Sleep tight
Definition 1 of 1
Pro Tip 1/3Goodnight Wish
Use 'sleep tight' to wish someone a good night in a friendly way. 
Goodnight kids, sleep tight!
Pro Tip 2/3Informal Setting
Suited for casual conversations, not for formal or business-related interactions. 
Had a great time tonight. Sleep tight, see you tomorrow!
Pro Tip 3/3Endearing Tone
Conveys warmth and care, often used towards loved ones or children. 
You've had a long day, sweetheart. Sleep tight.