We understand the mechanisms that cause sleep; the hypothalamus stops producing histamines, which normally keep you awake (think about getting drowsy on antihistamines). It stops producing histamines in response to your circadian clock, which is kept by another part of the hypothalamus, the suprachiasmatic nucleus, which sits atop the optic chiasm, and keeps track of how much light you’re exposed to. Other areas of the brain influence the hypothalamus and cortex, including the ascending reticular activating system and, of course, your conscious mind. Worry or anticipation can keep you awake. We don’t know the causal reason for thoughts regulating histamine output, but they do.
Once you are asleep the thalamus, which is the central switching station through which all sensory information has to pass before you became conscious of it, begins a slow, rhythmic firing of its neurons. That apparently blocks the sensory information, making you deaf to most sounds and unaware of the bed beneath you. In stage 1 sleep you probably think you’re still awake. Normally you pass from stage 1 to stage 2 sleep after a few minutes. In all stages of sleep the brain’s neural activity responds to external stimuli, even though upon waking you wouldn’t remember having experienced it. If someone makes a loud noise in the room – but not loud enough to wake you up – the neurons responsible for processing auditory input fire, but the information never becomes conscious. About a half an hour after you’ve fallen asleep you’ll have moved through stages 1-4 and then begin to return through the stages in reverse order. Your first experience of REM sleep is only a few minutes long, but as you cycle up and down through the stages, each REM period becomes longer.
So, yes, we do know what happens in sleep. What we don’t understand is why we sleep. Research has shown that people who get less sleep have lowered immune responses and some hormones, including human growth hormone, are produced primarily during sleep. But are those processes the reasons why we sleep, or are they evolutionary responses that take advantage of the fact that we do? If we only needed periods of forced physical rest, we could just collapse into physical paralysis each night, while our minds remained alert. Sleep certainly seems like an evolutionarily ‘expensive’ thing to do, since while we’re unconscious we are vulnerable to attack and unable to go about the important business of eating and procreating.
Some researchers speculate that during sleep our brains are consolidating memories. Memories from the last year or two are stored in your hippocampus. Whether it stores the entirety of a memory or acts as a sort of index, providing links to cortical areas where the memory is actually stored is unclear. What is clear is that there must be an active process for separating declarative memories (as opposed to non-declarative ones which refer to physical or cognitive skill, rather than conscious recollection of events or facts) from the hippocampus, to make room for new memories. You can probably remember your high school graduation pretty well, along with what various classmates went on to do in life. For weeks and months after graduation you also probably remembered what you wore that day, whom you ate lunch with and other particulars. Those declarative memories became much less detailed over time, which allowed for a more efficient use of your brain’s storage capacity. To manipulate those memories like that, however, requires firing the neurons which underlie the memories, which, it seems, should result in a conscious experience, but doesn’t. You don’t remember shuffling the details out of your hippocampus, do you?
Sleep, with its enforced isolation from sensory input and separation from consciousness might be the time when your brain shuffles memories from the hippocampus where newer memories are stored to cortical areas for older ones. Sleep might also provide cover for the brain firing the neurons necessary fro stripping detail from memories. Dr. Bob Stickgold – who should know, since he’s spent a lot of time studying sleep and dreaming – says “I have now taken to describing dreaming as the phenomenology that unavoidably arises when the neural networks and neural systems necessary for memory processing happen to include a sufficient portion of these which underlie consciousness.”