You may think that sleep's function is to give your body a break, but you'd be wrong. Our muscles, and body in general, do need rest, but actual sleep doesn't contribute anything to the process. The body only knows when it's at rest; it doesn't care whether you're sleeping or just relaxing in your recliner. But that doesn't mean that your brain doesn't need rest as well. That's why many researchers believe that sleep is just that -- a chance for parts of your brain to take a break. It's an opportunity for your neural connections to strengthen and recharge. However, even if there's some truth to this, science hasn't pinpointed the exact reason why sleep facilitates it. Some scientists feel sleep research has been overanalyzed, and that researchers are simply unwilling to accept the notion our brains need a break.
For example, the head of sleep research at the Walter Reed Army Institute of Research posits that the brain simply gets tired. He backs up his theory with data that shows the brain experiences a dramatic drop-off in performance after it's been awake for more than 24 hours. The brain runs on glucose, and test results show that even when there's plenty of glucose available, after the brain has been awake for a full day, it simply doesn't use it. After the 24 hour window, the use of glucose stabilizes, but the brain's performance continues to suffer. No one is sure why this happens, but the idea that the brain runs on a cycle is gaining credence. It appears the brain literally needs sleep in order to refuel.
We're not there yet, but we're closer than ever to uncovering the mysteries of what exactly happens when we sleep. Perhaps one day science won't have to fall back on its old joke: "We need sleep to cure sleepiness."