Have you ever gone to sleep at night and dreamt that you were falling? Then suddenly, your body jerks, and you are startled awake. An experience like this can be somewhat unsettling, but if it’s happened to you, you’re not alone. This strange occurrence is called a hypnic jerk, and a 2016 study found that sixty to seventy percent of people experience them.

The Stages of Sleep

Before getting into the cause of these hypnic jerks, let’s first do a refresher on the stages of sleep:

Stage One

This is what happens when you first start nodding off. This short period lasts several minutes. During this time your heartbeat, breathing, and eye movements slow, and your muscles relax with occasional twitches. Your brainwaves also begin to slow from their wakefulness patterns.

Stage Two

This is a period of light sleep before you enter a deeper sleep. Here, your heartbeat and breathing slow, and your muscles relax even further. Your body temperature drops and eye movements stop. Your brain activity also slows, but you experience occasional bursts of electrical activity.

Stage Three

You need stage three to feel refreshed in the morning. This is when your heartbeat and muscles are at their slowest, your muscles are relaxed, and it might be difficult to wake you.

REM Sleep

Rapid eye movement sleep occurs about ninety minutes after you fall asleep. Your eyes move rapidly from side to side and your brain wave activity becomes closer to that of wakefulness. Your breathing becomes faster and irregular, and your heart rate and blood pressure increase, almost to waking levels. This is when most of your dreaming takes place. You cycle through these stages four or five times every night. Depending on what cycle you’re in, the stages will vary in length.