What alcohol does to your HRV, resting heart rate and sleep

Twenty-six healthy adults aged 30 to 60 spent three nights in a sleep laboratory. Before bed on one night they drank a placebo. On another they drank one standard drink if they were women or two if they were men. On the third they drank three or four. On the low dose night their heart rate ran about 4 percent faster than on placebo for the first four hours of sleep. On the high dose night it ran about 14 percent faster for all six hours the researchers analyzed. RMSSD, a beat-to-beat measure of vagal activity, fell for several hours on both alcohol nights, more after the higher dose (de Zambotti et al., 2021). These were ordinary evening amounts, and the participants simply went to sleep afterward.

The same pattern appears in millions of nights of wearable data. In 20,968 WHOOP members contributing just over 5 million days, one drink more than a person's usual amount, compared with one less, went with a sleeping heart rate 2.4 to 2.8 bpm higher and an overnight HRV 3.3 to 3.8 ms lower (Grosicki et al., 2026). The effect on overnight HRV and resting heart rate rises with the dose, and it appears in laboratory studies and everyday data alike.

This article opens a series on what moves your numbers. Each piece takes one common factor, starting here with alcohol and later covering caffeine, illness and travel, and answers the same questions in the same order. What does it do in the body? How big is the effect at a given dose? What happens to sleep, and how long does it last? Who is affected more? How does it show up in Titan, and how do you test it on yourself?

01The short answer

The rest of the article supplies the sources and the mechanisms behind each row.

02What alcohol does in the body

Absorption and clearance

Alcohol is absorbed from the stomach and small intestine and distributed through body water. The liver clears it at a fairly steady rate that barely depends on how much is in the blood. Cederbaum's 2012 review puts the average at about 7 g per hour for a 70 kg adult. A US standard drink holds about 14 g of alcohol, according to the National Institute on Alcohol Abuse and Alcoholism, so the average person clears roughly one drink every two hours.

The rate varies widely between people. In Jones's 2010 survey of forensic and clinical studies, blood alcohol fell by 10 to 15 mg per 100 mL per hour in fasting subjects and 15 to 20 after a meal, with 15 as a reasonable average for moderate drinkers and up to 35 in people being treated for alcohol dependence. Food slows absorption and speeds clearance.

The arithmetic has a direct consequence for sleep. Three drinks finished at 10 pm leave alcohol in the blood for a few hours after an 11 pm bedtime. Roehrs and Roth (2001) estimated that at typical doses, metabolism is complete within 4 to 5 hours of sleep onset, which places the end of clearance near the middle of the night. That timing explains most of what happens to sleep and to overnight heart data.

The autonomic shift

At rest, and especially in early sleep, the parasympathetic branch of the nervous system slows the heart through the vagus nerve. That vagal activity produces the fast beat-to-beat variation that RMSSD and high-frequency HRV capture. Alcohol pushes the balance the other way.

Spaak and colleagues measured sympathetic nerve traffic directly with a needle electrode in the leg. In 13 volunteers aged 24 to 47, two drinks raised muscle sympathetic nerve activity by 9 to 10 bursts per minute, and red wine raised heart rate by 5.7 bpm (Spaak et al., 2008). In a companion analysis of 12 of the subjects, one drink had no effect on heart rate or HRV. Two drinks raised heart rate by 5.4 to 5.7 bpm, cut total HRV power by 28 to 33 percent and high-frequency power by 32 to 42 percent, and roughly doubled the ratio of low- to high-frequency power (Spaak et al., 2010). Red wine and plain ethanol had similar effects.

Those were daytime measurements in awake subjects. The same shift appears during sleep. Sagawa and colleagues gave 10 healthy young men 0, 0.5 and 1.0 g of alcohol per kg of body weight on separate nights, three weeks apart, and recorded their hearts overnight. As the dose increased, heart rate rose and HRV power fell in every frequency band (Sagawa et al., 2011). Pietilä and colleagues described the same shift in real-world data as increased sympathetic regulation and decreased parasympathetic regulation, in proportion to the amount consumed (Pietilä et al., 2018).

