The next question is why do we get sleepy? There is a part of your brain called the suprachiasmatic nucleus (or SCN, for those of us who don't have time to pronounce that and/or have a stutter) that regulates your sleep schedule, and makes you get your sleepy on. I think mine is working a double shift today. It's noon, and I want to go to bed.
Someone with a normal SCN has a routine sleep cycle: the same amount of sleep around the same time each day, but someone with an impaired SCN (or a lab mouse whose SCN was destroyed, say) sleeps at random times for random durations.
The SCN sets your internal clock to a 25-hour day. Earth days, as you probably know, are 24 hours (ish--but I'm not going to go into that whole business). Sunlight recalibrates our SCN daily, so that we keep on track with the day length our planet has set for us. People in a sleep study who were kept indoors and weren't exposed to any natural light started deferring to their brains' 25-hour day.
There is a great variety of sleep behaviors in the animal kingdom, but nearly all animals exhibit some kind of sleep-like behavior. Even sponges slow down at night, although it's not clear whether or not that constitutes sleep.
Sleep was obviously advantageous for our flashlight-less ancestors, as it kept them from stumbling around in the dark, getting eaten by lions or falling down bottomless pits. But it still seems like such a massive disadvantage to be unconscious and vulnerable for a third of the day.
But it's not all about staying safe when it's dark; after all, animals that have great night vision have to sleep, too. As I explained earlier, sleep is very important for the brain--the more complex the creature's brain, the more serious the sleep.
To make sleep yet more confusing and weird, scientists discovered that blind cave fish get by on almost no sleep at all. Maybe they evolved to need less sleep because their environment requires them to constantly be on the search for food, but I also read that this suggests that part of sleep's purpose is to process the visual input collected during waking hours. So no input, no need to process. But of course, blind humans still need sleep, so don't go and blindfold yourself so that you won't have to sleep. 'Twon't work.
Brainy aquatic species like dolphins and whales still need their sleep, and their situation is a difficult one. How do you find a safe place to sleep in the open ocean and refrain from drowning while you do it? The answer is unihemispheric sleep--sleeping half a brain at a time. The other half can keep an eye out for danger and make sure the animal breathes.
I'll leave you with what my sleep research source told me when I asked a million questions about why we must sleep:
"If sleep were not important then it is probably the most significant mistake, evolutionary speaking, ever made."Well, that and hairless rodents. Yeesh.