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This is why most vigorous twitching occurs in the last phase before deep sleep. Scientists have conducted studies during which they essentially bored sleep-deprived subjects to sleep by asking to push a button whenever they heard a tone. When the subjects had nodded off, and the tone sounded, and the subjects sputtered back to consciousness - but only after a delay of a few seconds. The conscious brain was still working, it just couldn't get through to the body until after the stimulus had gone. This experiments led scientists to believe that most twitch-awakes are in response to some small sound that eventually made its way to their motor response and got them to jerk awake. Most people associate the jerk-awake phenomenon with a sensation of falling or drifting. This could be what it feels like to lose control of motor function — it's just that usually we're too asleep to notice.
And how is motor function shut down? A lack of serotonin. It seems that serotonin doesn't just make us feel good, it also helps the larger muscles move. The body stops making it during sleep, which is why we generally don't wake up running into walls. (Although, technically some people do. Certain people have conditions that do not let them relinquish control of the larger muscles during sleep and wake themselves constantly and violently.) The smaller muscles, though, are not governed by serotonin. They twitch whether we're feeling good or not. This is why eyes, lips, hands, and feet twitch spasmodically while people dream in deepest sleep, while larger muscles don't move. The small motions don't run any risk of waking us or injuring us, so they are allowed to keep functioning, while the large powerful muscles in the body are paralyzed.
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