Alcuin intentionally slowed his breathing, but stayed intentionally awake until Aaron was asleep, listening to his thoughts.

This was the reason he'd been so disappointed that Aaron had had him leave, before. Someone's thoughts as they were falling asleep were often a gold mine. He also had some ideas for how to get more information with a conversation in the morning.

And then he did fall asleep, because he really had been through quite a lot today and was exhausted.