[OOC: Sorry that it took so long for me to reply to this.]
Vader, hearing her words, allowed himself a moment of rest. He could already feel that his usually iron control was no longer needed here. To feel whole again, even if it was only in this small part, was... great, really great.
He knew that the medic was beginning to tire, that in a short time she would be beyong the moment where mistakes begin to happen. To ensure his continure safety, as he tried to tell himself as the only reason, he began an old Jedi practice used in these situations. He took what fatique he could find in her into himself.
After all, nobody would think any worse of a surgery patient to be in need for some sleep.