i have, plugged into the outlet at the foot of my bed, a strangelooking device which looks rather like a baby monitor. it emits ultrasonic sound waves to repel pests. gives them a headache, i suppose.
at any rate, i can hear it. especially when i'm trying to fall asleep.

that, and the neighbors yelling at their dog.
and my housemate's tv.
and the toilet leaking in the bathroom.
and the sirens.
and the front door opening. and closing.
and opening again.
and closing.
again.
and then, only a very few hours after all those noises stop,
my alarm clock.