Bedtime

I've read somewhere that a sort of biochemical traffic along the neurons or axons flows backwards during sleep (maybe something like this Scientific American article). This Atlantic article has interesting information about how little we know about what's going on:

If you needed more proof that sleep, with its peculiar many-staged structure and tendency to fill your mind with nonsense, isn’t some passive, energy-saving state, consider that golden hamsters have been observed waking up from bouts of hibernation—in order to nap. Whatever they’re getting from sleep, it’s not available to them while they’re hibernating. Even though they have slowed down nearly every process in their body, sleep pressure still builds up. “What I want to know is, what about this brain activity is so important?” says Kasper Vogt, one of the researchers gathered at the new institute at Tsukuba. He gestures at his screen, showing data on the firing of neurons in sleeping mice. “What is so important that you risk being eaten, not eating yourself, procreation ... you give all that up, for this?”

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Sleep-inducing substances may come from the process of making new connections between neurons. Chiara Cirelli and Giulio Tononi, sleep researchers at the University of Wisconsin, suggest that since making these connections, or synapses, is what our brains do when we are awake, maybe what they do during sleep is scale back the unimportant ones, removing the memories or images that don’t fit with the others, or don’t need to be used to make sense of the world. “Sleep is a way of getting rid of the memories in a way that is good for the brain,” Tononi speculates. Another group has discovered a protein that enters little-used synapses to cause their destruction, and one of the times it can do this is when adenosine levels are high. Maybe sleep is when this cleanup happens.