggested that these doses may reduce the physiological recovery that sleep provides typically by 9.3 per cent5.
What Does This Mean In Practise?
In practical terms, 0.25 g/kg of alcohol represents 17.5 grams, for a 70kg person. In the UK, one alcohol unit is measured as 8g of pure alcohol. This means that, for our theoretical 70kg person, 2.2 units of alcohol could harm sleep.
The problem is, I’m rubbish at remembering what a unit of alcohol means in terms of a real drink. I looked into this, and it turns out that 2.2 units represents (approximately):
- A double (50ml) measure of whisky (40% ABV (Alcohol By Volume))
- 2/3rd of a pint of beer (5.5% ABV)
- A medium/standard (175ml) glass of red wine (12% ABV)
Not a lot.
Does It Matter How Late I Drink?
It takes approximately one hour to metabolise (break down) one unit of alcohol. While this rate varies according to body mass, sex, age, recent food intake and individual variations in metabolic rate, it could take a 70kg person around 2 hours to metabolise one of these drinks. Based on this evidence, if I was to increase the time between drinking and when I intended to go to sleep (i.e. drink earlier in the evening), this may reduce the influence of alcohol on my sleep, but it’s not guaranteed to eliminate the effects entirely.
We Can’t Win
Alcohol is usually classed as a depressant drug. While this is true, alcohol has a ‘biphasic’ effect. This means that alcohol can also have a stimulating effect, depending on the dose. At low doses, alcohol is stimulating. At higher doses, it acts as a sedative. Consequently, you can’t really win. If you don’t drink a lot, the stimulating effect will likely encourage you to stay up later. If you drink more, you may get to sleep more quickly, but your sleep is likely to be disrupted, and brain recovery will be diminished. Also, neither regular physical activity or age seem to be able to protect us from the adverse effects of alcohol on sleep.5
Make Sure It’s Worth It
I’m probably going to continue to enjoy an alcoholic drink, some evenings, but my scan through the research has challenged me to be more selective about when I choose to do this. The evidence is clear; a regular habit of drinking in the evening is almost certainly very detrimental to sleep, so I’m going to make sure that when I do choose to drink, it’s really worth it, rather than merely drinking out of habit, or under a mistaken impression that it could improve sleep. The solution seems to be to drink less, stop earlier. Ideally, don’t drink at all. If you’re struggling with sleep, you may find the infographic above to be useful.
- Hablitz LM, Vinitsky HS, Sun Q, Stæger FF, Sigurdsson B, Mortensen KN, et al. Increased glymphatic influx is correlated with high EEG delta power and low heart rate in mice under anesthesia. Sci Adv. 2019;5(2):eaav5447.
- Purves D, Augustine GJ, Fitzpatrick D, Katz LC, LaMantia A-S, McNamara JO, et al. The Possible Functions of REM Sleep and Dreaming. In: Purves D, Augustine GJ, Fitzpatrick D, Katz LC, LaMantia A-S, McNamara JO, et al., editors. Neuroscience. 2nd ed. Sunderland (MA): Sinauer Associates; 2001.
- Roehrs T, Roth T. Sleep, Sleepiness, and Alcohol Use Timothy. Alcohol Res Heal. 2001;25(2):101–9.
- Ebrahim IO, Shapiro CM, Williams AJ, Fenwick PB. Alcohol and Sleep I: Effects on Normal Sleep. Alcohol Clin Exp Res. 2013;37(4):539–49.
- Effect A, Intake A, Autonomic C, During R, Hours F, Sample LR, et al. Acute Effect of Alcohol Intake on Cardiovascular Autonomic Regulation During the First Hours of Sleep in a Large Real World Sample of Finnish Employees : Observational Study. 2018;51.