Sleep and HRV

There's a clear reason why Feelmo's Balance score is born mainly from your heartbeat during sleep. On this page, we walk carefully through the relationship between sleep and HRV (heart rate variability), with references, and share how Feelmo translates that night's data into "balance."

There's nothing complicated to do. By the time you finish reading, it should make intuitive sense why simply wearing it and sleeping at night delivers a quiet clue the next morning.

Sleep is not constant

A single night of sleep cycles repeatedly through light sleep, deep sleep (slow-wave sleep, N3), and REM sleep. The activity of the autonomic nervous system slowly shifts along with these stages, and it has been shown that during deep non-REM sleep the parasympathetic nervous system (the side that lets the body rest) becomes dominant (Trinder et al., 2001).

Figure: top = the sleep stages across one night (hypnogram); bottom = an illustration of parasympathetic activity / HRV. HRV rises during deep sleep (N3) and approaches waking levels during REM sleep.

As the figure shows, even within a single night the autonomic nervous system slowly travels back and forth between resting, regulating, and dreaming. Reviews have organized how HRV during sleep relates to sleep stages, sleep quality, sleep-disordered breathing, and more (Stein & Pu, 2012; Tobaldini et al., 2013). That is exactly why nighttime HRV becomes a gentle mirror of how well your body was able to rest that night.

Why we measure at night

During the day, HRV sways from moment to moment with many factors — exercise, meals, posture, caffeine, conversation, and more. This is not abnormal; it's proof that you're alive and moving. But when the swaying conditions stay scattered, it becomes hard to fairly compare yesterday's you with today's you.

Sleep is the time when these disturbances are at their fewest. The body lies down, activity settles into a uniform state, and the conditions line up nearly the same every night. In other words, night is the quietest measurement window, where each day can be compared with the same ruler. That's why Feelmo places nighttime data at the center of the Balance score.

• During REM sleep, autonomic activity approaches waking levels, while during deep non-REM sleep it settles down — and this back-and-forth difference softly reflects how recovery is going (Trinder et al., 2001; Tobaldini et al., 2013)
• Because a disrupted night tends to show up in HRV as well, the Balance score can also serve as a gentle clue about "sleep quality" (Stein & Pu, 2012)

Measuring at the wrist does have its weak points. The wrist is somewhat susceptible to body movement (rolling over or arm motion), so Feelmo values measurements taken while you're asleep — when readings are stable — or while you're sitting quietly. For more, see Apple Watch and Measurement.

How Feelmo handles it

Feelmo translates the fluctuations in your heartbeat during sleep (HRV) into a Balance score (0–100). This is a single indicator, computed by the shared platform Lumo Core, that smoothly expresses how well your autonomic nervous system is balanced.

What it is built from

The Balance score summarizes multiple HRV-derived clues, measurement quality, and movement from your own baseline. The exact internal recipe is not public.

From these clues, Feelmo sums up "how calmly you were able to recover across the whole night" into a single score. As for the final formula — how multiple indicators are integrated and smoothed onto the 0–100 scale — we don't disclose every detail, as it is the core of the shared Lumo Core platform.

How to read it

• Take it loosely as low = swaying / high = balanced.
• The most important thing is to read it not against other people, but against your own baseline (your usual self). Your 60 and someone else's 60 do not mean the same thing.
• There's no need to be elated or downcast over the number itself. Rather than turning the score into words of judgment, Feelmo translates it into the expressions of your six companions (Genki, Odayaka, Futsuu, Fuan, Panku, and Tsukare) and places gentle words of observation beside you.
• The flow over several days to several weeks tells you far more than a single night's value. With Premium, you can also see long-term trends and a forecast of tomorrow's condition.

For more on how the Balance score itself works, see The Balance score; for the thinking behind baselines, see Your Own Baseline; and for the basics of HRV, see Getting to Know HRV. For daytime records, please see Well-being and Habits.

References

1. Trinder J, Kleiman J, Carrington M, et al. Autonomic activity during human sleep as a function of time and sleep stage. Journal of Sleep Research. 2001;10(4):253–264.
2. Stein PK, Pu Y. Heart rate variability, sleep and sleep disorders. Sleep Medicine Reviews. 2012;16(1):47–66.
3. Tobaldini E, Nobili L, Strada S, Casali KR, Braghiroli A, Montano N. Heart rate variability in normal and pathological sleep. Frontiers in Physiology. 2013;4:294.