Why does barre/yoga/pilates make you sleepy?

Look this is just a question I have: why does an hour of certain kinds of flexibility/stretching exercises – such as barre, yoga, pilates – make you sleepy?

Obviously strenuous exercise (like running a marathon) can make you tired, but this is different – if you play it right I think you can get this sleep-inducing effect without even breaking a sweat, and it's very specifically sleepy/drowsy rather than exhausted-from-exertion.

It really feels like it's tied to the stretching or movement, somehow. I think I have this experience more often when I go to a new class after a while and have that "wow we are stretching muscles I had forgotten existed" feeling, though there's so many things going on there that I might well be misunderstanding.

I'm pretty sure I've only had this experience in group classes? Which is probably just because I push myself harder there than when practicing things alone, but you could come up with other stories about the social element I suppose.

Mythopoetically it really does feel like there are certain sensations trapped in our joints?fascia?lymphnodes?nerves, and that if you stretch and move in a novel way you can crack open a trapped tube of sleep-hormones and release it all at once, like turning your pants upside down and having a bunch of coins fall out of tiny side-pocket.

I have asked various people about this over the years, and some of those people have had theories, but I don't get the sense that there's a known answer to this. So if you have an answer, or a barmy speculation, please do let me know.