The phenomenon of the day-night rhythm is the most spectacular. This is quite difficult to disrupt, hence the phenomenon of jet-lag.
The day-night rhythm is not easily changing. Only 1 to 2 hours a day can be catched up. This is because, the brains this rhythm set at 25 hours, and that it can be adjusted. About 2 hours per day, So catching time (if one flies to the west, eg America) is pretty easy: nature fills itself for an hour, and we'll do another hour at. "Bringing forward" (as they fly east, eg India) is much more difficult, because one hour is lost, and you can catch up just one hour
That nature is at 25 hours is a classic phenomenon: there are many own rhythms in the body, but they are so defined that they are slower than the natural conditions, so they can easily be adjusted, eg own heart rhythm of 40 beats / min, slow bowel movements, minimal breathing rhythm.