It’s basically an iPhone which monitors more than your iPhone does passively, and most of them have Always On Displays. The fact that such a tiny, complicated device lasts as long as it does is a testament to Apple’s design and hardware team.

All it takes is about half an hour on charge and it’ll last me 24/30 hours. That means I can wear it to bed, to work and for the rest of the day, whack it on to charge while I do my night routine and stick it back on before bed, rinse and repeat