Why did the ancient Japanese shave their hair in the middle?

This is called the "Tsukiyo head". Ancient Japanese samurai wore their hair in the shape of a head, because in war, hair is often scattered for various reasons, and then the hair on the top of the head in the center of the front of the head will cover the face, blocking the line of sight, and affecting the battle. Therefore, some samurai shaved the front part of their heads so that even if their hair fell out in battle, it would only fall on the sides and back of their heads and would not interfere with their vision.

Because of its effectiveness, samurai followed suit, and over time it became an unwritten tradition, though this shaving was limited to the samurai class. As for those who did not shave, they were the exceptions or commanders who did not want to be in the battlefield.

Another widely circulated version is that the ancient samurai always had to wear armor and helmets for a long time in battle, and when the weather was hot, the head was too stuffy to bear, so some samurai shaved off the front of the head and scattered hair in the back of the head in the wearing of helmets gathered to the top of the head, and these hair soaked in sweat naturally formed a bun which evolved into a "Tsukiyo head", which is the name of the head of the samurai. The original form of the "Tsukiyo head".

Expanded Information:

History of the Samurai:

The Samurai of Japan arose in the Heian period, and beginning in the middle of the ninth century, a number of local lords began to establish private armies for their own defense and to use them to expand their power. This armament gradually matured into an institutionalized professional military organization based on clan and master-slave relationships. By the tenth century, the imperial court, unable to suppress rebellions by local forces, had to resort to the power of the local samurai, who were further recognized by the central government as the privileged ruling class of Japan.

The formation of the samurai group was inseparable from the disintegration of the centralized power system headed by the emperor and the development of the manor system. As a result of the widespread establishment of manors, there was an increasing number of conflicts and struggles between the manors. In order to expand their territories, the lords of the manor often had to take over other people's land; the lords whose land was taken over by other people had to resort to force in order to protect their own interests. So part of the manor people were armed, at the beginning or also agricultural and military, mainly agricultural; later became a professional samurai.

The samurai group was finally centralized under the Minamoto clan and the Heike clan. The Minamoto and Heike clans were both descendants of the imperial family. The Minamoto clan originated in 814, when Emperor Saga bestowed the surname Minamoto Asamune on the sons of the Emperor. Since then, 14 emperors, from Junwa to Sanjo, have bestowed the surname Minamoto on their children and grandchildren.

The most famous of the many Minamoto clans is the Seiwa Minamoto clan, which began in 961 with Minamoto Keiki, grandson of Emperor Seiwa, and was long based in the Kansai region. The Ping Clan was also a family name bestowed by the imperial family, and there were four Ping Clans, including the Huanwu, Renming, Wende, and Gwangxiao Clans. Among them, the Huanwu Ping Clan was the most powerful, and its founder originated from Emperor Huanwu's great-grandson, King Takamang, who had lived in the Kanto region for generations.

Baidu Encyclopedia - Tsukiyo Head

Baidu Encyclopedia - Japanese Samurai