From Longman Dictionary of Contemporary Englishheavy sleeperheavy sleeperSLEEPsomeone who does not wake easily → heavy
Examples from the Corpus
heavy sleeper• Some people who sleep next to snorers are able to tune out the sound, if they are heavy sleepers.• Mrs Fairfax is a heavy sleeper, like most old people, and didn't hear anything.• Or else she had become a heavy sleeper over the years. 