Don't say:
I 'll sleep early tonight.

Say:
I'll go to bed early tonight.

Note:
To ge to bed denotes the act of lying down on a bed in preparation for going to sleep. we can sav that a person went to bed at nine o'clock. out that he didn't sleep until eleven o'clock. Then he slept soundly. Go to sleep means to fall asleep He went to sleep while he was in the cinema.