Hi firmspear, and welcome BTW,
I guess the body does a lot of maintenance at night, of all types. (Nothing else to do
Repairs damage, and I guess in that process it would be "taking out the garbage". I think that's why they ask for urine test in the morning. But I guess one would have to do a specific investigation to see when it handles semen disposal.
Because of my vasectomy, I know that unused semen is absorbed processed and disposed of, via the liver/kidneys I believe. But the details of how and when, I don't think we've talked about much here.
If you become so inclined more information could be useful in understanding the complete process. If you find anything, let us know.