For a wearable, that means two correlated signals move together. Overnight HRV falls and sleeping heart rate rises. Both are the inputs that most recovery scores, including Titan's, are built on.

03How big the effect is at each dose

The largest dose-response dataset comes from Finland. Pietilä and colleagues (2018) analyzed 12,411 days of beat-to-beat heart recordings from 4,098 employees, 56 percent of them women, with an average age of 45. Participants wore the recorders through ordinary life and logged what they drank. The analysis compared each person's drinking nights with their own non-drinking nights, looking at the first three hours of sleep. A drink in this study was a Finnish portion of 12 g of alcohol.

Within-person changes during the first 3 hours of sleep. Source: Pietilä et al. (2018).

The recovery state is Firstbeat's HRV-derived measure of how much of the recording was dominated by parasympathetic activity. Its changes are percentage points of the recording classed as recovery, a separate quantity from a percent change in HRV. The authors gave a worked example. An 80 kg person drinking five portions, 60 g of alcohol, lands at 0.75 g/kg, the top of the moderate band.

The authors found the effects "similar for both genders and for physically active and sedentary subjects," and stronger in younger participants and in those with a lower sleeping heart rate at baseline. Their conclusion was blunt: "Regular PA or young age do not protect from these effects." A low sleeping heart rate usually reflects strong vagal activity, which suggests that people with more vagal activity have more for alcohol to suppress.

The WHOOP analysis by Grosicki and colleagues (2026) reached the same shape with a different design. It compared each member's nights against their own average intake. One drink above that average, against one below it, went with a sleeping heart rate 2.8 bpm higher in women and 2.4 bpm higher in men, and RMSSD 3.8 ms and 3.3 ms lower. Five drinks above average, against three above, cut HRV by a further 5.6 ms in women and 5.1 ms in men. Sleep duration and next-day physical activity also fell as intake rose.

A 4 bpm change can look small next to a watch's daytime readings. Against your own overnight baseline, it is large. Sleeping heart rate is one of the steadiest numbers a wearable records, so a change of a few bpm can sit well outside a person's usual night-to-night range.

04What alcohol does to sleep

Falling asleep and the first half of the night

Alcohol is a sedative. Ebrahim and colleagues' 2013 review of controlled studies in healthy volunteers concluded that at all doses it shortens the time to fall asleep and makes the first half of the night more consolidated. Slow wave sleep, the deep stage in most sleep stage systems, increases in the first half of the night across doses, ages and sexes. Sleep for athletes covers what each stage contributes to recovery.

A 2025 meta-analysis of 27 studies in healthy adults by Gardiner and colleagues is more cautious on the first point. It found shorter sleep onset latency and a faster arrival of deep sleep only after high doses of at least 0.85 g/kg, about five standard drinks. The two reviews agree that heavy drinking makes people fall asleep faster. They differ on whether a drink or two does, and the meta-analysis is the more formal of the two.

REM sleep

REM sleep is where the evidence is most consistent. Ebrahim and colleagues called delayed onset of the first REM period the most recognizable effect of alcohol on REM, present at all doses. Total REM fell at moderate and high doses in most studies. Gardiner and colleagues found that alcohol delays REM onset and reduces REM duration in a dose-dependent way, and that even low doses affect REM. High doses shorten the time to fall asleep while degrading REM, which may hide the later disruption from the sleeper.

The second half of the night

Ebrahim and colleagues described increased sleep disruption in the second half of the night at all doses. The timing matches clearance. By the early morning hours, alcohol is gone from the blood, the sedative effect has worn off and the sleeper wakes more easily. In 24 people aged 18 to 21 brought to a breath alcohol level near 0.10 percent, Chan and colleagues (2013) found more slow wave sleep and less REM in the first half of the night, then more time awake after sleep onset and lower sleep efficiency in the second half. In 95 young adults at a mean breath alcohol of 0.11 percent, Rohsenow and colleagues (2010) found lower sleep efficiency, less REM, more time awake and more sleepiness the next day. Bourbon and vodka produced the same sleep results, and only the hangover symptoms differed.

