Leia, abaixo, o início do capítulo sobre pesadelos, do livro Dreams and Nightmares, de J. A. Hadfield e responda à questão.

N I G H T M A R E S

PEOPLE may ignore their dreams, they cannot ignore their nightmares. For nightmares can be most distressing, casting their shadows throughout the following day. Hamlet shrank from taking his life because he would have ‘perchance to dream’. Nightmares are a common cause of sleeplessness, for many people, like the war-shocked soldier or civilian, dare not sleep because of the horrifying dreams that await them. One does not lightly submit oneself to the experience of getting blown up or buried night after night. The night terrors of children are of this type, for not only are they terrifying in themselves, but their effects persist, filling the day with apprehension and foreboding. The child who is frightened by a dog during the day may have a nightmare of the monster, and may continue to be frightened all the next day. (...) How to define nightmares as distinct from ordinary dreams is a little difficult: the very origin of the term is obscure.

The distinctive feature of a nightmare in the more restricted sense of the term is that of a monster, whether animal or subhuman, which visits us during sleep and produces a sense of dread. Sometimes it is a witch, sometimes a vampire, which is conceived as a reanimated dead person who returns to suck the blood of living people during their sleep; or it may be a night hag, an incubus, or a mare. The word nightmare originally referred to these monstrous creatures themselves and then came to be used of the dream in which these monsters appeared. (...)

Dreams and Nightmares, de J. A. Hadfield 1954.

Qual a explicação oferecida pelo autor do texto para o fato de que nós, principalmente as crianças, não conseguimos esquecer facilmente nossos pesadelos?