Have you ever asked yourself "Is the '24-hour cycle' enough for me to sleep and wake up without feeling tired?" or "Can you sleep at a specific hour and wake up at a specific hour everyday without feeling tired?"
It turns out that most people have a regular "24.9-hour" cycle and not 24!
Scientists have done several experiments, in one of them they kept a man in cave without sun light for months. His sleep-wake cycle was a very regular 24.9-hour. he was waking up everyday nearly an hour later.
Another interesting case was a young woman who was kept for 25 days. She shifted to a 30-hour cycle and felt well and relaxed!
At first scientists thought that it's because in the past the earth used to spin slower but it turned out that the earth used to spin even faster!
Something to note here is that the sleep-wake cycle is something that we are born with and which is biologically determined.
Reference: Awareness Biorhythm, sleep and dreaming