What does it mean that Paul and his co-workers worked night and day?

The phrase night and day, though it obviously emphasizes how hard Paul worked, does not indicate directly how long his hours were. Neither does the placement of night before day necessarily imply that Paul began working before sunrise. This Greek construction night and day means during the night and during the day,¹ thus showing how Paul used any moment available to him to ply his trade. He did not simply say, as the sun went down, That's enough work for one day; let's now take our rest. Truly, he was willing to give his whole being for the Thessalonian believers!