Understanding HRV: What Is Normal and What It Reveals About Your Sleep

Heart rate variability is one of the most telling markers for recovery and stress. Here's what HRV actually means, why it says more than sleep duration, and how to interpret it correctly.

What is HRV?

Heart rate variability (HRV) is the variation in time intervals between consecutive heartbeats. It's not the same as heart rate. A pulse of 60 bpm doesn't mean your heart beats exactly every 1,000 milliseconds. The intervals vary: sometimes 950 ms, sometimes 1,050 ms. That variation is HRV.[1]

Higher HRV means your autonomic nervous system is flexible and can switch quickly between tension and recovery. Lower HRV indicates a higher baseline load, with less room for adaptation.[1]

What is a normal HRV?

There is no fixed threshold. HRV varies widely from person to person and declines with age. Your value also depends on which measure your device uses.

Apple Health shows SDNN

There are several measures of HRV. [1] Apple Health stores it as SDNN, the standard deviation of the intervals between normal heartbeats. [9] Many studies and some other devices use RMSSD instead, which measures the differences between successive heartbeats. [1] Both are given in milliseconds but cannot be compared directly.

Values by age and sex

The table comes from a study of 1,906 healthy adults from the Augsburg region in Germany. HRV was measured by ECG for five minutes lying down, after five to ten minutes of rest. [6] It shows the mean and standard deviation in milliseconds, rounded. The standard deviation shows how widely the values spread.

Age	SDNN women	SDNN men	RMSSD women	RMSSD men
25–34	49 ± 19	50 ± 21	43 ± 23	40 ± 20
35–44	45 ± 21	45 ± 17	35 ± 19	32 ± 17
45–54	37 ± 14	37 ± 15	26 ± 14	23 ± 11
55–64	31 ± 12	33 ± 15	21 ± 12	20 ± 11
65–74	28 ± 12	30 ± 13	19 ± 12	19 ± 11

How to read the table: 45 milliseconds of SDNN is close to the mean at 25 to 44 and 12 to 17 milliseconds above it at 55 to 74. Because the spread is large, values well below or above are not unusual in healthy people. The calculator below helps you place your value.

And at night?

The table values come from short measurements at rest during the day. Over 24 hours, HRV behaves differently: in a study with 24-hour ECG in 166 healthy people, measures that reflect vagal activity, such as RMSSD, peaked at night. Hourly SDNN, on the other hand, was lower at night and rose in the morning. With age, the night-time rise became smaller. [7] Night-time values can therefore only be compared roughly with the table.

If your HRV is low at night

A single low value says little. One well-documented trigger is alcohol: in a study of more than 4,000 Finnish employees, the recovery index derived from HRV during the first three hours of sleep fell on average by 9, 24 and 39 percentage points after low, moderate and high alcohol intake. [8] Other influences are listed below under “What affects your HRV?”. If your HRV stays clearly lower than usual for a long time or you have symptoms, talk to your doctor.

How Circadian reads your HRV

Circadian compares your HRV not with a table but with yourself. In the sleep review, the app takes the median of the HRV readings that fall within your sleep for each night. The grey band shows the middle half of your values from the previous 30 nights, once at least seven earlier nights are available. Only readings from the same source count: if you switch devices, the comparison starts again, because devices can derive HRV differently. In the energy curve, HRV takes effect from seven nights of reference values and fully from 14 nights.

Why HRV tells more than sleep duration

Sleep duration is an input. HRV is an output. You can sleep 8 hours and still be poorly recovered because you had alcohol, were stressed, or are coming down with something. Sleep duration won't show you that. HRV will.[2]

Tobaldini et al. (2013) showed that HRV during sleep reflects recovery quality: in nights with poor recovery (fragmented sleep, high sympathetic tone), HRV drops measurably, even when total sleep duration is sufficient.[2]

What affects your HRV?

HRV responds to many factors. Some lower it, others raise it.[1][3]

What lowers HRV

• Alcohol: Even moderate amounts noticeably lower overnight HRV
• Psychological stress: Chronic stress keeps sympathetic tone elevated
• Illness: Infections put load on the autonomic nervous system
• Overtraining: Too much training load without adequate recovery
• Sleep debt: Chronically insufficient sleep lowers the baseline
• Late meals: Digestion during the night disrupts recovery

What raises HRV

• Consistent sleep: Regular bedtime and wake time
• Aerobic exercise: Cardio fitness improves HRV over time
• Recovery days: Deliberate rest after intense training sessions

How to interpret HRV correctly

The most common mistake: comparing your absolute numbers with someone else's. An HRV of 40 ms can be excellent for one person and low for another.[3]

What matters is your individual baseline: the average over 7 to 14 days. Day-to-day values naturally fluctuate. Only when your HRV stays below baseline for several consecutive days does it signal insufficient recovery.[3]

Plews et al. (2013) showed that trained athletes optimized their training load through rolling HRV averages, not single readings.[3] The same principle applies to everyday life.

HRV and the energy curve

Circadian doesn't use HRV as a standalone score. Instead, your HRV feeds into the energy curve as a recovery multiplier. When your HRV is below your personal baseline, the entire energy curve shifts downward. When it's above, the level rises.

This makes HRV tangible: instead of staring at an abstract number, you see directly in your day when you're performing well and when you're not. More in our energy curve article.

What you should NOT read from HRV

HRV is not a stress score, not a health grade, and not comparable between people. Single measurements are meaningless. Only trends over days and weeks matter.[4]

Many wearables present HRV as a color-coded number: green, yellow, red. That suggests precision that doesn't exist. A low daily reading can be harmless, and a high reading doesn't necessarily mean everything is optimal.[5]

The most useful way to use HRV: as a contextual factor in a bigger picture, alongside sleep data, training load, and how you feel.

Your HRV in Context