Like Jigsaw mentioned this is a sign that your physical body is beginning to fall asleep so that the astral body can separate from it, a very common experience known as vibrations.

Although the vibrations state and similar states of waking up and being unable to move can be very annoying and even scary to someone who doesn't understand it, it can be a very desireable state once someone recognizes what is happening, and uses it to their benefit, as it can be used to signal to them that their body is falling asleep so to be ready for what is about to follow.

Congratulations on reaching this state as this shows that you are very close to achieving not only a WILD state but a possible out of body experience.

Best thing to do when this happens is to realize what is happening, and then lay back and enjoy the ride.