From Longman Dictionary of Contemporary Englishsleep in phrasal verb informalWAKE UP/GET UPto let yourself sleep later than usual in the morning We usually sleep in on Sunday mornings. → sleep→ See Verb table
Examples from the Corpus
sleep in• Where are Diane and Mike? Sleeping in again, huh?• She doesn't even like sleeping in, even on Sundays.• I think I'll go to bed late tonight and sleep in tomorrow. 