---
title: "Sleep tracking with a wrist wearable vs a chest strap"
language: en
topics: ["apple", "data", "health", "heart", "sensors", "watch"]
full_article_url: https://willem.com/en/2019-07-16_limitations-of-sleep-tracking-using-a-wearable/
---
# Sleep tracking with a wrist wearable vs a chest strap

*Why optical sensors miss heart rate spikes at night.*

> In my 2019 test a wrist wearable sampling heart rate every 5 minutes measured a nightly maximum of 69 BPM, while a chest strap recording continuously caught spikes up to 111 BPM. Wrist data is incomplete.

Optical heart rate sensors need LEDs, a photo sensor and processing, which costs power. Because wearable batteries are small, heart rate during sleep is often sampled only once every 5 or even 10 minutes: 6 to 12 measurements per hour, while at 60 BPM your heart beats 3600 times an hour.
A chest strap measures the heart's electrical activity directly and captures multiple accurate readings per second. I wore both for a night: a Biostrap on my wrist at a 5 minute interval, the same interval Apple Watch used, plus a chest strap.
Both agreed on average and minimum heart rate, but the strap showed short spikes the optical sensor missed, probably from dreaming, turning over or briefly waking. Gyroscope and microphone cannot fully make up for that, so I question whether wrist based sleep tracking is worth the effort.