Ever been woken up by a feeling of falling down? Learn more about why that happens.

That strange falling sensation you get when you’ve nodded off for a little bit is a muscle twitch known as hypnagogic myoclonic twitch or “Hypnic jerk.” It’s perfectly normal, and about 70% of people experience it at some point in their lives.

While most experts agree that it’s normal, it’s not completely known why it happens. The general consensus is that when you begin falling asleep, your muscles begin to slack and go into a restful state. However, this happens before your conscious brain shuts off. When the conscious brain feels your muscles relax, it misinterprets the signal thinking you’re falling down. Your brain then sends a signal to the muscles in arms and legs in an attempt to jerk you back upright.

While it’s common for you to suffer hypnic jerks, it’s more common in people with sleep anxiety, fatigue and discomfort. It’s especially more common in people who are fighting sleep or have been sleep deprived for more than 24 hours.

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