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Dreaming, someone has said, is the third state of existence. Its importance in the maintenance of physiological and psychological homeostasis is still largely a matter of conjecture. For that matter, sleeping, our other state of existence, is understood still only as a complex biorhythmical phenomenon unfolding in recurring phases; during each night's sleep our bodies and our minds dip into ever deepening oblivion only to return close to the waking state in repeating cycles. But is it oblivion? Sleep may be restful in a sentient way; it is, rather, a most laborious piece of physiological work.
If to be awake is to be aware of the world within our conscious horizon and if to be asleep is to permit the return of keen awareness, to dream is to be conscious of another world—the fantastic harbor of our worries.
This book makes a contribution of the greatest value to this
Hess HJ. The Psychology of Sleep.. Arch Intern Med. 1967;119(5):542-543. doi:10.1001/archinte.1967.00290230180023