I agree and it’s not the only alert that seems counter to its purpose.
The bedtime notification haptic is similarly harsh and has actually woken me up before when I was super tired and went to bed a few minutes before my scheduled bedtime.
The low battery alert keeps the watch screen on at full brightness until you dismiss it.
There is a way to do it because I accidentally do it a couple of times a month. I charge my watch before bed and sometimes forget to unlock it after putting it back on.
When that happens my alarm on my phone goes off instead of the watch haptic alarm and it always scares the absolute shit out of me. So, make sure your watch is locked when you go to bed.