Not on its own. A low overnight heart rate variability (HRV) cannot tell you that you have sleep apnea, the condition where breathing stops and starts during sleep. People with severe apnea do have lower HRV on average: in a study of 1,247 people, RMSSD RMSSD Root mean square of successive differences: an HRV metric built from how much each heartbeat interval differs from the next, in milliseconds. It mainly reflects vagal activity. More about RMSSD measured while awake was 21.5 ms with severe apnea against 27.9 ms in people without apnea (Qin and colleagues, 2021). But those are group averages with a large overlap, and alcohol, age, illness and a hard day lower HRV in the same way.
What apnea does to HRV
Each time breathing stops, oxygen falls, the brain briefly rouses, and the “ fight or flight Sympathetic nervous system The "fight or flight" branch of the autonomic nervous system. It speeds up the heart and makes it pump harder, but it acts too slowly to drive beat-to-beat HRV. More about Sympathetic nervous system ” side of the nervous system fires. Heart rate swings, blood pressure rises and the calming vagal signal is pushed aside. Researchers have used these swings in the heartbeat to study apnea for years, and a 2012 review lists them among the main uses of HRV in sleep research (Stein and Pu, 2012).
The 2021 study puts numbers on it. The researchers recorded 5 minutes of heartbeats while people lay awake before a sleep study, in 426 people without apnea and 821 with it. Time-domain measures such as SDNN SDNN The standard deviation of all NN intervals in a recording, in milliseconds. It measures overall variability, and its value depends strongly on how long the recording lasts. More about SDNN and RMSSD fell as apnea got more severe, and the heartbeat pattern became less complex. Frequency measures such as LF/HF LF/HF ratio LF power divided by HF power, with no unit. It was proposed as a gauge of balance between the sympathetic and parasympathetic systems, but research shows it cannot measure that reliably. More about LF/HF ratio did not differ overall; they only shifted toward the sympathetic side in people who were also obese. The authors suggested HRV could add information about the heart in people who already have apnea, not that it could find apnea in the general population.
Why your number cannot diagnose it
- Overlap. A 6 ms difference between group averages is smaller than the spread within each group. Plenty of people with apnea have ordinary HRV, and plenty without it sit at 20 ms. See what a low HRV means.
- Many causes. Alcohol, illness, short sleep, a hard session and age all pull overnight HRV down. The number does not say which.
- Different recordings. The study measured people awake in a clinic. Your ring or watch estimates HRV from a light sensor during sleep. The cut-offs do not carry over.
Watches that screen for apnea do not use your HRV number. Apple’s sleep apnea notifications, for example, use the accelerometer to look for breathing disturbances while you sleep, analyse the data every 30 days, and notify you only if it consistently shows “Elevated” breathing disturbances. You need to wear the watch to bed for at least 10 nights in that 30-day period. Apple says the feature is not intended to diagnose, treat or help manage sleep apnea and that not everyone with apnea will get a notification (Apple Support). A 2024 review of 38 studies of wearable algorithms found pooled sensitivity for apnea events of about 79% and specificity of about 95%, and concluded the performance was not yet good enough for routine clinical use (Abd-Alrazaq and colleagues, 2024).
Does CPAP raise HRV?
CPAP, a mask that keeps the airway open with gentle air pressure, is the standard treatment for moderate to severe apnea. A 2025 review that gathered the HRV studies in sleep apnea found that CPAP tended to raise vagal measures such as RMSSD and lower LF/HF, in studies of 52 to 62 people, with the clearest changes in severe apnea and a slower response in smokers (Zabara-Antal and colleagues, 2025). The review reports no pooled effect size. Some changes appeared after a single night of CPAP and others over one to two months, and in smokers the response was slower and not complete. So a rise is plausible, but not guaranteed, and the studies are too small to say how big.
If you start CPAP, note the date and compare weekly averages before and after. A better night’s breathing may also show up in resting heart rate Resting heart rate How many times your heart beats per minute while you are fully at rest. For adults it is usually 60 to 100 beats per minute, and lower in well-trained athletes. More about Resting heart rate and breathing rate Breathing rate How many breaths you take a minute. It moves HRV on its own: slower, deeper breathing makes the beat-to-beat swing larger, and below about 9 breaths a minute it shifts from the HF band into LF. More about Breathing rate , which your device may track. See HRV vs resting heart rate.
When to talk to a doctor
Go by symptoms, not the number. Talk to a doctor about a sleep test if you snore loudly, if a partner has seen you stop breathing, if you wake up gasping or with a dry mouth and headache, or if you feel very sleepy during the day after a full night in bed. Do the same if you have high blood pressure that is hard to control. A normal HRV does not rule apnea out, and a low one does not rule it in. More on how sleep shapes your number is on HRV and sleep.