For me, overnight in coach is a no-no. I did it a number of times when I was younger, but one unbearable night with a leering seatmate did me in. I was frankly too nervous to sleep given his inappropriate behavior, and the coach seating attendant would not reseat me unless he actually touched me. That was it for me and overnight coach. Now I probably wouldn't be subject to that kind of shenanigans, but my balky hips probably couldn't take coach.
My advice for surviving a coach trip in the summer is the same as above--bring a blanket, blow-up pillows, and warm clothing. I always brought a warm hat--it's surprising how much difference that makes, and they take up little room. Snacks help deal with cafe car and dining car disappointments--I used to splurge on nuts, fancy chocolate candies, dried fruit like cherries and blueberries, and even hard cheese bits like aged gouda. Again, light weight but tasty and 'special.' Don't try to sleep until you are good and tired; there's nothing worse than tossing and turning in those recliners because your body just isn't quite ready for sleep. I used to cheat myself on sleep the night before so that sleep would come more easily. (Okay, the real reason was that I always packed at the last minute, but I rationalize that it was a plan to get better sleep.) One last recommendation--bring a travel size packet of baby wipes for a quick sponge-down in lieu of a shower, since there aren't any in coach. And if you have a seat mate, feel free to offer one to them--you'll both be happy you did. Just don't try to flush them, regardless of what the package says--they aren't flushable!