The Science Behind Why Your Body Jerks As You Fall Asleep

Have you ever been on the edge of sleep and suddenly felt like you were falling—only to be jolted awake by a sudden twitch? This strange and sometimes unsettling sensation is known as a hypnic jerk. You’re far from alone: a 2016 study found that 60–70% of people experience these involuntary movements.

Understanding the Stages of Sleep

Before diving into the cause of hypnic jerks, it’s helpful to review the stages of sleep:

Stage 1:
This is the lightest stage of sleep—the transition from wakefulness. It lasts just a few minutes. During this time, your heartbeat and breathing slow, muscles begin to relax (often with small twitches), and brainwaves start to shift from their active patterns.

Stage 2:
This is a deeper stage of light sleep. Heart rate and breathing slow further, body temperature drops, and eye movement stops. Brain activity slows, but occasional bursts of activity (known as sleep spindles) still occur.

Stage 3:
This is deep, restorative sleep. It’s the stage that leaves you feeling refreshed in the morning. During this phase, it’s harder to wake up as your heartbeat, breathing, and brain activity reach their lowest levels.

REM Sleep:
Rapid Eye Movement (REM) sleep begins around 90 minutes after falling asleep. Your eyes move rapidly beneath your eyelids, brain activity increases to levels similar to being awake, and breathing and heart rate become irregular. Most dreaming occurs during REM sleep. You cycle through all these stages about four to five times each night.

What Are Hypnic Jerks?

Hypnic jerks—also called hypnagogic jerks or sleep starts—are involuntary muscle contractions that occur as you begin to fall asleep. Though their exact cause isn’t fully understood, they tend to happen more frequently when you’re sleep-deprived or trying to stay mentally active while your body wants to rest.

According to Dr. Rafael Pelayo from the Stanford Sleep Medicine Center, hypnic jerks occur because different parts of your nervous system are transitioning at different rates—some parts may already be asleep while others are still active. Essentially, your brain sends mixed signals, causing your body to “misfire” with a sudden twitch.

These jerks usually happen during the shift from wakefulness to Stage 1 sleep. If you're extremely sleep-deprived, you may skip straight to REM sleep, bypassing Stages 2 and 3. Dr. Pelayo suggests that a hypnic jerk is your body’s way of signaling that it's time to sleep—“Nothing else will satisfy that urge,” he says.

Are Hypnic Jerks Harmful?

The good news is no—they’re completely harmless. While they may startle you or make it harder to fall back asleep, they don’t indicate any underlying health problem. If it happens, try not to worry. Simply reminding yourself that it’s normal can help you relax and drift off again.

Hypnic Jerks and Sleep Deprivation

These twitches can also happen when you’re exhausted but unable to sleep—like during a boring lecture or a long meeting. In these situations, Dr. Pelayo offers a few practical tips:

• Shift your focus to something engaging in the moment.

• Make eye contact with the speaker to stay alert.

• Drink coffee ahead of time if you know you’ll struggle to stay awake.

• Most importantly, aim for 7–9 hours of sleep each night to avoid overtiredness.

While hypnic jerks aren’t dangerous, they’re often a sign that your body isn’t getting enough rest. If you notice them frequently, consider what might be disrupting your sleep and take steps to prioritize rest.