Hey,
I have not had my waters break before, so can't give any specific advice. However, apparently a small % of women actually have a spontaneous rupture, something like less than 10%. So, given that you spend maybe 8 hours sleeping (if you're lucky ) you have a less than 5% chance of waters breaking in bed.

Having said that, there is always a chance! Perhaps sleep on old towels, or even get a plastic sheet and put it under your fitted sheet. I guess you could even use a change mat (for bubs later on) under your butt so if it does go, it wont go into the mattress.

Interesting question anyway, perhaps something to think of in a few weeks time for me!

Good luck