"sleeping outside in September-October, when there's almost no risk of raining."
If you have never slept outside before in this manner you will be quite unusual if you actually sleep very much at all, that is unless you've done a 10 miler the day before with a big bergan..
Health effect nil if you stay warm and dry. If it rains you'll find keeping your face and head dry a real pain if you have to start wrapping yourself up in ponchos or waterproof coats. Buy a second hand umbrella (from a charity shop perhaps ?) Cut the 'J' handle off leaving just a short length of handle. The handle shaft will almost certainly be hollow...tubular. Cut a piece of stick or dowel and ram it in the hollow end then sharpen a point on it.
Take the brolly with you and if it rains in the night, open it up and stick it in the ground near your head. Result...instance overhead cover in about 2 seconds. Night.. night...
It does work...(well it did on a Portugese beach with an old beach umbrella )..