Twice.

At LGW we were traveling on two separate tickets and I wanted to avoid missing the flight to the States. I scheduled us to arrive in LGW late and then take the morning flight home. My wife and I decided to save hotel money and slept on the couch type benches that LGW had at the time.

At CPH: we wanted maximize time with the in-laws so we flew AAL-CPH at 10 pm and slept at CPH. That was awful as they had plastic, bucket type seats. We decided we weren't going to do that again.

Came close to sleeping at KEF during what became a 6 hour delay, but that was daytime.