In environments in which they cannot perceive day nor night, humans do not typically follow the 24-hour sleep schedule. The actual schedule followed by humans is dependent upon the sleep habits of the individual.
Velikovsky first declared that the human sleep cycle is in exact accordance with the Martian day of 23 1/2 hours, rather than the Earth day. However, Velikovsky is perhaps not one of the most credible scientists ever to speculate on the subject (though perhaps one of the most imaginative). Later studies concluded that the sleep cycles of humans more often range from 24-28 hours, meaning that people in seclusion will frequently go to sleep as much as four hours later each night, depending again upon the individual.
It is speculated by many that the tendency toward a longer sleep cycle is related to the individual's rate of metabolism.