Sleep and HRV

Heart rate variability (HRV)—variation in time between heartbeats—changes across sleep, but it is not a direct sleep-quality or recovery score. This page explains what Apple Watch records and Feelmo's proprietary Balance display can and cannot tell you.

Why am I still tired after a full night of sleep?

Sleep duration is only one part of sleep. Brief awakenings, sleep timing, breathing disturbances, illness, medication, and many other factors can affect how you feel. Neither HRV nor an Apple Watch reading can identify the cause of persistent tiredness. If it continues or affects daily life, consult a qualified healthcare professional.

Can I use the watch to increase deep sleep?

Deep sleep, also called stage N3, shifts naturally across the night and varies between nights. Wearable sleep stages are estimates, so trying to maximize one night's N3 number can be misleading. A steady wake time, daytime activity, and a dark, quiet sleep setting are general sleep-hygiene measures, but no app reading can guarantee more deep sleep.

How do sleep stages relate to HRV?

HRV is the variation in time between heartbeats. During one night, sleep cycles through lighter non-rapid-eye-movement (NREM) sleep, deep NREM sleep, and rapid-eye-movement (REM) sleep. Heart rate and autonomic patterns also change with stage.

At group level, deep NREM sleep often shows slower heart rate and greater influence from the parasympathetic branch, which can slow the heart, than wakefulness or REM sleep. REM sleep can be more variable and include short bursts from the sympathetic branch, which helps mobilize the body (Trinder et al., 2001; Tobaldini et al., 2013). Different HRV measures do not all change in the same direction, and the pattern is not identical in every person or night.

Figure: a simplified sleep-stage sketch. The lower line only shows that autonomic patterns can change; it does not show a measured HRV value, a fixed direction, or a normal night.

Studies and reviews link sleep stages and some sleep disorders with HRV patterns (Stein & Pu, 2012; Tobaldini et al., 2013). Those group-level relationships do not make nighttime HRV a stand-alone measure of an individual's sleep quality, recovery, or sleep disorder.

Why does context still matter at night?

During the day, movement and rapidly changing conditions can make optical wrist data difficult to compare. Large movement may be lower during parts of the night, but this alone does not make a record accurate.

Night is still not a controlled experiment. Sleep stage, awakenings, breathing, posture, alcohol, exercise, medication, illness, sensor fit, and room conditions vary. Apple Watch also decides when to create HRV samples, so their number and timing may differ each night. Nighttime measurements should not be described as identical or automatically more accurate.

What are the limits of Apple Watch and HealthKit?

Apple Watch uses an optical pulse sensor. HealthKit makes selected HRV and sleep estimates available to apps, but these data are not continuous polysomnography (PSG) — the multi-signal laboratory test used to assess sleep — and they are not a continuous clinical electrocardiogram.

Movement, sensor contact, recording length, breathing, hardware, software, missing samples, and irregular beats can affect HRV estimates. Wearable results therefore need context and should not be used alone to diagnose insomnia, sleep apnea, an arrhythmia, or another condition (Carter et al., 2026).

How does Feelmo present the data?

Feelmo uses proprietary on-device processing to summarize HRV-derived information available through Apple Watch / HealthKit and patterns in your own past into Balance (0–100). Processing and display rules are not public.

• The score is not a standard HRV metric, medical reference range, or validated measure of whole-night recovery.
• The six companions are product characters, not detected sleep, emotion, fatigue, recovery, or health states. Their display rules are not public.
• Comparing with your own baseline avoids some between-person confusion, but it does not remove changes in sleep stage, breathing, routine, health, or measurement quality.
• Several days can be more informative than one isolated value, but a longer trend still shows association rather than cause.
• The tomorrow-morning view (called “Condition Forecast” in the app) is an in-product reference, not a medical prognosis or a validated prediction of next-day condition.

For more detail, see The Balance score, Your Own Baseline, Getting to Know HRV, and Apple Watch and Measurement.

References

1. Carter JR, et al. Guidelines for rigor and reproducibility of heart rate variability within human cardiovascular research. American Journal of Physiology-Heart and Circulatory Physiology. 2026. doi:10.1152/ajpheart.00041.2026. PMID: 42495990.
2. 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.
3. Stein PK, Pu Y. Heart rate variability, sleep and sleep disorders. Sleep Medicine Reviews. 2012;16(1):47–66.
4. 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.