Bring earplugs and an eye mask - seriously, the difference between miserable and actually getting some rest is huge. The terminal lighting never fully dims, and there's always some announcement or beeping cart going around.
Since you've got a valid onward ticket, you're allowed to stay, but here's what people don't mention: bring a small blanket or use your jacket, because airport AC cranks hard at night and those terminal chairs aren't made for actual comfort. Find a quiet gate area, ideally away from restrooms and information desks. Scout it out when you first arrive so you know where the least-trafficked spots are. Wear a crossbody bag or keep your stuff close - not because Frankfurt is sketchy, but because 14 hours is a long time to sleep lightly and you don't want to stress about your belongings.
Also, charge your phone while you can, hit a bathroom, maybe grab some water. If you can't sleep much, don't sweat it - power through the connection flights fine even on no sleep. And heads up: if you need to move your luggage to your next flight, do that early before you settle in for the night so you're not scrambling at the last minute.