How does the main character of Midnight Mass, a vampire, gain the ability to operate in the daytime

This is not my question, actually, but the fellow here didn't follow up when I suggested he go to this Stack Exchange and now I'm a bit curious. I have his question stated verbatim below:

So I no longer own a copy of Midnight Mass by F. Paul Wilson (author of the Repairman Jack novels, which I haven't read) and I really don't want to go out and buy a copy just to trope something I vaguely remember, but I was hoping maybe someone here owns it or has read it and has a better memory than I.

I doubt anyone would sustain an actual discussion about the book, as obscure as it is, so here's my question:

The main character gets turned into a vampire. Then something happens to him that makes him able to stay conscious later into the morning, and wake up earlier in the evening, than other vampires, giving him an advantage over them. He has all of their abilities and strengths, but he can be awake longer than them, allowing him to kill them while they're vulnerable.

The problem is I can't remember what it was that gave him this ability. I know it involved him being in the sun, but it was not just that.

I would really appreciate it if anyone knows.

I have a copy of the book requested from my library, but I also don't have a huge amount of time to read additional books at the moment, so I don't know if I'll be able to get him an answer.