The Sea of Night.
Can you imagine how often we have a nightmare during our lifetime? To what extent can a nightmare affect a person? Actually it will affect your physical and mental health. More seriously, it might increase the possibilities of contracting certain disease. Since everyone is exposed to such a threat of nightmare, why not combat it? Besides, a night-mare may be the true reflection of what you want to ward of or long for. Therefore, a common understanding and sharing of nightmare may count a lot.