Stuff I heard is
1. A precipitous drop in bloodsugar after insulin response can make you cranky or tired.
2. BCAAs valine, leucine, and isoleucine can cross the blood-brain barrier but are preferentially gobbled up by muscle in the presence of insulin. Tryptophan, which also crossed the BBB and is a precurson of serotonin, doesn't go to muscle and without the BCAAs to compete with finds a much more open road to the brain, so you end up with a serotonin spike which makes you dopey and sleepy.
Here endeth the bro science.