He'd completely forgotten about Caleb's dome. Yussa has been reluctant to cast too much in here, but the dome is a reasonable protection and so he nods when Caleb offers it. He wants to sleep. He doesn't think he'll need very long, but the dome will ensure nothing gets to them in the meantime and it will allow Caleb to fully rest as well.

Yussa stays awake long enough to be sure that Caleb has no trouble casting.

Once the dome is up, however, the archmage allows himself to drift off, giving in to the exhaustion pulling at him - also seemingly more potent here in the Feywild, or perhaps just a testament to how thoroughly Caleb Widogast can wear him out.