we say that a lot, and maggie says "sleep tightly." as in, "tonight i'm going to sleep tightly in my bed all night!"
at first i said to myself, awww how cute. but then i thought about it, and i realized she's using "tightly," an adverb, instead of "tight," the adjective (stay with me dad and erik!). because "sleep" is a verb, it is technically grammatically correct to use an adverb and not an adjective. i teach high school kids who don't know this yet! of course, she's not aware that "sleep tight" is some bizarre reference to the ancient practice of using ropes strung thought a wooden frame as a mattress support (to replace the modern box spring i presume), and tightening the ropes at night to prevent sagging.
such a smart little girl!