The darkness inside of him.
He's awake, watching the light of the lamp play on the ceiling. He's trying not to think about anything, just letting his mind stay blank for a moment. As a demigod his thoughts were usually always consumed by death. He has good things in his life, yes, but sometimes all of the bad things that have happened to him just pile up and it's all he can see. Annabeth is usually the one that distracts him away from this thoughts, but in the middle of the night she can't do anything to help him. He closes his eyes and is instantly taken by sleep.