Why does sema make me SO tired??

Taking the question as asked, rather than the general version of it. Take it one variable at a time. Almost every unanswerable question in these threads is unanswerable because three things changed in the same fortnight, and no amount of subsequent argument can untangle them after the fact.

If somebody has the primary source to hand I would rather cite it than paraphrase it.

LarryQC_SD said:

Almost every unanswerable question in these threads is unanswerable because three things changed in the same fortnight, and no amount of subsequent…

Agreed, and the size of the effect matters as much as its existence. Something real and small gets treated here as though it were real and decisive.

SleepFixSam said:

I am tired in a way that does not respond to sleep, which is different from the tiredness I had before any of this started.

Same pattern here, and in the same order.

Clinical perspective, offered as context rather than as advice. The honest answer is that the effect is real, the magnitude is contested, and the individual variation is larger than either. Those three things can all be true at once, and most arguments here are two people holding different parts of that.