What a wearable sees and misses

Consumer wearables estimate sleep stages from movement and heart rate, without recording brain activity. That limits how much of the change in sleep stages a watch can register. In Strüven and colleagues' 2025 study, 40 healthy adults wore a Withings ScanWatch through three alcohol-free days, three evenings of drinking 40 g (women) or 60 g (men), and three days after. Nocturnal resting heart rate rose from 63.6 to 66.6 bpm during the drinking evenings. The watch's light sleep, deep sleep and awakening counts did not change significantly, even though participants rated their sleep as worse.

The practical reading is that on a wearable, heart rate and HRV detect alcohol more reliably than staged sleep does. The Apple Watch sleep accuracy article covers how well watch staging matches laboratory polysomnography.

05Timing of the last drink

The evidence for stopping earlier is consistent in direction, and the size of the benefit depends on the dose.

In the WHOOP data, when women drank 60 minutes earlier than their usual time, they had a sleeping heart rate 0.87 bpm lower and HRV 1.5 ms higher, and the authors described drinking about 2 hours earlier, such as before dinner rather than after, as associated with better overnight autonomic regulation (Grosicki et al., 2026). That is a modest improvement next to the effect of the drinks themselves.

A small laboratory study shows the limit of timing. Landolt and colleagues (1996) gave 10 healthy men with an average age of 62 a moderate dose of 0.55 g/kg six hours before bedtime. By the time they went to sleep, breath alcohol had returned to zero in every subject. Their sleep efficiency, total sleep time and REM sleep still fell, and wakefulness in the second half of the night doubled. The study was small and limited to older men, but it shows that a zero breath reading at bedtime does not guarantee an undisturbed night.

A reasonable reading of the evidence is that finishing earlier reduces the part of the effect that comes from alcohol still circulating during sleep, and that the rest depends mainly on how much you drank. The clearance figures above give a working estimate. At an average of about one standard drink cleared every two hours, each drink still unmetabolized when you stop adds roughly two hours before your blood is clear, with wide variation between people.

06How long the effect lasts

This is the question people most want answered and the one with the least direct evidence.

Within the drinking night, the heart data is clear. Pietilä's effects were measured in the first three hours of sleep. In de Zambotti's laboratory study, the low dose raised heart rate for about the first four hours and the high dose for all six hours analyzed. As alcohol clears, heart rate falls back toward normal, and heavier doses keep it elevated for more of the night.

Across nights, the evidence is thin. The WHOOP analysis covered only the night after drinking. In the Withings study, nocturnal resting heart rate on the three days after three evenings of drinking averaged 64.9 bpm, close to the 63.6 bpm baseline. The paper did not report a statistical test of that difference, so it shows a quick return in that sample without proving complete recovery. We found no controlled study that tracked HRV over several nights after a single drinking night in healthy adults.

Some effects outlast the night. Roehrs and Roth (2001) reported that with drinking on consecutive nights, tolerance to alcohol's sedative and sleep-stage effects developed within three nights, while suppression of growth hormone secretion persisted through all three. Muscle performance after hard exercise, covered below, was still impaired 36 hours after a single dose of 1 g/kg. Chronic heavy use is a different situation. Karpyak and colleagues' 2014 review found that HRV in alcohol-dependent people improves with abstinence but remains below that of controls.

For a single evening of light or moderate drinking, the available data suggest that overnight heart rate returns close to baseline within a night or two. Heavier or repeated drinking likely takes longer, by an amount no study has pinned down. Your own records can measure it for you, which the self-experiment section explains.

07What alcohol means for training and adaptation

Muscle protein synthesis

Parr and colleagues (2014) had eight physically active men complete a session of resistance exercise followed by continuous and interval cycling. One hour after the session they began drinking 1.5 g/kg of alcohol, about 12 standard drinks, over three hours. Compared with protein alone, myofibrillar protein synthesis over the recovery period was 24 percent lower when the alcohol came with protein and 37 percent lower when it came with carbohydrate. Protein partly offset the effect without removing it. The dose was large and the sample small, so the result describes a heavy night of drinking after training.

