Has this ever happened to you? You’re just about to drift off into a deep sleep when suddenly you feel like you are falling or coming down quickly on a roller coaster. You then fully wake up to realize that you are safe in your bed. You may have been confused or frightened for a second or 2 and your heart may be beating faster. You may have also felt the muscles in your body jerk when this all happened. Did you ever wonder why?

This falling sensation, together with a body movement known as a “sleep start,” is what doctors and scientists call a hypnic or myoclonic (say: my-eh-kla-nik) jerk, and it usually happens right before going into deeper stages of sleep.

Here’s why doctors think the falling feeling and myoclonic jerks happen. When you fall asleep, normally your body temperature goes down and your heart starts to beat just a little slower. This is because your brain signals your body to relax to prepare for sleep. When your muscles are relaxed, you go into the very first stages of sleep, but you’re not sleeping too deeply yet.

This brain activity may cause your larger muscles to contract a little bit. In response, another part of your brain will make up a very quick mini-dream to go along with what is happening. You might think this would take a while, but your brain works so lightning fast that it makes up the story almost at the same instant that your muscles jerk.

During this light sleep, you can be easily awakened – like if the phone rings or if your dog licks your face. A lot of people who do wake up during this light sleep believe they haven’t really fallen asleep yet. That’s why you may think that your body jerked right before you were about to fall asleep, but really, you were lightly asleep the whole time – just not deeply asleep.

Myoclonic jerks are a normal part of sleep. Most people (80%) have them at one time or another. But many people do not realize their muscles are jerking at times during light sleep and it doesn’t wake them up.

Try watching someone who is in the early stages of sleep. If he’s twitching a little bit, it means he’s experiencing a myoclonic jerk but isn’t being startled awake.