That's a really great question because it's really about our brains and consciousness. I don't think anyone knows the answer for sure, but here's the explanation I think is most likely.

Your brain is part of your body, but let's imagine your brain and the rest of your body as two different things for a minute. Your body sends signals to your brain about what is happening. Your brain sends signals to your body about what to do. Think about some of those underwater rovers that people use to explore places they can't go like the Titanic. The person on the research ship gets pictures from the cameras on the rover, then sends signals telling the rover where to go.

Let's say they control the rover with a cool virtual reality system where they have a helmet showing the images from the rover and they wear special gloves that turn their hand movements into movements of the rover. Now let's say the person wants to stop for lunch. The person has to take off the helmet and gloves. Otherwise they couldn't see their sandwich and when they reached for it, the rover might smash its arm into the side of the Titanic.

When you go to sleep, especially when you dream, your brain is "taking a break" and has to turn off most of the incoming signals. It also has to "take off the gloves" so that the body doesn't respond to what the sleeping brain tells it to do. So when you're dreaming, the images you see come from your brain instead of your eyes. When you think about running in your dreams, your body stays on the bed. The controller (your brain) has cut off the connection to the rover (your body). (It doesn't cut off completely. A loud noise, a cat jumping on your bed, or a sudden light can all get through and wake you up. You also twitch and turn during your dreams.) Normally this happens quickly, but during the change from connected to disconnected, we sometimes feel like we're falling and our muscles twitch. Anyway, that's my story.

Do you think time passes in our dreams at the same rate that it passes in the real world? How would you test that?

There are several reasons people might "twitch" while trying to fall asleep. The most common kind of twitch is called a "hypnic jerk". They happen on occasion, especially if a person is over-tired or sleep deprived. Some researchers think it has to do with the muscles and nervous system reacting suddenly during the transition phase your body goes through while shifting from being awake to sleep. I had a difficult time finding very much research on the topic, probably because it doesn't seem to be linked with any kind of illness or medical condition. There are, however, more serious sleep disorders that also cause twitching. The symptoms are more serious, either in terms of the frequency of the twitching (for example if it happens every night for weeks on end) or in the severity of the twitching (for example if the twitching lasts for many seconds or minutes). These conditions may indicate a person is having night seizures or other problems. One of these conditions is called "myoclonic jerks" or is also called "periodic limb movement"

You can do more research on this topic on the web. It is important to be certain you are reading information from a reliable source. I would recommend looking at WebMD (www.webmd.com); a medical information website with a very good reputation for valid information, or the American Academy of Sleep Medicine (www.aasmnet.org), a professional organization that focuses on sleep research.
Sleep well!

Sleep is very poorly understood, so I don't know if there's a good answer to your question. Our nervous system do many different and mysterious things when we're asleep, and the transition from awake to asleep is even more mysterious, I think. Additionally, different people's nervous systems do different things as they fall asleep, and the environment you're in and your emotional state as you fall asleep all add additional complications.

For example, many people experience what's called hypnagogic imagery as they fall asleep. This is kind of like hallucinating. Some people see weird things, others hear voices or sounds, and others feel sensations of falling. Twitching or movement can be a reaction to any of those things. People who are stressed or uncomfortable are probably more likely to twitch or move around, but there are exceptions to that rule, too. Some people's twitches are very small, involving small areas in the skin or face instead of the movement of whole limbs. I think most neurologists would tell you that the nervous system is too complicated for us to know why this happens.

People who study the nervous system in sleep usually focus on brain activity, sleep disorders, or dreams. Sometimes information about twitching may come out of experiments on these other aspects of sleep, but it's not what the scientists usually focus on.

Great question! It would be interesting to survey one's friends and see how different they are in how they move as they fall asleep. Let us know if you conduct such a survey!