As you drift off to sleep, you may have experienced the feeling of falling, accompanied by an involuntary jerk that forces you awake again.

If you have, you are not alone. It has been a topic of discussion on Twitter this week, after author Holly Seddon tweeted about it, and others said they'd had the same thing.

As Seddon notes, the feeling is known as a "hypnic jerk", which the National Institute of Neurological Disorders and Stroke define as "sudden muscle contractions" similar to jumping when startled, which often come after a falling sensation. Studies exploring the phenomenon have noted that they generally happen during light sleep, don't always wake you up, and often are accompanied by vivid dreams or hallucinations.

Around 70 percent of people experience the harmless jerks at some point, but what causes them? Well, the annoying answer is: we don't know for sure. One suggestion is that your brain misinterprets your body relaxing as it drifts off as your body physically falling, waking you up so that you can deal with the gravity situation. 

Another fun add-on to this theory is that the jerk is left over from a time when our ancestors lived in trees.

"A common hypothesis is that it is an archaic reflex to the brain’s misinterpreting the muscle relaxation accompanying the onset of sleep as a signal that the sleeping primate is falling out of a tree," one review of the topic explains.

"The reflex may also have had selective value by having the sleeper readjust or review his or her sleeping position in a nest or on a branch in order to assure that a fall did not occur."

There is little in the way of evidence behind this claim, but rest assured that hypnic jerks are harmless, most people get them, and they could be telling you to adjust your position in your nest.