It could also slightly have to do with that we don't have any of the day's tasks in head. Not that this is the main reason, but just one of the factors. From a

It could also slightly have to do with that we don't have any of the day's tasks in head. Not that this is the main reason, but just one of the factors. From a biological/endocrinological standpoint though, from what I understand, that cortisol levels are highest in the mornings.