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.)