Ah. I understand now. Insomnia. I just sent along an owl with some Hot Chocolate - it's a special recipe of Albus's that he gave me when I first started teaching. Drink it. It's an instant cure for insomnia.
You need more sleep when you're younger, Severus, something you found out a little too late. She has to get up early, too. She still teaches during the day. And I've never seen her awake at night before this, only you. I was curious.