Strength recovery after muscle damage

Barnes and colleagues ran a series of small trials on the quadriceps. After 300 maximal eccentric contractions, 11 men drank either 1 g/kg of alcohol or a matched non-alcoholic drink. At 36 hours, isometric strength had fallen 34 percent in the alcohol condition against 12 percent in the control condition. Concentric strength had fallen 40 against 28 percent, and eccentric strength 34 against 19 percent (Barnes et al., 2010a). A second trial in 10 men with the same dose found similar losses at 36 hours (Barnes et al., 2010b). When the dose was halved to 0.5 g/kg, the difference disappeared (Barnes et al., 2011). Barnes's 2014 review concluded that about 0.5 g/kg is unlikely to affect most aspects of recovery in male athletes.

For a 75 kg person, 0.5 g/kg is 37.5 g of alcohol, between two and three US standard drinks. The trials involved men only, and a woman of the same weight reaches a higher blood alcohol from the same dose, as the next section explains.

Hormones and the wider picture

Lakićević's 2019 systematic review of 12 studies on alcohol after resistance exercise found that most recovery markers, including creatine kinase, heart rate, lactate, inflammatory markers, force, power, soreness and perceived exertion, did not change meaningfully. Cortisol rose, while testosterone, plasma amino acids and muscle protein synthesis fell. Add Roehrs and Roth's finding on growth hormone during sleep, and the evidence points in one direction. Heavy drinking after training appears to reduce the building side of recovery, even when soreness feels normal.

If you are in a block aimed at muscle growth, the timing that matters most is the evening after a hard session. The HRV and training readiness guide explains why a Recovery drop that follows alcohol should be read differently from one that follows training.

08Who is affected more

Sex and body size

Women reach a higher blood alcohol than men from the same dose per kilogram. Frezza and colleagues (1990) found that in women, first-pass metabolism of alcohol in the stomach was 23 percent of men's and gastric alcohol dehydrogenase activity was 59 percent of men's. Baraona and colleagues (2001), studying 65 people, found that women also had a 7.3 percent smaller volume of distribution for alcohol, with lower gastric metabolism as the main driver. Body water scales with lean mass, so a smaller or leaner person of either sex has less volume to dilute the same drink.

The heart data on sex differences is mixed. Pietilä and colleagues found similar effects in women and men. In the WHOOP cohort the effects were generally greater in females, and in the Withings study they were larger in women and in people with a BMI under 25. De Zambotti's study gave women fewer drinks than men to reach similar blood levels. Counting doses in grams per kilogram of body weight removes much of the sex difference in exposure.

Age

Vestal and colleagues (1977) infused alcohol into 50 subjects aged 21 to 81. Peak blood concentration rose with age, because lean body mass and the volume that alcohol spreads into both fall, while the rate of elimination did not change with age. Older people therefore reach higher levels from the same dose.

The heart data runs the other way. Both Pietilä's and Grosicki's analyses found larger HRV effects in younger adults. In the WHOOP data, five drinks above average reduced HRV by 3.0 ms more in people in their twenties than in people in their thirties. The likely explanation is baseline. HRV declines with age, as the HRV by age article shows, and a higher vagal baseline leaves more to suppress in absolute milliseconds.

Tolerance

Regular drinkers adapt to part of the effect. Stein and Friedmann's 2005 review concluded that up to two to three standard drinks before bed initially promote sleep, but that these effects diminish in as few as three days of continued use. Colrain and colleagues (2014) described a nine-night study in which slow wave sleep had returned to baseline by the ninth night. Those findings describe tolerance to the sedative and sleep-stage effects. We found no evidence of tolerance to the heart rate and HRV effects, and Pietilä's result that regular exercise does not protect points the same way.

09How a night of drinking shows up in Titan

Titan scores each night against your own history, so the question is how far a drinking night moves your numbers from your usual range. The descriptions below follow the current Titan app.

