Why Do We Jerk As We Fall Asleep?

Story at a Glance.

  • Once you are asleep your body experiences sleep paralysis.
  • Scientist believe that as you fall asleep your waking brain battles with your asleep brain and fights the paralysis, causing your muscles to jerk.
  • People who drink too much alcohol or coffee and who are stressed or very tired tend to jerk more as they fall asleep.

It’s an interesting question. Most of us, actually up to 70% of us have experienced twitching or sudden jerking movement as we’re falling asleep. Sometimes that happens as a result of dreaming we are falling, or tripping over, and other times our bodies simply jerk us awake without a corresponding dream feeling.

So what’s going on?

There’s a few theories to this one. The most predominant one is that our brain works in two modes. Awake and asleep. We’re all fairly well versed in the physical attributes of being awake, that is our body moving and getting us from A to B. But when we’re asleep we’re not so familiar with what goes on. Not surprisingly though, the asleep version is for the most part, the exact opposite.

Once you are asleep your body experiences what we call sleep paralysis. Your brain pretty much puts your body in a state of immobilisation. Your body is a complex piece of machinery and uses different systems to get you through your day, from walking around, to breathing even down to digesting a burger.

We call part of this machinery the Peripheral Nervous System (PNS). The PNS is divided into two parts, the Somatic Nervous System (SNS), which is the system you use to voluntarily control your muscles, say to walk across a room, and the Autonomic Nervous System (ANS), the part that does all the unconscious work, like breathing and controlling your heart rate. Needless to say the ANS stays active whilst you are awake and asleep.  But the brain turns off your Somatic Nervous System while you’re asleep, stopping your muscles from being able to move. It’s an inbuilt safety device to stop you wandering around or driving a car while you’re fast asleep. Handy!

So whilst you’re awake your SNS has been running all day, but at bedtime when you settle down for sleep the part of your brain responsible for getting you ready for sleep kicks in. Now remember one of the jobs of the brain is to turn off the Somatic Nervous System switch. That is, shut down your muscle activity and send your body into paralysis. Scientists have proposed that as part of your brain is trying to paralyse the muscles, the other part that is used to telling the muscles to move, fights for control. And each night, as the shift changes over, there’s a bit of toing and froing as the two systems swap.

As your dream brain kicks in and you sense yourself falling, the part of your brain that controls your waking life moves the muscles in your legs or your arms to protect you from the fall. Of course, you’re not really falling, you’re lying on your back but the awake brain is interpreting the data from the sleeping brain and overriding the sleep paralysis.

Ergo, you jerk.

Sometimes the jerking will be so intense that it will wake you, other times it’s just a gentle twitch. And you don’t necessarily need to be dreaming you’re falling. Sometimes just the changeover between being awake and being asleep is enough to trigger the awake brain into jerking your body as the asleep brain completes the immobilisation process.

Scientists have found that when you’re physically or emotionally exhausted, it increases your propensity to jerking or twitching before you fall asleep. The current theory behind it is that people who are exhausted go through the first stage of sleep too quickly, precisely because they are so tired. During the first stage of sleep, which only lasts a couple of minutes, your body starts to slow down your breathing and heart rate. This stage of sleep is still very light. If you rush through this phase, which exhausted people tend to do, you shut down too fast. It triggers your brain into believing that your vitals are falling and in response, jerks you awake. The theory is that it’s a protective mechanism of the brain.

People who drink a lot of alcohol or caffeine are also more prone to it. As are people who are anxious or stressed. And people who vigorously exercise before bed are more likely to twitch too.

The general consensus is that twitching before you fall asleep is a perfectly normal part of the brain’s neurology. If it’s starting to cause a problem, take a look at how much stress you’re under, and how many hours of sleep you’re getting each night. Cut back on the glass of wine to unwind at the end of the day and ease off on the coffee while you’re awake. And maybe take up a bit of meditation to unwind yourself at the end of the day. A calm brain is far less likely to jerk you back awake.

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