HRV, resting heart rate and temperature

What your overnight HRV, resting heart rate and skin temperature mean, what moves them, and why to compare them only with your own baseline.

• Evidence: Some
• Low effort
• Free
• Works: 1–3 weeks

Why it matters

HRV (heart rate variability) is the variation in time between heartbeats. Wearables usually report RMSSD, a measure of beat-to-beat change that mainly reflects the “rest and digest” (vagal) side of your nervous system [1]. WHOOP’s developer data reports HRV as RMSSD [8]. Oura measures HRV only during sleep, in 5-minute samples, and shows the average [2]. Garmin’s watch reads your wrist heart rate while you sleep [6].

Resting heart rate is your heart rate at rest; overnight, it’s usually at its lowest. Oura reports both your average and lowest overnight value, sampled every 10 minutes [3].

Temperature on a wearable is skin temperature, not core temperature. Oura measures it at the finger during sleep and shows it as a difference (+ or −) from your own baseline [4]. Apple Watch measures wrist temperature on Series 8 or later and any Ultra [7].

These numbers vary a lot between people. HRV falls with age and differs by sex, and norms from one kind of recording don’t transfer to another [1]. Oura says typical HRV runs from under 20 to over 200 milliseconds, and tells you to compare only against your own trends [2].

How to do it

1. Let your device learn your baseline. Apple’s Vitals app needs 7 nights [7]. Garmin needs about three weeks of sleep data before it shows HRV status [6]. Oura sets your temperature baseline over the first couple of weeks [4] and compares resting heart rate with roughly the last 2 months [3].
2. Wear it every night, so the baseline is built from your normal life.
3. Look at 7-day trends. Garmin’s HRV status is based on a 7-day average [6], and Oura’s HRV Balance compares 14 days with 3 months [5].
4. When a number moves, ask what changed the day before: drinks, a late meal, a hard session, a cold coming on, or your cycle.
5. Act on patterns, not single nights.

Common mistakes

• Comparing your HRV with a friend’s. A higher number doesn’t mean a healthier person [1][2].
• Panicking over one low night. Day-to-day noise is normal. In data from over 21,000 wearable users, you needed at least 5 nights out of 7 to estimate a week’s HRV variability reliably [15].
• Reading a temperature rise as illness when it’s your cycle. In 22 women wearing a ring, nightly finger temperature was about 0.3 °C (0.5 °F) higher in the luteal phase (after ovulation) than in the follicular phase (before it) [12].

Effect on your sleep score

• Alcohol: in 4,098 Finnish employees, drinking was linked to a dose-dependent drop in recovery-related HRV measures and a shift towards “fight or flight” activity in the first 3 hours of sleep, even at low doses. Compared with the same person’s nights without alcohol, heart rate was on average 1.4, 4.0 and 8.7 beats per minute higher, and RMSSD 2.0, 5.7 and 12.9 ms lower, after low, moderate and high intake (up to 0.25, 0.25–0.75 and over 0.75 g of alcohol per kg of body weight) [9].
• Hard exercise late in the day: in 14,689 WHOOP users, later and harder exercise was linked to higher overnight resting heart rate and lower HRV; sessions ending at least 4 hours before sleep showed no link [10].
• Late meals: in a 7.5-week trial in 39 adults with overweight or obesity, finishing eating at least 3 hours before sleep lowered nighttime heart rate and raised HRV compared with usual eating [11]. Oura also lists late meals, caffeine, alcohol and late exercise as things that can raise resting heart rate [5].
• Illness: in 50 people with COVID-19, wearable skin temperature rose and matched self-reported fever [13]. In another study, 26 of 32 infected people showed changes in heart rate, steps or sleep, often before symptoms [14].
• Menstrual cycle: temperature rises in the luteal phase [12]. Oura lists this phase among causes of temperature change [5].

Evidence

Rated Some. The physiology is well described [1], and large real-world datasets link alcohol and late hard exercise to worse overnight HRV and heart rate [9][10].

Limits: most of these are observational. The alcohol and exercise studies show links, not proof of cause. Two authors of the alcohol study were affiliated with Firstbeat, a heart-rate analytics company, and two authors of the exercise study work for WHOOP [9][10]. The meal trial was in middle-aged and older adults with overweight or obesity, not young healthy ones [11]. The menstrual cycle study was a 22-person pilot, and two of its authors were affiliated with Oura [12]. The illness studies were early, small COVID-19 studies [13][14]. None of this makes a wearable a medical device: Apple says its Vitals app is not intended for medical use [7].

General information for healthy adults, not medical advice. If you have a sleep problem that lasts, read when to see a doctor. Sources checked 4 October 2026. Spot a mistake? Tell us.