[koliber]

This kind of correlates with a wisdom I've heard before:

You don't suffer from a lack of time, but rather a lack of energy. And actually, you don't suffer from a lack of energy, but a lack of activation energy.

We colloquially refer to activation energy as motivation. But maybe that's not the whole story. Some of it is willpower and personality, but maybe some of it is a buildup of adenosine.

This squares with my personal experience. It's harder to start things after day's worth of intense mental effort.