Yan Zhenqing, his ancient poem "Encouragement to Study", contains a verse: Black-haired people don't know how to study early, white-headed people regret studying late. So the words of this ancient poem were written by Yan Zhenqing. And the verses you mentioned are derivatives of that.
The original text of the work is "Encouragement to Study: The lights are on at three o'clock and the chickens are on at five o'clock. It is the time for men to study. Black-haired people do not know how to study diligently early, and white-haired people regret studying late."
The background of the creation is: Yan Zhenqing’s father passed away when he was three years old, and his family was in a state of decline. His mother had high hopes for him, implemented strict family education, and personally supervised his studies. Yan Zhenqing was also diligent and studious, and studied every day. Read hard, this poem is what Yan Zhenqing encouraged future generations to do.
This poem is a seven-character ancient poem by Yan Zhenqing. The first two sentences express through the description of the learning environment that you should be diligent when you are young and studying. The last two sentences express through the change of hair color that it is too late to study when you are older. He advised young people to cherish their time, study diligently and make a difference, otherwise they will achieve nothing and regret it when they grow old.
The third update is from 11:00 to 1:00 midnight. The third watch of the lights means reading until late at night, and the fifth watch means getting up early to read at about five o'clock after sleeping for six hours. Young people, if they don’t know that hard work will make their hair gray, they will regret not studying hard in the first place.
Introduction to Yan Zhenqing
Yan Zhenqing was born in Linyi, Langya. He was a famous calligrapher in the Tang Dynasty. Together with Zhao Mengfu, Liu Gongquan and Ouyang Xun, he is known as the four masters of regular script. Together with Liu Gongquan, they are also called Yanjin Liugu.
Yan Zhenqing has exquisite calligraphy and is good at running and regular script. He first studied under Chu Suiliang, and later under Zhang Xu, acquiring his brushwork. His regular script is dignified and majestic, and his running script is powerful. He created the "Yan style" regular script and had a great influence on later generations. He was also good at poetry and prose, including "Yunhai Jingyuan", "Liyue Collection", "Wuxing Collection", "Luling Collection" and "Linchuan Collection", all of which are lost. The Song Dynasty people compiled "Yan Lu Gong Ji".
Yan Zhenqing first learned calligraphy from Chu Suiliang, and later learned calligraphy from Zhang Xu, and also discussed calligraphy with Huai Su. He conducted in-depth research on the calligraphy of Er Wang, Chu Suiliang and others, absorbed their strengths, completely got rid of the style of the early Tang Dynasty, and created a new calligraphy style of the era. Yan Zhenqing's calligraphy style is called "Yan style".