It is not that working 365 days is hard. It is the job in today's world are very different, Muti tasking, context switching and anything but simple even in the lowest rank.
This is mentally tiring and I have also wondered if it is a recent thing. My argument is that cutting cost over the past 20 - 30 years meant what used to be a 5 person's job is now a 1 to 2 person job.