Often the different areas in the brain synchronize their patterns of activity, resulting in what you might call “brainwaves.” If other people’s brains start synchronizing too, this is called a “Mexican brainwave.”? There are several types of brainwaves, and each NREM stage has specific ones that occur.
In NREM Stage 1 the brain displays largely “alpha” waves; NREM Stage 2 has weird patterns called “spindles,” and NREM Stage 3 is predominately “delta” waves. There is a gradual reduction in brain activity as we progress through the sleep stages, and the further you progress the harder you are to wake up. During NREM Stage 3 sleep—“deep” sleep—an individual is far less responsive to external stimulus such as someone yelling, “Wake up! The house is on fire!,” than at Stage 1. But the brain never shuts down completely, partly because it has several roles in maintaining the sleep state, but mostly because if it did shut down completely we’d be dead.
Then we have REM sleep, where the brain is as active, if not more so, as when we’re awake and alert. One interesting (or sometimes terrifying) feature of REM sleep is REM atonia. This is where the brain’s ability to control movement via motor neurons is essentially switched off, leaving us unable to move. Exactly how this happens is unclear; it could be that specific neurons inhibit activity in the motor cortex, or the sensitivity of the motor control areas is reduced, making it much harder to trigger movements. Regardless of how it occurs, it does.
And that’s a good thing, too. REM sleep is when dreaming occurs, so if the motor system was left fully operational people would be physically acting out what they’re doing in their dreams. If you can remember anything you’ve done in your dreams, you can probably see why this would be something you’d want to avoid. Thrashing and flailing while asleep and unaware of your surroundings is potentially very dangerous, for you and any unfortunate person sleeping nearby. Of course, the brain isn’t 100 percent reliable, so there are cases of REM behavioral disorders, where the motor paralysis isn’t effective and people do in fact act out their dreams. And it’s as hazardous as I’ve suggested, resulting in phenomena such as sleepwalking, which we’ll get to shortly.
There are also more subtle glitches which will probably be more familiar to the everyday person. There’s the hypnic jerk, where you twitch suddenly and unexpectedly while falling asleep. It feels as if you’re falling suddenly, resulting in spasm while in bed. This occurs more in children and gradually declines as we age. The occurrence of hypnic jerks has been associated with anxiety, stress, sleep disorders and so on, but overall they seem to be largely random. Some theories state it’s the brain mistaking falling asleep for dying, so it tries urgently to wake us up. But this makes little sense as the brain needs to be complicit in us falling asleep. Another theory is that it’s an evolutionary holdover from a time when we slept in trees and sudden tilting or tipping sensations meant we were about to fall out, so the brain panics and wakes us. It could even be something else entirely. The reason it occurs more in children is likely to be due to the brain still being in the developing stages, where connections are still being wired up and processes and functions are being ironed out. In many ways we never truly get rid of all the glitches and kinks in such complicated systems as those used by our brains, so hypnic jerks persist into adulthood. Overall it’s just a bit odd, if essentially harmless.17
What’s also mostly harmless, but doesn’t feel like it, is sleep paralysis. For some reason, the brain sometimes forgets to switch the motor system back on when we regain consciousness. Exactly how and why this happens hasn’t been confirmed, but the dominant theories link it to disruption of the neat organization of the sleep states. Each stage of sleep is regulated by different types of neuronal activity, and these are regulated by different sets of neurons. It can happen that the differing activity doesn’t alter smoothly, so the neuronal signals that reactivate the motor system are too weak, or the ones that shut it down are too strong or last too long, and as such we regain consciousness without regaining motor control. Whatever it is that shuts down movement during REM sleep is still in place when we become fully alert, so we’re unable to move.18 This typically doesn’t last long, as once we wake up the rest of the brain activity resumes normal conscious levels and overrides the sleep system signals, but while it does it can be terrifying.
This terror is not unrelated either; the helplessness and vulnerability of sleep paralysis triggers a powerful fear response. The mechanism of this will be discussed in the next section, but it can be intense enough to trigger hallucinations of danger, giving rise to feelings of another presence in the room, and this is believed to be the root cause of alien-abduction fantasies, and the legend of the succubus. Most people who experience sleep paralysis do so only briefly and very rarely, but in some it can be a chronic and persistent concern. It has been linked to depression and similar disorders, suggesting some underlying issue with brain processing.
Even more complex, but likely to be related to sleep paralysis, is the occurrence of sleepwalking. This has also been traced to the system that shuts off motor control of the brain during sleep, except now it’s the reverse—that the system isn’t powerful or coordinated enough. Sleepwalking is more common in children, leading scientists to theorize sleepwalking is due to the motor inhibition system being not yet fully developed. Some studies point to hints of underdevelopment in the central nervous system as a likely cause (or at least contributing factor).19 Sleepwalking has been observed as heritable and more common in certain families, suggesting that a genetic component might underlie this central nervous system immaturity. But sleepwalking can also occur in adults under the influence of stress, alcohol, medications and so forth, any or all of which might also affect this motor inhibition system. Some scientists argue that sleepwalking is a variation or expression of epilepsy, which of course is the result of uncontrolled or chaotic brain activity, which seems logical in this instance. However it’s expressed, it’s invariably alarming when the brain gets the sleep and motor control functions mixed up.