How do nightmares occur during sleeping?

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Across
  1. 2. Scary sleeping experience
  2. 4. Emotions often felt during nightmares
  3. 6. State of tension or anxiety
Down
  1. 1. A constant feeling of discomfort
  2. 3. A supernatural creature that often appears in nightmares
  3. 5. Something that we fear during nightmares