Smartwatches: HRV, SpO2, and Sleep Data

What does your watch actually measure?

Modern smartwatches and fitness bands measure an increasing number of health metrics: heart rate, HRV, blood oxygen saturation (SpO2), sleep stages, step count, and skin temperature. These devices have become accessible to millions of people and generate enormous amounts of data daily.

However, it is important to understand what these numbers mean, what their limitations are, and when it is worth seeing a doctor. Consumer devices use optical sensors (photoplethysmography) that measure blood flow changes through the skin. Their accuracy is limited -- they are suitable for tracking trends but not for medical diagnostics (Source -- Henriksen, 2019).

The best strategy is to use this data as a supplementary source of information that can help you notice changes and prompt timely consultation with a specialist. They should not replace professional medical tests.

HRV -- heart rate variability

HRV measures the variation in time intervals between heartbeats in milliseconds. This is not heart rate (pulse), but rather its irregularity -- how much the time between each beat varies. Paradoxically, greater irregularity is a good sign.

Higher HRV generally indicates better autonomic nervous system balance and the body's ability to adapt to stress. When the parasympathetic (rest) nervous system is more active -- HRV is higher; when the sympathetic (stress) system is active -- HRV is lower.

HRV is a highly individual metric. Younger people typically have higher HRV (50-100 ms RMSSD), older people -- lower (20-40 ms). Comparison with others is not informative -- what matters is tracking your personal trends over weeks and months (Source -- Shaffer, 2017).

HRV decreases with: physical or psychological stress, insufficient sleep, alcohol consumption (even a single large dose can reduce HRV for 24-48 hours), onset of illness, and overtraining. If your HRV consistently decreases for several days -- it is a signal to slow down, allow more recovery time, and check whether you are getting sick.

HRV increases with: regular physical activity, better sleep, meditation and slow breathing exercises (4-6 breaths per minute), and good hydration levels.

SpO2 -- nighttime oxygen saturation

SpO2 shows what percentage of hemoglobin is saturated with oxygen. The normal value during sleep is 95-100%. Brief drops to 92-94% can occur due to body position, deep sleep phase, or sensor displacement on the wrist.

Regular SpO2 drops below 90% at night may indicate obstructive sleep apnea (OSA) -- a condition where soft tissues of the upper airway temporarily close during sleep. Each such episode causes a brief awakening (often unnoticed) that fragments sleep. Sleep apnea increases the risk of cardiovascular disease, stroke, diabetes, and has a significant impact on quality of life (Source -- Punjabi, 2018).

If your watch consistently shows SpO2 drops below 90%, especially if you snore, feel tired during the day despite adequate sleep duration, or experience morning headaches -- discuss this with your doctor. They may order polysomnography -- a professional sleep study that will accurately diagnose the condition.

It is important to know that a wrist sensor is not as accurate as a medical finger pulse oximeter. Isolated low readings may be sensor errors -- what matters more is a recurring pattern.

Sleep stages

Sleep consists of several stages that repeat in cycles of 90-110 minutes (typically 4-6 cycles per night):

Light sleep (N1 and N2) accounts for about 50-60% of sleep. This is the transitional phase during which the body gradually relaxes. In the N2 phase, sleep spindles appear -- specific brain wave patterns important for memory consolidation.

Deep sleep (N3, slow-wave sleep) accounts for about 15-25% and is the most important phase for physical recovery. During this phase, growth hormone secretion, muscle repair, immune system strengthening, and tissue repair occur. The proportion of deep sleep naturally decreases with age -- a 50-year-old typically has less deep sleep than a 25-year-old.

REM sleep (rapid eye movement phase) accounts for about 20-25% and is important for memory consolidation, emotional processing, learning, and creativity. Dreams mostly occur during the REM phase (Source -- Haghayegh, 2019).

For healthy adults, 7-9 hours of sleep is recommended. However, quality matters as much as duration -- whether there is enough deep and REM sleep and whether there are too many awakenings.

Practical usage plan

1. Track your personal trends -- use a 7-14 day rolling average rather than daily numbers. Day-to-day fluctuations are normal. 2. Check HRV in the morning -- if it is significantly lower than your 7-day average, consider a lighter day or a recovery workout. 3. Review nighttime SpO2 reports periodically -- if you see regular drops below 90%, see your doctor. 4. Evaluate sleep structure -- if deep sleep is consistently less than 10% of total sleep, consider sleep hygiene practices: cooler bedroom temperature, limited screen time before bed, regular sleep schedule. 5. Do not rely solely on watch data -- it supplements but does not replace a doctor's consultation and professional testing.

When to act immediately

See your doctor if: the watch consistently shows SpO2 drops below 88% at night, you notice a very low resting heart rate (below 40 beats per minute) or an unusually high one (above 120 at rest), your HRV has dropped dramatically and this lasts more than a week without a clear reason, or you notice an irregular heart rhythm with possible pause or extra beat notifications -- this could be a sign of atrial fibrillation that requires medical evaluation.

This is not medical advice. Consult your doctor for health decisions.