Very likely true. There've been a few cave studies where they shield the experimental subjects from all natural light, and then time how long their "biological" day is. It usually ranges from about 23.5 hours to around 28, with an average of about 25. It varies significantly by person, but was not at all unusual for someone's cycle to be longer than 24 hours.
(Personally, my natural sleep cycle seems to be about 25-26 hours; I'll usually go to bed 1-2 hours later each night until I reset at some point.)
I wonder if there's some control-theory reason why one's unsynchronized circadian rhythm is on average slightly longer than a day? Intuitively it seems that one might actually have more consistent synchronization with the solar cycle if upward pressure from a natural setpoint is balanced by downward pressure from end-of-day environmental signals (light levels, etc). Or in other words, the body has evolved a slight margin to allow for adjustability "in the field" via feedback loops.
But that's just a guess and I can't seem to find anything in a quick search... anyone know more about this?
(Personally, my natural sleep cycle seems to be about 25-26 hours; I'll usually go to bed 1-2 hours later each night until I reset at some point.)