Just gonna tell you a couple of things that happen to me. Personal experience:
1-) While ejaculating, doesn't matter how fast or deeply I breathe, I feel like I'm out of breath.
2-) After ejaculation, if I try to move fast (or run), I feel really dizzy. Even if I stand (not sit) where I am, a little bit of diziness comes into the play.
3-) After I ejaculate, the last thing I want to think about is sex. It just doesn't feel good to think about it anymore.
4-) Around 1-2 minutes after ejaculation, I turn to normal (like right before the erection), both physically and mentally. I just go straight back to my homework and start doing it (and I can concentrate to it really well.)
5-) If its later than 01:00 AM and I have just ejaculated, lying in the bed, well, I hardly find the energy to go wash my hands. I just have to sleep after that. It is like the "resting time" after doing some sports.
6-) One of the last things I want to do after ejaculation is to get out of my room, because I don't want to talk to anyone. I just want to enjoy the moment of peace and quiet.
These are probably true for most of the guys, meaning that these may be the reasons that he sleeps after sex.