Someone who sleeps late is often called a "sleepy head". For example - If a person came to breakfast late, others might comment, "You're a real sleepy head this morning".

I would assume the answer would be "late sleeper", that would be the opposite of an "early-riser" in my opinion.

A "night owl" is someone who stays up late into the night. If you're asking for the opposite as in someone who sleeps late, I'm not so sure of a common expression for that. Maybe "bed-head" or "bed bug"...?