Yes - the tired stuff is a substance called adenosine.

I used to think Garmin watches were way overpriced but then I got involved in wearable fitness monitor validation studies as part of my graduate degree lab and they were by and far the most reliable and accurate for everything we tested. I went from a Fitbit Versa to a Garmin Instinct and loved it so much that after it broke, I got their Fenix 7X. It’s literally everything you could ever want In a fitness tracking watch with smart features. A full charge lasts me nearly 20 days due to the solar charging.