On the first night he joined Orochimaru, sleep came easily, which came to him as both a surprise, and not. A surprise, because abandoning one's village is no light matter. Not, because he just fought a battle against his best friend with full intention of killing him.
The second night is when it really sinks in, and Sasuke finds himself restless. He lies awake on his back, staring at the ceiling, but it's so dark that he can't even see that much. When there is nothing to see, the mind likes to create its own images.
His memories ask himself why he left Kakashi, who protected him; Sakura, who loved him; Naruto, who befriended him. Anyone could see that it didn't make sense. He had a team who became family whether he intended for them to or not, and part of him wonders how he could even think about betraying them the way Itachi did to him.
He wonders why he fought so relentlessly against Naruto, even after Sakura risked her own life to stop them on the rooftop. He wonders if she would have done it again if she was at the Valley of the End.
He wonders why he stopped to see how much he had hurt Naruto, even though his mind and body didn't stop even once for the duration of the battle. He wonders if he would have stopped if Naruto was stronger, or maybe it was him who needed to be stronger.
He wonders why he left after all, even though Kakashi said he had already lost everybody he had ever cared for. He wonders if he would have stayed if Kakashi just said "I care about you, too."
Sasuke closes his eyes and wishes his thoughts away, but he knows they'll be here to haunt him for years to come.
Mostly, it's a burden, because he won't be able to focus on getting stronger. Yet, in a way, it was reassuring, because at least the guilt makes him feel a little less cruel.
What eventually put him to sleep is the fact that even if he wanted to go back, he couldn't. There was only one place that would accept him after what he had done, and he was already there.