If you’ve ever been happily falling asleep only to startle yourself with a jerk/ twitch of the leg, then you have experience what is known as a hypnic jerk, or a hypnagogic jerk. Don’t worry, these are common, don’t indicate any illness, and are experienced by about 70% of the population. It is completely involuntary, and in this article, I’ll explain why it happens.
Before we look into the causes of these hypic jerks, its best to understand some of what happens when you fall asleep. You see, throughout your sleep your brain is very active and plays out dreams you won’t remember. To your mind, these dreams are quite real, so to avoid you acting them out and hurting yourself and anyone you share a bed with, you produce 2 chemicals which inhibit motor neurons, which stop your muscles moving, and effectively make you paralysed, and this is called sleep paralysis. This paralysis doesn’t come into full effect for about an hour, and so this is why you experience these twitches as you fall asleep, but less so during your sleep. That being said, some people can still experience them during their sleep, but it is much less common.
There are two main theories for the hypnic jerk:
It’s thought that the hypnic jerk is an evolutionary kickback from when we slept up trees. As we fall asleep our muscles relax, and our brain might interpret this as falling, which if you sleep in a tree, is very bad, so instinctively our muscles contract in an attempt to grab onto something and stop us falling. This theory is supported by the fact that many hypnic jerks are accompanied by a feeling of falling, and I know from personal experience that when I have a hypnic jerk, it is just as I feel like I’ve started falling down (which just adds to the disruption of my sleep).
Another theory, which is also very likely to be the cause of some hypnic jerks, is that the nerves ‘misfire’, which cause a sharp muscle contraction. Throughout your day, you nerves are constantly ‘firing’, and causing muscle contractions, particularly in muscles in the legs. The firing of these nerves is something the body is equipped to do very regularly, but is controlled by your focused and conscious mind (thankfully). However, as you drift off to sleep your muscles and mind relax, you might even start drifting off into a dream, and your mind becomes less pre-occupied with muscle movement. Nerves which have been firing all day may then ‘misfire’, almost out of habit, which causes your muscles to twitch.
This is kind of similar to boiling water on a stove. Continuous energy from the stove caused the water to boil continuously in the same way continuous signals from our brain cause nerves to move muscles continuously. When you remove the water from the heat, most of the water will quickly cool and stop boiling, but some of the water will still be hot enough to boil, and will still form bubbles for a short period of time. These parts of the water which continue to boil after the heat is removed is is similar to our nerves continuing to fire even though our brain isn’t signalling them to.
This would explain why hypnic jerks can be experienced without the feeling of falling.
These nighttime twitches are not bad for you, and don’t mean you are ill, but they are associated with stress or sleep deprivation, which obviously isn’t good for your health for a number of reasons. They are also associated with alcohol consumption and caffeine, which ideally should be avoided around bed time because they can disrupt your sleep in more ways that just causing hypnic jerks.
It’s worth noting that just because you experience hypnic jerks doesn’t mean you are stressed, but being stressed can increase the chances of experiencing hypnic jerks. Probably because it will interfere with your natural sleep process (possibly in a similar way to caffeine/ alcohol).
A twitch or jolt as you fall asleep is called a hypnic jerk. It is quite common and doesn’t cause you any harm (unless you hit something/ someone). The actual cause isn’t known for sure, but there are 2 accepted theories, both of which can cause the twitches. The first is that the muscle contraction is an evolutionary kickback from sleeping in trees, where we needed to grab branches to stop ourselves falling out of trees. The second is simply that our nerves misfire as our body relaxes, which causes a muscle contraction.
This Youtube video will give an overview of the information found on the article tab. If you want to know more about the topic, or want to see where the information came from, have a read of the article after you watch the video.
As your muscles and mind relax, muscles which have been regularly ‘firing’ may ‘misfire’, and cause a twitch. It is also possible that as we relax our mind feels like we are falling, and so twitches to grab onto something.