sleep tight

:  the natural periodic suspension of consciousness during which the powers of the body are restored — compare REM sleep, slow-wave sleep

:  a state resembling sleep: such as

:  a state of torpid inactivity

:  to rest in a state of sleep

:  to be in a state (as of quiescence or death) resembling sleep

:  to be slumbering in

:  having elements close together

:  so close in structure as to prevent passage or escape (as of liquid, gas, or light) — compare lightproof, watertight

:  fitting very close to the body

:  fast, tightly, firmly

:  in a sound manner :  soundly