We had a 5:55 AM flight from FCO to AMS, so we opted to forego the night bus and spend the night at FCO. We knew the night bus picked up somewhere in front of Termini, but we didn't know exactly where and we didn't want to take a chance on missing it.

We caught one of the later Leonardo Express trains (sometime between 9 and 10 PM) from Termini and got to FCO not long before a second-floor cafeteria-style restaurant closed. We ate, then got some snacks for the night at a snack-bar type place right next to the restaurant.

Most of the seats in the terminal (both floors) were already occupied with people sleeping across multiple seats. We found a couple of second-floor seats (around a corner from the restaurant and snack bar) to sit in, but couldn't get comfortable enough to sleep there. So we walked over to some large windows that had wide marble (or marble-esque) surfaces in front. We stretched out on the marble surfaces and had a few hours of fitful sleep. We saw security people walking around the airport from time to time, but we didn't see them force any sleepers to wake up or move.

Not a horrible way to spend the night, but next time we'll probably plan to get a later flight so we don't have to do it again. My wife's a good sport, but she wouldn't want to make a habit of sleeping at FCO or any other airport if she can avoid it.

Hope this helps...