Concentrated meditation retreats have been conducted in Japanese Zen monasteries since the Middle Ages (1185–1600). The Zen retreats held during the Middle Ages, known as jōza 定坐, differed somewhat from the type of retreats held during the Edo period (1603–1868), known as sesshin 摂心, a form of concentrated meditation practice similar to present-day Zen retreats (still known as sesshin). The sesshin format appears to have developed under the influence of the Ōbaku school of Zen, a Chinese Zen school transmitted to Japan from Ming-dynasty China during the early seventeenth century. Evidence for this is found in the Ōbaku sannai shingi 黄檗山内清 (Detailed regulations of Mount Ōbaku) and the recorded sayings of early Ōbaku-school monks. The prototype of the sesshin was probably a Song-dynasty seven-day form of retreat known as the xiaoxian 小限.
