A teacher introduces the correct sleeping posture for children at a kindergarten in Anping County, north China's Hebei Province, March 21, 2017. Tuesday marks the World Sleep Day. (Xinhua/Li Xiaoguo)

Teachers adjust sleeping postures for children at a kindergarten in Anping County, north China's Hebei Province, March 21, 2017. Tuesday marks the World Sleep Day.(Xinhua/Li Xiaoguo)

A teacher introduces the correct sleeping posture for children at a kindergarten in Anping County, north China's Hebei Province, March 21, 2017. Tuesday marks the World Sleep Day. (Xinhua/Li Xiaoguo)

Nurses introduce the correct sleeping posture for children in Handan City, north China's Hebei Province, March 21, 2017. Tuesday marks the World Sleep Day. (Xinhua/Hao Qunying)

A teacher corrects sleeping postures for children at a kindergarten in Anping County, north China's Hebei Province, March 21, 2017. Tuesday marks the World Sleep Day.(Xinhua/Li Xiaoguo) 

A teacher introduces the correct sleeping posture for children at a kindergarten in Weinan City, northwest China's Shaanxi Province, March 21, 2017. Tuesday marks the World Sleep Day. (Xinhua/Cui Zhengbo)

Children and their teacher play a sleeping game at a kindergarten in Weinan City, northwest China's Shaanxi Province, March 21, 2017. Tuesday marks the World Sleep Day.(Xinhua/Cui Zhengbo)

A teacher introduces the correct sleeping posture for children in a kindergarten in Xiajiang County, east China's Jiangxi Province, March 20, 2017. Tuesday marks the World Sleep Day. (Xinhua/Chen Fuping)

A teacher introduces the correct sleeping posture for children at a kindergarten in Donghai County, east China's Jiangsu Province, March 21, 2017. Tuesday marks the World Sleep Day.(Xinhua/Zhang Kaihu)