For many people, it may not be, I can only speak for myself. There's another thing, called 'idiopathic postprandial syndrome' which is essentially the symptoms above, but without actual low blood sugar (<60mg/dl), which some people think is another form of insulin resistance, where your blood sugar is normal but your body "wants" more sugar in the blood.
Talking with endocrinologist, they say that the insulin sensitivity for most people is much higher in the AM and daytime than at night, so it makes sense that you might have more problems in the afternoon, but you should probably talk to a doctor rather than taking my word for it!
It's often difficult (in the US at least) to get primary care and endocrinologists to take you seriously if you are not actually dying of diabetes or passing out from low blood sugar - this is where dipping into the realm of concierge medicine can be helpful, or at least, it has been for me. They are often much more willing to investigate thoroughly.
Missed this the first time around. They do this, for research, it's called a glucose clamp test.
https://en.wikipedia.org/wiki/Glucose_clamp_technique
Unfortunately it's almost impossible to find someone willing to administer it to you. It's an outpatient hospital procedure lasting a number of hours, and almost no insurance would cover it, as far as I am aware.
Responses are by readers, so take with a pinch of salt
What? That is quite a boring and obvious observation because your insulin sensitivity is much higher in the morning.
Not discounting people's experiences, but trying to suss out the science here. If you were, for example, to inject sugar directly into someone's blood stream would the result be fatigue every time?
It seems to me that there's more involved in this. In my experience (more anecdata!) I'm able to eat anything in the morning. Giant bolus of carbs and sugar, and I feel great. That same meal in the afternoon will give me such a fatigue that I need to lay down.
Clearly there's some other factor at play for me in the function whos result is fatigue.
FATIGUE_LEVEL = CARB_GRAMS * (HOUR_OF_DAY / 24)