What Causes Stiffness In Body On Waking Up From Sleep?

I wake up out of my sleep and I can not move my body,my feet or my hands for a few seconds.I know I am awake. What is this and what causes this.I heard something on the Doctors tv show that discribe what I am feeling but I caught the tail end of it.Will you educate me on this matter.Thank You.My email address is YYYY@YYYY