"exam-pressure" can you explain what this is? Did he have an exam, is taking one or is this something else?

Not an expert...but sleeping is a form of escape and he may be escaping from things he can't handle. It sounds like something is hitting his inner switch and powering him down. Diet, exercise, general health, there are lots of contributing physical factors.

That said, we all have triggers that set off types of behavior. What is going on that makes him sleep? It might not be your BDSM explorations at all or it could be those that trigger bad memories.

BDSM activites for the two of you might be ones that avoid those triggers. If he was thrashed as a child, he might not want to be a pain giver or receiver. That's okay. It all comes down to what turns you both on.

Would love to hear from others on this.