When it has week long battery life!
It supports silent alarms (and of course has the battery life to avoid charging every night).
Got the watch ordered but still need to use the fitness band just for this feature.
I'm pretty sure it already supports silent alarms, you'll just need to be wearing it (of course).
I've been thinking about this, and I, too, would love to have week long battery life to support both silent alarms and sleep tracking... However, I don't foresee this ever happening. Battery life in iPad/iPhone has mostly been stable through it's existance (iPhone has gone up some, but for me, even with the 6+ I've never been able to comfortably go for more than a day without charging). As the watch progresses, it'll get more power efficient sure, but it will also get thinner and add capabilities (GPS?) to drain power.
Without some battery revolution, I can't imagine the watch being able to sustain so much as 2 full days of use (under a regular load, on a consistant, comfortable basis) for many years.
I think the much more likely scenario would be charging wirelessly (like witricity) where I can be sleeping in bed with the watch on my wrist as it charges. Sure this will slow the charging process, but if the watch takes 2 hours now, I can imagine wireless charging for 6 hours would provide approx a full charge. Another added perk would be that the iPhone would (almost surely) add this capability as well. Although, I suppose recharge time would be more important with iPhone