Not exactly an answer to your question, but if a character sleeps in medium or heavy armour, they are fatigued the next day. It seems not too farfetched to apply the same penatly to people who go without sleep. If you really need a mechanic, I'd say something like the forced march mechanic (with a penalty of becoming fatigued or even exhausted instead of taking non-lethal damage) might be appropriate, but I'll leave it up to you to come up with a reasonable time scale after which fortitude saves have to be made.