This is actually one of the most important uses I have right now for the Garmin vivosmart HR+. Sleep app is very useful to me. But the Garmin (if you do not use th GPS, and I rarely do), will last 4 or 5 days on a charge. But the Apple watch alas still has to be charged almost daily, so I've held off on buying it. The Garmin is super ugly, but I don't really care, until Apple improves the duration one can go without a charge.
I just would like to note with you that the Apple Watch actually really works pretty well for this (and in a moment I’ll even argue that the worse battery life could be considered a positive against these longer lasting battery devices. I know, don’t get too antsy!)
So on my series 1, I’ve found I only require approx 60 minutes of daily charging, because it charges so quickly. So I generally split this up to about 30 minutes while getting ready for bed and again for about 30 minutes in the morning. Works really well for me. (Note: charging slows above 80% so I generally only charge up to 80. My watch almost always stays roughly in the 40-80% ballpark which is actually ideal for increasing battery longevity). Apparently series 2 and 3 have improved battery life, so it should require even less charging. So my point is it’s surprisingly do-able, and certainly not something I would worry too much about.
“But you said the worse battery life could be a positive! That’s nuts!” I agree it sounds weird, but follow me here. My gf has a Garmin Forerunner and only has to charge it every 5-7 days. What this results in is just forgetting to charge it altogether. Sure enough randomly every week there’s a “oh... my watch is dead” and then comes the search for the charger. Is it at my place or hers? Where did she put it exactly? And often it ends up staying drained for a few days as she just sets it aside and forgets about it. I’ve heard of similar stories from other people I know. The point is when it isn’t part of your daily routine you tend to forget about things and misplace things. If you just remember to charge your watch while getting ready for work/school in the morning you’ll never (or rarely) forget or misplace your charger. And I’ve found that the battery life is good enough that even if I forget to charge it once, there is enough battery left to get me to my next pre-determined charge time (like I said morning and night on a series 1)