Translation:Schlaf dich aus!
I think ausschlafen means sleep as much as you need, The Englsh translation does not make any sense.
It's an extra verb: schlafen is "to sleep", but ausschlafen is "to sleep in".
(There is also einschlafen which is "to fall asleep")
Mmh it can be by itself, but in this sentence it comes from the verb. ausschlafen is a separable verb which makes the prefix "aus" split off and go to the end of the sentence. It seems complicated at first, but you will get the hang of it I'm sure. :-)
Just think it as the separable prefix in this case and not the stand-alone definition.