To Sleep Immediately Since You are Tired

The Following three expressions all mean to go to sleep very deeply and quickly since you are very tired.

FLAKE OUT
I feel fine in the mornings, but in the afternoons I just flake out at my desk.

GO OUT LIKE A LIGHT
Anita was so tired that she went out like a light as soon as she went to bed.

AS SOON AS YOUR HEAD HITS THE PILLOW
As soon as Jack's head hit the pillow, he fell into a deep sleep.