I remember going to conferences,especially the really small ones, the professors and students would spend all day listening to extremely draining lectures; this one particular conference started at 8.30 am, went till 1 pm, you get a few hours break, and then it goes till 9 pm.
I'd go back to my room in the noon time to take a nap, and in the evening maybe go to the bar for a small bit to get a drink and go back to the room to get ready for the next day, but the majority of professors would spend most of the midday break meeting each other, and also continue socializing in the bar fairly late into night, and they'll still come back the next day as if this is a regular day.
I had a talk with my professor on how they're doing it and his response was just "you just have to train yourself to do it and get used to it." Probably one of the main reasons why I chose not to continue down that path!