Among people with healthy circadian clocks, there are "larks" or "morning people" who prefer to sleep and wake early, and there are "owls" who prefer to sleep and wake at late times. But whether they are larks or owls, people with normal circadian systems:
Researchers have placed volunteers in caves or special apartments for several weeks without clocks or other time cues. Without time cues, the volunteers tended to go to bed an hour later and to get up about an hour later each day. These experiments demonstrate that the "free-running" circadian rhythm in humans is about 25 hours long. To maintain a 24 hour day/night cycle, the biological clock needs regular environmental time cues, e.g. sunrise, sunset, and daily routine. Time cues are what keep our body clocks aligned with the rest of the world.
Biological Timing Tutorial from the National Science Foundation Center for Biological Timing