If you wear a fitness tracker, there is a good chance it shows you a number called HRV each morning. It moves up and down in ways that can feel random, and most apps say little about what it means. Here is what that number measures, why it changes, and how to read your own.

Your heart does not beat like a clock

Take your pulse and the rhythm feels perfectly steady. It is not. The gap between one beat and the next changes constantly, by a few milliseconds each time. Heart rate variability, or HRV, is a measure of how much it changes.

You can feel this yourself. Find your pulse and breathe slowly. As you breathe in, your heart speeds up slightly. As you breathe out, it slows down. That rise and fall is your nervous system adjusting your heart beat by beat, and it is the clearest example of HRV at work.

Why more variation is a good sign

Your nervous system has two branches that pull your heart in opposite directions. One prepares you for action and pushes your heart rate up. The other, working mainly through the vagus nerve, slows it down and helps you rest and recover.

When you are rested, the calming branch has the upper hand and keeps making small adjustments, so the gaps between beats vary more. HRV is high.

When your body is dealing with something, the action branch takes over. It holds your heart at a steadier, faster rhythm so you stay ready to respond. The gaps even out and HRV falls. Across many studies, periods of psychological stress are consistently linked with lower HRV.

What lowers it

Most things that lower HRV give your body extra work to do overnight, and you will often see the effect in the next morning's reading.

Alcohol. Your body spends the night processing it, which keeps your heart rate raised. Even a couple of drinks often shows up as a lower number.

Hard training. After a demanding workout, your muscles are repairing, which takes energy and keeps your system working while you sleep.

Short or broken sleep. Sleep is when the calming branch of the nervous system is usually most active, so less of it means less time to reset.

Illness. HRV can fall while your body fights off an infection, sometimes before you notice symptoms.

A stressful stretch. Ongoing worry keeps your body on alert, even while you sleep.

How to read your own numbers

Most wearables measure HRV while you sleep, because that is when nothing else, such as moving, talking or coffee, is affecting your heart. The number appears in milliseconds. A reading of 45 ms means the gap between your beats changes by roughly 45 milliseconds from one beat to the next.

Resist comparing your number with anyone else's. Two healthy people of the same age can be tens of milliseconds apart, and HRV naturally declines as we get older. The only useful comparison is with yourself, over weeks rather than days.

Say your HRV usually sits around 50 ms.

A 40 the morning after a late night out is your body processing alcohol and lost sleep. It should recover within a day or two.

Three or four nights in the 30s with no obvious cause suggests your body is busy with something you may not have noticed yet, such as a cold coming on or stress building up. That is the moment to go to bed earlier and ease off hard training.

A reading above your usual range usually follows a good night's sleep or a rest day. Your body had what it needed to recover.

If your device shows a normal range or baseline, that band is your usual. The readings worth paying attention to are the ones that fall below it and stay there.

Measure the same way every time. Overnight readings are the most reliable. Look at the weekly average rather than reacting to a single morning.

Try slow breathing. Breathing at around six breaths a minute, roughly five seconds in and five seconds out, makes the natural rise and fall in your heart rate larger. It is the basis of HRV biofeedback practices.

Protect your sleep. Regular bed and wake times give your nervous system its best chance to recover overnight.

Notice what alcohol does. Check your reading the morning after a drink or two. For many people it is the clearest cause and effect their HRV shows.

Balance effort with rest. Training, long workdays and late nights all add to what your body has to recover from overnight. When your HRV trends down for several days, it is often the first sign that the balance has tipped.