Sleep like a baby
To sleep very soundly and peacefully.

Catch some Z's
To get some sleep.

Burn the midnight oil
To stay up late working or studying.

In the land of nod
To be asleep.

Hit the hay
To go to bed.

Sleep on it
To think about something before making a decision.

Sound asleep
Deeply asleep.

A cat nap
A short sleep, especially during the day.

Sleep tight
A wish for someone to sleep well.

Out like a light
To fall asleep suddenly and deeply.

Sleep like a log
To sleep very deeply and heavily.

Not sleep a wink
To not sleep at all.

Sleep through the night
To sleep for an entire night without waking up.

Dreamland
A state of sleep where one dreams.

Sleep it off
To recover from something (like a hangover) by sleeping.

Drowsy
Feeling sleepy or lethargic.

Catch 40 winks
To take a short nap.

Sleep like a stone
To sleep very deeply and undisturbed.

In your dreams
Something that is unlikely to happen.

Sleep off
To get rid of something (like tiredness) by sleeping.