In other words, the Mute switch silences sounds that the user didn’t ask for, but not those that the user explicitly requested.
Marco is dead on. Where do you put the iPhone while sleeping? In another room? In the same room? Even next to your bed?
I usually mute the phone while sleeping. Imagine what a bad design mistake it would be if the alarm would’t wake me up because I don’t want to be disturbed while sleeping.