If you were to live deep inside a cave, with no exposure to the outside world, eventually you would start sleeping roughly every 24 hours and 18 minutes, instead of exactly every 24 hours.
Under normal circumstances, our eyes sense light from the Sun, and our brains adjust to the standard 24 hour day, but we'd really always like to sleep that extra 18 minutes. Why is that? Scientists don't really know, but maybe it explains why everyone is so crabby all the time.
Some completely blind people, who sense no light at all, naturally live on a 24 hour, 18 minute day, and slowly march their schedule around the clock, out of phase with everybody else. I'm speaking from personal experience when I say that it's pretty annoying when you have a blind neighbor who mows his lawn in the middle of the night.
Our built-in tendency to live on a 24 hour, 18 minute day might seem to make sense if that was how long days were in the distant past, and the length of each day has been slowly speeding up. However, exactly the opposite is occurring. The length of each day has been gradually increasing, as the rotation of the Earth slows down because of the Moon's tidal forces. Half a billion years ago, days were 22 hours long. Four and a half billion years ago, they were six hours long. This didn't allow for a lot of variety in the television programming schedule.
A need for a longer day might also make sense if we were all from Mars. (We're not. Most of us are not. Let's just say that all the important people you know are not. If you don't already know that you are, you're not. It's best if you don't bring this up with your parents. Forget I mentioned any of this.) On Mars, each day is 24 hours and 39 minutes long. Scientists working on the Spirit and Opportunity Mars rover projects initially lived on a schedule based on Mars days, because the solar-powered rovers could only operate during daytime on Mars. The scientists wore special Mars watches so they'd know what time it was there. After a few months of this, and always sleeping weird hours, they were all pretty grumpy. Eventually they switched to a more practical, Earth-based schedule.
Sleeping in one continuous block during the night is an extremely recent development. Humans only started doing this when artificial lighting become readily available, and it was easier for everybody to stay up late. Before that, people tended to sleep in multiple chunks throughout the night, separated by periods of activity. Presumably, they would use this time to make trips to the ice box or watch crude infomercials performed by nocturnal travelling minstrels, which were quite popular during the Renaissance.