Body clock

Our bodies think there are 25 hours in a day, not 24. It's only by means of chemical changes in our bloodstream that our internal clock is reset each day. When daylight hits the eyes, hormones are released that end the sleep cycle and start the waking cycle.

They did tests with volunteers who lived in a cave for a month with no watches. After a month, they were fully a day behind in their reckoning of how much time had passed. Crazy, isn't it!