Recovery

Titan's Recovery score belongs to the day you wake up. It reads two numbers from the main sleep session that ended that morning, or from 10 pm to 10 am when no sleep session was saved. Overnight HRV is the average of all the HRV readings recorded inside that window, taken on a log scale. Apple Watch records HRV as SDNN, and Titan uses SDNN by default. The studies above mostly report RMSSD, a different measure on a different scale, so compare your changes with your own baseline and leave the study figures in milliseconds aside. Sleeping heart rate is the lowest average over any 30-minute stretch that has at least three heart rate readings. On a night with sparse data, Titan uses the 10th percentile of all readings instead.

Titan compares each value with your own baseline, the median of your measured nights in the Recovery window before that night. The window defaults to 60 nights, and a baseline needs at least 7 measured nights covering a quarter of the window, which is 15 nights on the default. The distance from the median is measured in units of your own typical night-to-night spread, the median absolute deviation scaled by 1.4826, so a given change counts for more if your numbers are usually steady. The score is 100 / (1 + e^−(0.25 + 1.5 × HRV distance − 0.75 × heart rate distance)). A night exactly at both baselines scores 56. HRV counts twice as much as heart rate, and each distance is capped at 3 spread units so that one bad reading cannot pin the score.

Alcohol moves both inputs in the unfavorable direction at once, which is why Recovery reacts so strongly. The table below uses illustrative inputs sized to match the changes reported in the studies above. The baseline is an overnight HRV of 48 ms and a sleeping heart rate of 52 bpm, with a median absolute deviation of about 12 percent in HRV and 2 bpm in heart rate.

Illustrative values, not measurements. Recovery bands are Low 0 to 39, Moderate 40 to 74 and High 75 to 100.

Someone whose numbers are more variable from night to night would see smaller drops for the same changes, because Titan divides by that person's wider spread. The last two rows show that either signal alone moves the score substantially.

To see the inputs for any night, tap Recovery on Today. Under What shapes your score, the Recovery screen lists Overnight HRV and Sleeping heart rate for the night next to your Recovery baseline for each. The Recovery history chart below shows the score across 7, 30 or 60 days, and an occasional drinking night shows up as a single low bar.

Three details of the calculation matter for alcohol.

The first is where the heart rate reading comes from. Alcohol raises heart rate most in the first hours of sleep. Titan's sleeping heart rate is the lowest 30-minute stretch of the night, which can fall later, after alcohol has cleared. After a drink or two, that lowest stretch may sit close to normal while HRV readings taken earlier in the night still show the disruption. After heavier drinking that keeps heart rate elevated all night, both inputs move.

The second is how drinking nights enter your baseline. Each night's baseline uses only the nights before it, so a drinking night never changes an earlier score. It does join the pool for later nights. A median ignores a single unusual night, but if you drink on two or three nights most weeks, those nights make up a large share of your 60-night window and pull the median toward them. Your baseline then describes your usual life, alcohol included, and a drinking night looks less unusual against it than it would against an alcohol-free baseline.

The third is what happens on a night without HRV, such as when the watch sits on the charger. Titan then estimates HRV by carrying forward a weighted average of recent measured nights, with the most recent weighted most and the value pulled 30 percent of the way back toward your median for each night since. If the last measured night was a drinking night, the estimate for the next night starts lower. The Recovery screen labels such a score as Estimated, with the reason.

Sleep

The Sleep score works differently. It compares time asleep with your Sleep Goal, which defaults to 8 hours, and adds two measures of the night's contents. Duration carries 30 of the 68 weighting points, about 44 percent of the score. Restorative sleep carries 24, about 35 percent. Titan counts Deep plus REM as restorative and treats 40 percent of time asleep as a typical night. Most of that component rewards reaching the 40 percent share, and 18 percent of it rewards an even split between Deep and REM. Awake time carries the remaining 14 points, about 21 percent, and earns no credit once time awake reaches 20 percent of time in bed.

