Sleepyhead is a warm, teasing name for someone — especially a child — who looks ready to fall asleep, or who will not get out of bed. A parent might stand in the doorway and say “Come on, sleepyhead.” It is not a doctor’s word and not a real insult if the voice is kind; it is closer to a nickname. You would not call your teacher a sleepyhead, and you almost never use it in a serious description of a stranger. Though actually, adults sometimes say it to a partner in the morning, in that soft, baby-ish way people use private nicknames. The “head” here does not mean a body part you are talking about; it is the same nickname shape as in other old teasing words that end in -head. It names the whole person.