If you don’t have cues for sleep time, your sleep cycle becomes 25 hours long.

We’ve all wished there were more hours in the day before. We just have too much to do and 24 hours isn't long enough. Well there may be a scientific reason that we feel this way.
Humans may be programmed for a sleep-wake cycle that is out of sync with the world's 24 hour day-night cycle. In 1938, two men, Nathaniel Kleitman and Bruce Richardson, decided they wanted to know how cave men slept.
In other words, how long would humans sleep and then stay awake if there were no external cues to tell them when to go to bed and when to wake up.
After searching for the perfect location that would completely isolate them from the outside world, they settled on a cave. Just like the cave men of prehistoric times. For 33 days, they stayed in the cave and let their bodies tell them how long to sleep.
What they found and what subsequent experiments have found is rather odd. They discovered that, when left to its own devices, the human body actually follows a 25 hour sleep-wake cycle rather than the 24 hour one that the world's turning dictates.