Sleepy course 😴 Day 26

Day 26 already, that’s crazy!

You have already learned so much about sleep! That knowledge will allow you to feel at home even among biohackers. 😏

For the last bit, I saved a few more advanced topics. And this is one of them.

Heart rate variability ❤️

A couple of days ago we talked about heart rate. It’s a good indicator of immediate body reaction to activity and arousal. But it can’t tell much about added stress and tiredness. That’s where heart rate variability (HRV) comes in handy.

This metric is harder to understand, but your buddy Sleepy has an easy explanation for you.

In very simplified terms heart rate variability is the measure of how nonregular your heartbeats are.

Let’s say, your heart rate at the moment is 60 bpm. This, however, doesn’t mean there is a heartbeat per second. Here’s how it actually looks like:

Source: Welltory

Let’s look closer at this example. The difference between the first and the second beat is 1.17 seconds. The difference between the seconds and the third beats os 0.88 seconds. Using only these three beats, we need to subtract 0.88 seconds from 1.17 seconds. The variability, in this case, equals 0.29 seconds.

You may wonder what it all means.

One would think that hear working like a clock and beating evenly is a good thing. However, it’s not that simple. The greater the variability is, the better is your overall functional state. Low values, on the contrary, correlate with added stress and tiredness.

This parameter is widely used by professional athletes, but it first was applied in space travel to evaluate astronauts’ condition.

Variability measure takes into account both physical and psychological state, reflects your current readiness for work and physical exercise. It can also be used to predict the existence of cardiovascular diseases.

HRV and sleep

Speaking of sleep, HRV is helpful for us for two reasons:

Research shows that evening HRV measurement is a good predictor of the quality of subsequent sleep.
Variability is synchronized with nightly sleep cycles. During SWS variability rises drastically and at its peak may reach 130-140 ms. If in the morning you see that HRV value was high throughout the night, know that it’s a good indicator you had a restful sleep. 👍

How to measure it?

There are two main approaches to measuring HRV - SDNN and RMSSD. Both are calculated as a root mean square. The difference is in the case of the first approach it’s the root mean square of the average, and in the second approach, it’s the root mean square of successive interval differences.

Notably, the values are wildly different for those two approaches. You should now, that Apple Watch uses SDNN, and Oura ring uses RMSSD.

What is normal?

The norm depends not only on the quantifying approach but also on sex and age.

You can use this table for reference.

Measuring devices

We need a device to measure HRV during sleep. Unfortunately, not all fitness trackers that measure heart rate can also measure variability. But Apple Watch and Oura ring can do it.

To just measure HRV during the daytime you can use an excellent app Welltory. They managed to measure HRV using a phone camera and a flash. Despite the simplicity of their method, the precision is good.

On their website you can find the full list of devices, that can measure variability.

Task:

We’ll continue changing our regime. Go to bed a little bit earlier today than you did yesterday.
Obviously, only after completing the evening ritual ❤️
If you are interested in HRV and its measurement, try Welltory or buy one of heart-rate measuring gadgets.

The studies:

* Some of the information for this day is taken from the Welltory blog