There was a study done to answer this exact question - I read it several years ago though so I can't remember what it was called. I believe it took place in conjunction with testing people in enclosed environments, such as those you'd see on a ship headed to Mars. They were kept in a space where they were deliberately were unable to tell night from day (plus no clocks, calendars, etc). I remember it being found that people tended to default to wake/sleep cycles longer than 24 hours, but I don't remember how much longer. Two days? I'm having difficulty finding it, but obviously NASA is extremely interested in this question as well.