---
title: "WHOOP 4.0, what the sensor tracks and what changed"
language: en
topics: ["data", "health", "heart", "review", "sensors", "sport", "watch"]
full_article_url: https://willem.com/en/2022-01-14_wearing-whoop-4-0/
---
# WHOOP 4.0, what the sensor tracks and what changed

*Smaller, waterproof charging, and more body signals.*

> WHOOP 4.0 tracks heart rate, heart rate variability, respiratory rate, skin temperature, blood oxygen and sleep. It is 33% smaller than before, can be charged in the shower with a waterproof pack, and can be worn in apparel, not just on the wrist.

WHOOP 4.0 is the sensor I wear day and night. It measures heart rate, heart rate variability, respiratory rate, skin temperature, blood oxygen saturation, and sleep duration and cycles. It uses 5 LEDs across 3 wavelengths of light and 4 photodiodes to take its readings. Against the earlier strap it is 33% smaller, adds a vibration function that can wake you silently when your body is rested, and charges from a waterproof battery pack you can leave on in the shower. It can be worn on the wrist or elsewhere using apparel such as shorts, leggings, bras and tops. It collects data all the time, so unlike Apple Watch there is nothing to start or stop for a workout.