Quoted from Wikipedia (Circadian_rhythm - Humans):
Early research into circadian rhythms suggested that most people preferred a day closer to 25 hours when isolated from external stimuli like daylight and timekeeping. However, this research was faulty because it failed to shield the participants from artificial light. Although subjects were shielded from time cues (like clocks) and daylight, the researchers were not aware of the phase-delaying effects of indoor electric lights. The subjects were allowed to turn on light when they were awake and to turn it off when they wanted to sleep. Electric light in the evening delayed their circadian phase. These results became well-known.
More recent research has shown that: adults have a built-in day, which averages about 24 hours; indoor lighting does affect circadian rhythms; and most people attain their best-quality sleep during their chronotype-determined sleep periods. A study by Czeisler et al. at Harvard found the range for normal, healthy adults of all ages to be quite narrow: 24 hours and 11 minutes ± 16 minutes. In normal subjects outside the laboratory this "clock" is reset, primarily by exposure to light, so that it follows the 24-hour light/dark cycle of the Earth's rotation.
Other recent studies indicate that the average intrinsic circadian rhythm in DSPD patients is longer by perhaps half an hour (see e.g. the Micic reference below).