The HLTH Journal·The Science
What is heart rate variability? A complete guide to HRV
A heart that changes its rhythm slightly with every beat is a healthy heart. One that beats like a metronome is a heart under stress.
HLTH Science Team· May 2026 · 5 min read
Your heart does not beat at the same speed every second. The gap between each beat changes slightly, even when you are sitting still. That variation is called heart rate variability, or HRV.
Key takeaways
- Higher HRV generally means your body is well-recovered; lower HRV is a signal it needs more rest.
- HRV declines naturally with age, so your own trend matters far more than anyone else’s number.
- Sleep, alcohol, training load, stress and illness are the four biggest drivers.
- The HLTH Band measures it overnight rather than continuously, because movement adds noise.
What HRV actually measures
Your heart rate is controlled by your autonomic nervous system, which has two sides. The sympathetic side speeds things up when you are stressed or exercising. The parasympathetic side slows things down when you are resting and recovering. HRV reflects the balance between these two systems. When recovery is winning, the gaps between your heartbeats vary more widely. When stress is winning, those gaps become more uniform. A higher HRV generally means your body is well-recovered and ready to handle what the day brings. A lower HRV is a signal that your body needs more rest. HRV also naturally declines with age, which is why tracking your own trend over time matters more than comparing yourself to others.
How HRV is measured
The most common measurement used in consumer wearables is called RMSSD, which calculates the variation between consecutive heartbeats. The higher the number in milliseconds, the more variability between beats. Wrist-worn devices, including the HLTH Band, measure HRV using optical PPG sensors that detect blood flow through the skin. The HLTH Band takes HRV readings during rest and sleep, not continuously throughout the day. This is worth understanding clearly. Movement creates signal noise that reduces accuracy, so measuring during sleep gives you the most reliable and consistent number to track. After each night, the app displays your HRV as a daily score alongside your minimum, maximum, and average values. Your score is placed within one of four bands. Low means your body is under strain from poor sleep, illness, stress, or accumulated training load, and you should prioritise rest. Normal means your recovery is on track. Good means your body has recovered well and is ready to train or take on a demanding day. Excellent means your nervous system is in strong balance, which is where consistent health habits show up in the data.
HRV is deeply personal. A number that sits in the excellent band for one person may be normal for another. What matters is your own trend over time.
How reliable is the number?
Flatt AA, Hornikel B, Esco MR. J Sci Med Sport. 2017;20(6):606-610An independent accuracy study found the HLTH Band’s overnight HRV readings produced an R² of 0.93 against the Polar H10 reference standard, meaning the trend data you see in the app reflects what is genuinely happening in your body.
What affects your HRV
HRV is sensitive to almost everything your body experiences. These are the four biggest drivers. Sleep. Poor sleep is the fastest way to drop your HRV. Even one bad night produces a measurable fall the following morning. Consistent, high-quality sleep is the single most reliable way to keep HRV elevated over time. Alcohol. Even one or two drinks significantly suppress overnight HRV. Alcohol reduces parasympathetic activity during sleep, which shows up clearly in your morning score, often before you notice you feel worse than usual. Training load. Aerobic exercise improves HRV over weeks and months, but hard training temporarily suppresses it. A lower score the morning after a tough session is normal. A consistently low HRV across several days signals your body needs more recovery time before the next hard effort. Stress and illness. Both psychological stress and physical illness activate the sympathetic nervous system and suppress HRV. Many people notice their score drops one to two days before they feel unwell. That early warning is one of the most practically useful things consistent HRV tracking reveals.
How to improve your HRV
HRV responds to lifestyle changes over weeks and months rather than overnight. The changes with the strongest evidence behind them are consistent sleep timing including weekends, reducing or cutting out alcohol, building a base of aerobic exercise at a pace where you can hold a conversation, and managing chronic stress through breathwork or reduced workload. Hydration is consistently underrated and has a more direct effect on HRV than most people expect.
