Has this happened to you? You're about to drift off to sleep when suddenly you feel like you're falling or coming down on a roller coaster. You then wake up to realize that you're safe in your bed. You may have been confused or frightened for a second or two 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 myoclonic 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 start 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 _ . You might think this would take a while, but your brain works so fast that it makes up the story almost at the same instant that your muscles jerk. During this light sleep, you can be awakened easily. People who wake up during this light sleep believe they haven't fallen asleep yet. That's why you may think that your body jerked right before you were about to fall asleep. Myoclonic jerk are a normal part of sleep. Most people have them at one time or another. But many people don't realize their muscles are jerking during night sleep if it doesn't wake them up. So the next time you dream about falling and wake up in a sweat, don't worry! Your body is just telling your muscles to relax and get some rest! The author thinks myoclonic jerks during sleep are   _  .
worrying usual abnormal useful