Quite frequently I start having a nice dream which ends as a nightmare. For example, I frequently dream of going for a vacation on a seashore. I enjoy waves when suddenly waves turn into getting bigger and results into a Tsunami. I can feel the giant waves hitting me. Then I wake up anxious and find it difficult to fall asleep again. Another example is sometimes I dream of having a nice conversation with my father when in due course the conversation turns into a very aggressive dispute where I am trying to shout at the top of my voice but no sound is coming out. Again I wake up sweating and anxious. What can be the reason I am having this kind of problem?