Done it thousands of times. Just because someone is "on duty" doesn't mean they're actually working. Three major places I worked required 24 hr ATC cover but from midnight to 5am there was little or no traffic so I and my colleagues would sit at the desk having a snooze, reading a book..... maybe play games (Formula 1 was favourite at one unit). What do you expect... controllers to sit staring at radar tubes all night?