It probably does have enough battery life if my watch is any indicator. And if it doesn't a small change to your daily routine, such as charging your watch both at night while you prepare for bed as well as in the morning while you get ready for the day, will make getting through the day + night no problem. You get most of a charge in something like 30 minutes.

Have people worked out how much various tasks and usage patterns affect battery life?

With a Series 4, I can get through the night. I suspect that a large part of that is that I keep "Wake Screen on Wrist Raise" off most of the time. Generally I only enable it when doing a workout (~30 minutes a day) and sometimes in the kitchen when using timers. Another factor is probably that I don't use it for much more than telling time, checking my activity level, checking weather, and reading the occasional text message.

If I charge to 99% or 100% during breakfast, I'm almost always above 60% at bedtime, and overnight with the Sleep++ app tracking uses under 20%.

Depends on the usage. My series 4 can easily last two days.

If it tells you that you've been waking up a lot or that your oxygen level drops precipitously every minute or so, you go see a doctor.

There are people trying to "optimize" their sleep, of course, but the real benefit is getting a large chunk of the population to notice something is wrong.

The same with the ECG. A one lead ECG isn't much, but it means a lot when you give it without any hassle or additional cost to millions of people.

I use AutoSleep on Apple Watch 4. It is good at poking me after a few days of not sleeping enough with a message like: "You slept for 5:38 last night, and have a sleep debt of 12%". That's enough of a poke to get my sleep back on track.

I've also used it to experiment a bit with how drinking at night affects my sleep. (noticeably!). I could do that with just how I feel, but the hard data is easier to correlate later.

Apple is rumoured to be working on sleep tracking. Of course it already exists by third parties: I use SleepWatch.

I thought the charging thing would be deadly for my use of it, but it is so much less of a thing than I imagined it would be. Firstly, sleep tracking uses surprisingly little battery. Often I'll see less than an 8% drop over night. Additionally I just drop it on the charger in the morning while doing my morning routines and that gets it quickly to 100%. Very occasionally I do a short burst in the evening.

I never thought it would be worth that sort of hassle, but it is a thousand fold. I send and receive messages from the watch. Answer calls. Look up things. Use the activity track. Use it to control media. It is my alarm clock. With the recent betas of 6.0 the usability has improved dramatically -- before all sorts of operations would yield "I can't do that" sorts of responses, whereas now I'm getting just brilliant results.

I was a naysayer of the thing (I strongly felt that if you couldn't go days without recharging it was a dud). The health features got me to give a shot, and I am a very strong believer now. It is an absolutely wonderful device.

I’ve been using mine for sleep tracking for years?

My routine is to charge my Apple Watch when I come back from work and put it back on before I go to sleep.

Or is there some other sleep related feature that requires a sensor other than HR that I’m not aware about?