By lowering brain temperature and maintaining thermal homeostasis, the thermoregulatory theory suggests that instead of prompting sleep, yawning actually serves to maintain focus and attention, thereby antagonizing sleep.
So not concrete but it is one of the prevailing theories for one of the reasons we yawn
there are quite a lot of articles about yawning cooling the brain. in this princeton research they observed how much people yawn outdoors in the winter vs summer, and it was more common in winter. personally i don't quite believe this, because aren't we more tired in the winter anyway? https://www.princeton.edu/news/2011/09/19/more-sign-sleepiness-yawning-may-cool-brain