During my freshman year of college, I took a supremely boring class called La Belle Epoque. In fact, it was so dull that I almost always nodded off during the lectures. Unfortunately, while drifting to sleep, I often experienced that unpleasant full-body convulsion that jerks you awake in the most embarrassing way possible. Good thing it was a 200-person lecture.
It turns out that this phenomenon has a name, and it is not, as I usually call it, “that awkward sleep twitch thing.” It’s actually known as hypnic myoclonia or a hypnic jerk, and is often preceded by a sensation of falling. (In my case, I usually dream of tripping—I guess the brain likes to pull from real life experience.) The majority of the population experiences this, although it is more commonly reported in those with irregular sleep schedules.
Like many brain-related conditions, the exact cause of hypnic myoclonia is unknown. In fact, the only thing we know for certain is that it’s super annoying. For now, the popular theory among sleep scientists is that as we drift off to sleep, our brain misinterprets our slackening muscles as falling and forces us to react.
Even more puzzling than hypnic myoclonia is periodic limb movement disorder (PLMD), previously known as nocturnal myoclonus. People with this condition suffer from excessive daytime sleepiness and have trouble falling asleep at night. When they do fall asleep, they display involuntary limb movements at bizarrely precise intervals, usually between 20 and 40 seconds. These intervals are so precise that you can extrapolate out five minutes and predict perfectly when they will twitch. Like hypnic myoclonia, the cause of PLMD is unknown, but it is related to stress, snoring, sleep apnea, and narcolepsy.
And finally, speaking of uncontrollable sleep movements, let’s discuss sleepwalking. I have only seen someone sleepwalk once in my life, and I will never forget it. What began as a high school, basketball-team-bonding, hair-brushing, boy-discussing sleepover eventually took a turn for the creepy. We were all awoken to one of our teammates walking around the dark house saying, “Mom, don’t leave me! I’m cold!” over and over in a strange voice. It was all very Exorcist.
The specific cause of sleepwalking is unknown, but we do know that it generally occurs during non-REM (NREM) sleep. Brain-wise, sleep is divided into REM sleep, during which we sometimes experience vivid dreams, and four stages of NREM sleep, distinguishable by their EEG rhythms. During REM sleep, the brain stem systems responsible for sleep processes actively inhibit spinal motor neurons, essentially leaving us paralyzed. The inhibition keeps us from acting out our dreams, thus protecting ourselves and others. During NREM, the inhibition stops and we are no longer paralyzed. However, with the exception of the occasional position shift, the brain does not usually send movement signals to the body during NREM.
I hope, for the sake of those who suffer from these sleep disorders (and of course, future generations of basketball-playing high school girls), sleep scientists can develop effective therapies and treatments. In the meantime, maybe we should just follow the example of bottlenose dolphins and put only one brain hemisphere to sleep at a time—it would certainly give a whole different meaning to being half-asleep.
Bear, M.F, Connors, B.W., Paradiso, M.A. (2006). Neuroscience: Exploring the Brain (3rd edition). Lippincott Williams & Wilkins.