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 [1].
The Stages of Sleep
Before diving into the causes of hypnic jerks, let’s first review the stages of sleep.
Stage One
This is the initial phase when you start nodding off. It lasts only a few minutes, during which your heartbeat, breathing, and eye movements slow. Your muscles relax, sometimes twitching, while your brainwaves transition from wakefulness.
Stage Two
This is a light sleep stage before deeper sleep. Your heartbeat and breathing slow even more, your body temperature drops, and eye movements stop. Brain activity slows, but occasional bursts of electrical activity occur.
Stage Three
This deep sleep stage is crucial for feeling refreshed. Heartbeat and breathing reach their slowest, muscles fully relax, and waking up becomes difficult.
REM Sleep
Rapid eye movement (REM) sleep starts about 90 minutes after you fall asleep. Your eyes move rapidly, brain activity increases, and breathing becomes irregular. Most dreaming happens in this stage. You cycle through these stages four to five times per night.
Hypnic Jerks
Hypnic jerks, also called sleep starts, are involuntary movements when transitioning from wakefulness to sleep. They often happen due to sleep deprivation, signaling that the body needs rest. Though startling, they are harmless.
