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.
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