Two articles about the studies were to be published by China-based international journal National Science Review on Thursday.

An increasing number of people are suffering from circadian disorders in modern society, with frequent overtime work, staying up late and traveling across time zones. The researchers said people without a regular daily schedule are prone to problems in their mental health and interpersonal relations.

Most studies on circadian rhythm disorders so far have been conduced on mice, which are not the best animal model because mice are nocturnal and their brain structures and functions are unlike humans, the researchers said.

"The cloned monkeys solved the problems of the long breeding cycle and small number of babies born to macaques. These monkeys, with their unified genetic background, are well-suited to be animal models," Poo said.