Sekka Yufu, Ouka Miko, Nijimine Kakoi - Sleep, Sleep (眠れ眠れ) by wtnb
Romaji Title: Nemure, Nemure
Music & Words: wtnb / UkiyoP (浮世P)
Image: Gaifuu Kaisei, from "26 Views of Mt. Fuji" by Katsushika Hokusai
Reprinted from Nico Nico Douga: http://www.nicovideo.jp/watch/sm25221646
This would have easily been at home in an old '80s or '90s game as a lullaby tune.