I use the watch as an alarm clock and have never overslept. The watch also registers whether you are actually up. So if you tend to turn off the alarm when you are half asleep, the watch simply wakes you up again a short time later until you are effectively up.

Whether the function is light, medium or strong depends, of course, very much on the wearer's sensibilities - it certainly got me out of every deep sleep. (so far). As far as I know, the function cannot be adjusted.