I think thats a really difficult question to answer. There is an answer obviously, but all horses are so individual, and the circumstances can vary so much.
I believe at the end of the day it all boils down to the relationship you have with your horse and how you deal with the problem. I don't think napping should be confused with spooking, though it can be an after effect of mishandled spooking.
I also don't agree with the opinion that on some days your horse has just got out of the side of the bed - that is just condoning lack of respect.
I think most napping problems stem from either a break down in respect and trust between horse and rider or during establishing confidence in a new relationship, or a change in home.
The cure is to either re-instate or establish the horses respect and confidence in you.
When my youngster napped, I went back to ground work, long lining and lunging, restored willingness and obedience on the ground. I never disciplined her, just asked questions, ignored the bad behaviour and praised the good. The first couple of days were pretty horrendous, she had definitely got too big for boots, but when she realised that regardless of what she did, it was a wasted effort - I would still keep asking, the penny soon dropped.
When I started riding her again I got a friend to walk out with me, partially to give Breeze a lead, but also for me - a bit of a security blanket. After a couple of rides we went solo and we have never looked back. I always ride with a schooling whip, I use my voice alot, I only ever ask with my legs twice, quickly followed by a whack of the whip across my own leg - then if need be I give her a smack, but that is very rare. She will now go anywhere, through mud, puddles, herds of cows, on the road - I'd honestly trust and take her anywhere.
I don't know if this has answered your question, but this worked for me.
Lesley