How do you quell last minute nerves?

I’ve got an exam in the morning which is worth 100% of my overall grade for this particular module. I had one Tuesday which didn’t go according to plan as I was overcome with last minute nerves and panic.
I’m usually quite calm during exams but this time around is proving to be quite different. How do you deal with nerves? My exam isn’t until 3.30pm tomorrow. It’s 23.30 now and I still have a lot of reading to do.
Any adivce?