Have you ever been drifting off to sleep in your bed, all quiet at night, and suddenly
it feels like you’re falling and your whole body twitchesl?
What is that?
Well, that’s a jerk.
A hypnic jerk, to be precise.
A hypnic jerk, sometimes called a sleep start, is the scientific term for what happens when
you jump in your sleep.
They’re the involuntary muscle twitches that happen in your arms, legs, or even your
entire body as you drift off to dreamland.
Most people get them: roughly 70 percent of the population jumps in their sleep.
But why do they happen?
Well, there are two main theories, based on neuroscience and evolution.
The most common explanation is that as you fall asleep, there are two different systems
in your brain that are basically battling each other.
System #1, a network of nerve cells called the reticular activating system, is just below
your cerebral cortex, the big, wrinkly part of your brain.
When this system is in full force, you feel alert, it essentially helps keeps you awake.
System #2, called the ventrolateral preoptic nucleus, or VLPO for short, is located near
the optic nerve and influences your sleep cycle. The VLPO is sometimes referred to as
the “sleep switch.”
As you fall asleep each night, or during naptime, the VLPO system begins to take control from
the reticular activating system.
During this handoff, from one system to another, the alert part of your brain can still sometimes
stimulate your muscles causing twitches. These hypnic jerks are last-ditch attempts by the
reticular activating system to maintain control of your body.
As you more or less “choose” to sleep, though, system #1 usually cedes control to
system #2, the VLPO, and the hypnic jerks eventually stop.
So these jerks are thought to be the result of a sleeping/waking war happening in your
body each time you try to get some shut-eye.
Another, less common, theory for what causes hypnic jerks involves our primate ancestors.
Some scientists think that the ancient primate brain associated relaxation, or the beginning
stages of sleep, with falling out of a tree.
Hypnic jerks were a way of waking the primate up quickly so they could take action and possibly
break their fall.
So, these muscular twitches might have stayed with us as we’ve evolved over the years.
Whatever causes hypnic jerks, they’re nothing to worry about, according to most experts.
But if you don’t love getting jolted awake, feeling like you’re plummeting to your death,
there are some steps you can take to limit how many sleep starts you experience.
Things like high caffeine consumption, working out in the evening hours, high stress levels,
and lack of sleep can cause more frequent and more intense hypnic jerks.
So cutting back on the coffee, getting your workout done in the morning, and managing
your stress levels and sleeping patterns might result in fewer hypnic jerks.
But like I said, they’re not dangerous, so if they don't bother you, no need to ditch
your afternoon latte.
There are bigger jerks out there to battle, after all.
Thanks to all of our patreon patrons for not being jerks, you keep these answers coming. If you’d
like to submit a question to be answered, or get some videos a few days before everyone
else, just go to patreon.com/scishow. And don’t forget to go to youtube.com/scishow
and subscribe!