Homework, especially in mathematics, is what junior and senior high school students most often blame for their lack of sleep, according to a recent survey.

The survey, conducted by the city's Xuhui District Education Bureau, polled more than 20,000 students from 39 junior high schools and found that about 60 percent of them spend from two to four hours a day on homework. It also found that senior high school students sleep fewer than seven hours a day on average, which is one hour fewer than the Ministry of Education recommends.

In China, students from the ages of 13 to 16 go to junior high school and from 16 to 19 to senior high school.

"People are becoming concerned more about the workloads of high school students," said Pu Zhengquan, deputy director of Xuhui District Education Bureau. "That's why we conducted this survey. We think we should reduce the pressures students are under and give them more spare time."

Of all school subjects, the survey respondents said they spend the most time on mathematics. Many complained that their math homework is too difficult.

An extreme example of what school pressures can drive students to occurred on Oct 24, when two girls in an Anhui province primary school attempted to commit suicide by drinking poison in their classroom. One of them left a message on the blackboard, saying "if we die, it's our math teacher's fault. Please call the police and have her arrested".

An investigation found the teacher had laughed at them after they had made low grades on a test.

"I hate math," said Wang Jingying, a high school student in Anhui province, who will take China's university-entrance examination next year. "Even though I've already worked hard at it, I still can't do lots of those problems."

Wang said she fell behind in her math classes after she had entered high school.

"The math problems are getting more difficult," she said. "Our math teacher only cares about those students who achieve excellent scores. She always loses her patience if we approach her to ask 'simple' questions."

Wang said she finds it difficult to understand what her teacher is discussing in class and has to pay extra for after-school courses.

Mathematics courses in China are widely believed to be more difficult than those found in the United States and many other countries.

"My son always did really well in math in the United States," said Wang Huayun, whose family migrated to the United States on 2009.

"But he was pretty weak at math when he was in China."

Mathematics is considered one of the three most important subjects for Chinese students, the other two being Chinese and English. No student can hope to enter college without being good at math.

Many students go so far as to give themselves extra math homework to help improve their performances at school.