Keflavík International Airport, pretty much the only way in and out of Iceland, is apparently a terrible place to sleep. According to the website Sleeping in Airports, if waking airport sleepers was a sport, the security guards at the airport would all be covered in medals.
The website describes the sleep style conditions as "Guantanamo", security guards will track down sleepers, no matter how many times they creep into the shadows to get some shut-eye.
However, if you are waiting for an early morning flight, the guards are more likely to give you some slack.
For a full list of airports where it is ill-advised to try to sleep, go to Sleeping in Airports.