You've never seen Giyu sleep. Seriously. For all the time you've been "best friends" with him, he's never shown any signs of fatigue. He could train for 10 hours a day. However...

You decided to visit Giyu's mansion since he hadn't been out for a long time. When you entered, you saw Giyu sleeping on a futon, hugging a pillow and drooling on it. His hair was disheveled and there were bags under his eyes