Don't go back to sleep!

When you lack sleep, you are apt to be careless.

Lack of sleep is telling me.

A few minutes after he finished his work, he went to bed.

He was just on the verge of going to sleep.

Tom is always sleeping in class. It almost seems like he goes to school just to catch up on his sleep.

I don't get enough sleep.

Because of my lack of sleep I'm going to bed early tonight.

Even when I set an alarm clock before sleeping, I still oversleep until I'm late.