What is HRV in sleep health?

The full name of HRV, which is often talked about in sleep scenes, is heart rate variability. To put it bluntly, it is the slight fluctuation in the time interval between two consecutive heartbeats - it is not the average heart rate we usually talk about how many times it beats per minute. The difference is the subtle changes in tens of milliseconds.

Don’t underestimate this slight fluctuation. It is actually an “invisible status report” given to you by your heart, which reflects the activity of your autonomic nerves: that is, the nerves that control your heartbeat, breathing, and digestion, which are physiological activities that do not require your deliberate control. They are divided into sympathetic nerves that make you nervous and excited, and parasympathetic nerves that help you relax and rest. When we fall asleep, the parasympathetic nervous system is supposed to be dominant, and the body is completely relaxed and repaired. At this time, the HRV will be in a reasonable range that can flexibly fluctuate. If the HRV stays at a very low level all night, even if your wristband shows that you have been in deep sleep for a long time, it is most likely that you have had a "fake sleep" and your body has not really rested.

Last week, a girl who worked in an e-commerce operation came to me to adjust her sleep. She said that she could sleep for 7 hours a day, but she was still too sleepy to open her eyes during the day. I pulled her wearable device data for a month in the background. All other indicators were normal, except that the HRV during sleep was 40% lower than the average baseline for the same age group. After questioning, I found out that she had to check the real-time data in the store's background for half an hour before going to bed every day. Her nerves were tight. Even if she lay down and fell asleep, she was still subconsciously concerned about performance, and her autonomic nervous system had not switched to rest mode at all.

It's a bit like listening to a professional singer singing. You don't just count how many words he sings per minute. You have to listen to whether the breathing and pauses between each sentence are flexible and natural. If the intervals between each sentence are as stiff as a stopwatch, it must be that you are too nervous to perform abnormally. The same is true for the beating rhythm of the heart.

However, there are now different opinions on the reference value of sleep HRV in the industry. Some researchers who are engaged in chronic disease prevention believe that long-term sleep HRV continues to be low, which can provide an early warning of fatigue accumulation and reduced immunity 1-2 months in advance, and can even assist in the early diagnosis of sleep apnea.； However, many clinicians feel that the accuracy of HRV measured by ordinary users using consumer-grade wristbands is limited. Occasional fluctuations in the value during the day may be caused by drinking half a cup of milk tea before going to bed or getting up late at night. There is no need to worry about the value of a certain day alone.

When I provide sleep guidance to users, I usually don't focus on the HRV value of a single day, but look at the overall trend for at least two weeks. If you find that your sleep HRV is lower than your normal level for more than half a month, and you still can't wake up and have no energy during the day, you can just adjust your work and rest or find a doctor to troubleshoot the problem. There is no need to dwell on the HRV value for half a day every day when you wake up.