The component weighting makes the Sleep score less sensitive to alcohol than the Recovery score. The extra deep sleep early in the night partly replaces the lost REM in the restorative share, and only the balance term notices the swap. The second-half waking shows up in awake time. Here is the same person on two nights, calculated with an 8 hour Sleep Goal.

Illustrative values, not measurements.

In these examples, the drinking night costs 10 Sleep points, while three or four drinks cost 45 Recovery points. The gap fits the research, which shows the heart signals moving more reliably than watch-staged sleep. To see the stages, tap Sleep on Today. Your night shows the stage timeline with Deep, REM, Core and Awake durations, and Sleep history has tabs for Score, Duration, Restorative, Debt and Consistency. The Sleep score help article lists the components, and Recovery scores explained works through more examples.

10Tagging alcohol in the Journal

The Journal lets you mark days and then compare your scores on marked and unmarked days. Alcohol is one of the 15 built-in tags. Tag the day you drank, and Titan lines the tag up with that evening and the night that followed.

Journal Insights then compares scored days with and without the tag over the last 60 days. Pick Recovery or Sleep at the top, then a metric from the menu below it. Recovery Score is the default under Recovery, and Sleep offers Sleep Score and Sleep Debt. Impact is the average on tagged days minus the average on untagged days, divided by the untagged average. If your Recovery averages 60 on untagged days and 42 on days tagged Alcohol, the impact is −30 percent. Tags used on fewer than 3 days in the window appear under Low Confidence. Tap a factor to see every scored day in the window, with the tagged days highlighted.

A day holds each tag once, so the built-in tag cannot record dose. Custom tags solve that. Tap + Add/Edit at the bottom of the Journal and add tags such as "1 to 2 drinks," "3 or more drinks" and "Last drink after 10pm." Because Insights ranks each tag separately, dose tags let you see whether your own response scales the way the studies say.

11Test it on yourself

Journal Insights compares averages, and averages from everyday life mix alcohol with everything that tends to come with it, such as late dinners, later bedtimes, weekends and social events. A deliberate self-experiment removes most of that. Here is a protocol that fits the way Titan calculates.

Establish a clean reference

Spend two to three weeks without alcohol, with your usual training, bedtimes and caffeine. Wear the watch every night. This gives you at least 14 alcohol-free nights to compare against. It does not reset Titan's baseline, which covers the last 60 nights and still contains any earlier drinking nights, so make the comparison against your alcohol-free nights directly as well as against the score.

Fix everything except the alcohol

Pick one dose, count it in standard drinks and keep it the same. Keep the time of the last drink, the bedtime and the food the same. Avoid drinking on the evening of a hard training day during the test, since hard training also lowers overnight HRV. The stress and HRV guide covers the other common confounders.

Repeat and tag consistently

Aim for at least five drinking nights on one dose, spread over several weeks, and tag each drinking day. Three is the minimum for Journal Insights, but five or more give a steadier average. To test the night after, add a custom tag such as "Day after alcohol" and put it on the day after each drinking day. If the second morning's scores match your alcohol-free nights, your carryover is short.

Read the raw numbers as well as the score

On each tagged morning, open the Recovery screen and note Overnight HRV and Sleeping heart rate against their Recovery baselines. Compare the changes with your ordinary night-to-night range. A 3 bpm rise means more if your sleeping heart rate normally varies by 1 bpm than if it varies by 4. Check Sleep for the awake time and the Deep and REM split, knowing the watch registers stages less reliably than heart rate.

Interpret with care

A handful of nights from one person can mislead. Regression to the mean makes a very good night likely to be followed by a less good one, with or without a drink. Knowing that you drank can change behavior, such as going to bed later, even though it cannot change the sensor. If the drinking nights stand clearly apart from the alcohol-free ones in both HRV and heart rate across repeated trials, you have found your personal dose response. If they overlap, that is also a result, and it tells you that at that dose alcohol is not the main thing moving your numbers.

Whatever you find, the result describes your own recovery signals. It says nothing about the wider health effects of alcohol, which are outside the scope of this article.

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