The day becomes more ragged as it moves from morning to night.
The first time I came across this idea was in an introduction to the I Ching. (I was an adolescent and stupid with it.)
I can remember a Chinese philosopher was quoted as saying something like:
"A man wakes up in the morning full of energy and good intentions, but there is only tiredness, irritation and failure by the end of the day."
(Being a teenager I harumphed at that and threw the three coins to tell the future. I eventually woke up and realised that the I Ching is totally useless at predicting the future or being a guide to future actions. At most it can be used as a seed for lateral thinking.)
The next time I remember coming across the same sort of idea was when reading "The Evolution of Consciousness" by R Ornstein.
He asked the question of why we can't keep to diets, or stop drinking or smoking. His answer was that we are not one unity, we are many people all mixed up in the same body, and the person who swears to go on a diet is not the same as the person who overeats.
And finally, a few years ago, I read The Origin of Everyday Moods by R. Thayer.
It is not well known, but I think it is a brilliant theoretical and practical book. Very very simplistically there are morning people and evening people (not his words) and he has a technique where you note down how you are feeling at different times over several days. You come to know when you have the most energy, when you are most tired and so on. For me the day becomes more ragged as it moves from morning to night. I'd better do the important things in the morning. And the routine grunt work (tax returns, bills, timekeeping etc.) should be done in the afternoon.
Thayer has invented this simplification of mood which is very clever:
At every point during your waking life you are somewhere in the space shown above.
Calm Tiredness comes when you have been working hard, but have had good results. You can relax, satisified with a good day's work done.
Calm Energy comes as you are working well and feel a little stretched but have the ability to complete the job.
Tense Energy is when you are working hard, maybe against an almost impossible deadline, but you have the energy to confront the task.
Tense Tiredness is the one to avoid. If you are tense and you are tired you are not going to be able to fly that passenger plane well.
To be honest I try to avoid Tense Energy too, I like the Calm side of the map.