get (a number of) hours of sleep

When you're talking about how much you slept at night, you usually use the expression "get ___ hours of sleep":

I usually get 7 or 8 hours of sleep every night.

You say "I slept for ___ hours" when you sleep for an extra long time, or when you fall asleep at a strange time of day:

I laid down after lunch and slept for 4 hours.