On a similar note, something I've observed is that the feeling of tiredness in the afternoon doesn't necessarily correlate with how much I've done in the hours prior.

Recently I started doing more things in the morning hours, with the hope that I'd be more effective at those tasks (especially the more mentally taxing ones) earlier in the day than when I had been doing them previously, which was typically in the afternoon or evening.

Intuitively, this would lead to better productivity in the earlier half of the day at the cost of reducing it in the latter half. Instead, my afternoon and evening energy levels remained unchanged or even improved a bit, meaning that I now get more done with no apparent cost. This is of course welcome but it was quite unexpected, and suggests that some amount of the "tired" feeling is more a result of conditioning than energy expenditure.