夕 is now regarded as the classifier of 夢. Actually, however, it is strongly related to the institution of the 媚 ‘miko: female shaman.’ As in 蔑, the upper part also shows a female shaman with eyebrow decoration. In antiquity, dreams were regarded as something mystical and thought of as being brought about by female shamans.
夕 points to the form of the moon at night. The early tortoise plastron and bone character has no 夕, but instead a 爿 showing the bedstead. Interestingly, all characters including 爿 are not displayed horizontally, but vertically.
In ancient China, a construction method called 版築 ‘hanchiku: board construction’ using boards existed for making clay walls. 爿 and 片 show such boards. Probably because such a board very much resembles a bedstead, in the early tortoise plastron and bone characters also a vertical 爿 board is contained as an element.
Among the tortoise plastron and bone writings a lot of examples of divination by means of dreams can be seen. The answer always took the form of an ‘auspicious’ - ‘inauspicious’ judgment. Among them a great number asks for the meaning of the appearance of deceased persons in dreams.
As a classic of dream interpretation based on ‘auspicious’ - ‘inauspicious’ judgment, ancient China has produced the 周公釈夢 ‘Zhōu Gōng Shì Mèng: Duke Zhōu’s Explanation of Dreams,’ which later also came to Japan and was widely read in the Edo period.