Transformation.
After Eliana proposed going on a vacation, Sebastian didn't have any tangible reason to resist it so he simply went with the flow.
He actually needed the vacation more than she even needed.
After spending so much time and effort into intensive research on his 3 priority experiments in preparation for the creation of his ultimate vessel, and adding his subsequent adventure in Manapedia, Sebastian was mentally tired.
Though he was not in a coma at the moment, Sebastian knew that he needed to rest. If he continued like this, all the stress could easily accumulate and turn into something that he would not be able to curb with his methods.
Besides, what was the reason why he worked so hard? He worked so hard to fulfill his goal to make his ultimate vessel as strong as possible.
After his adventure in Manapedia, he indirectly fulfilled a significant part of his goal in a way that he did not expect.