I’ve been consistently waking up with a fast heart rate, tension, and general feeling of anxiety about an hour after falling asleep.  I am not consciously anxious about anything.  Generally I have no difficulty falling asleep and my mind isn’t preoccupied with any thoughts.  My body is very tired at those moments but I cannot fall back asleep for at least an hour after suffering through very agitated and uncomfortable symptoms. Any idea what could be causing these symptoms?