Hi Italkian friend! How have you been? Let me introduce myself, my name is Getulio. I'm from Brazil. It's nice to meet you. Well, I happen to have some "sluggish" relatives who are always slowly, dull and tired. Thus I guess you can refer to them as "sluggish" Cheers!

"Sleepyhead" works, but it's usually only used for children. Otherwise, for adults, we'd use another structure to talk about them being tired.

Context matters for this one. As the earlier poster said, "sleepyhead" is the most literal single term for what you ask. But it has usage restrictions. It's typically used as a form of address; that is, it's more natural to CALL someone "sleepyhead" directly than to refer to someone AS a sleepyhead. And, as mentioned, it carries a silly or childish connotation. You would never say to your boss, "Lets not hire him, he's a sleepyhead." Some adjectives you might use to describe a sleepy person: Lethargic, dopey, narcoleptic, fatigued. A friend of mine, when he's sick or un-rested, will say, "I'm really dragging ass today." This sounds completely natural to a native English speaker's ear, but is highly vernacular.