People who get less than seven hours of sleep a night may be healthier than those who get eight or more, a new study conducted by the American Cancer Society said.

A six-year study published by the Archives of General Psychiatry monitored the sleeping habits of 1.1 million Americans. Study participants were asked how long they slept and how frequently they experienced insomnia.

Participants who slept seven hours per night had the best survival rate, while the mortality rate for those who slept over 8.5 hours or less than 4.5 hours increased 15 percent. However, researchers found no excess mortality rate among people who reported having insomnia.

Most people surveyed slept fewer than eight hours per night.

Sleep experts have criticized the study, particularly the conclusion it draws.

“The conclusion of this study is poor sleep may cause disease or contribute to disease,” said Craig Berridge, UW-Madison associate professor of psychology. “It seems like it may be the other way around. How do they know it’s sleep that’s causing the illness?”

Berridge said abnormally long periods of sleep are often symptoms, not the source, of medical problems.

“Some people sleep excessively because they’re depressed,” he said. “They’re not depressed because they sleep excessively.”

Ruth Benca, professor of psychiatry, said the study misleadingly links increased amounts of sleep with increased death rates.

“In this paper are associations, not causalities,” she said. “We don’t know if they’re related.”

Critics also say terms used are too vague. Benca said the study does not clearly define the word “insomnia,” but simply asked participants if they suffered from it.

Steven Weber, professor of neurology, said the amount of sleep reported by participants might be inaccurate.

“People don’t report how much they sleep, but really how much time they spend in bed,” he said.

Many also doubt the credibility of the study’s conclusion because it diverges from traditionally accepted beliefs about sleep and decades of scientific studies on the subject.

Weber said extensive research shows people’s performance, including their emotional, physical and cognitive functions, deteriorates when they are deprived of sleep. Thus, a lack of sleep can create health risks. The new study does not assess these functions.

Benca said despite past research, the new study may lead people to reduce the amount of time they sleep.

“It’s very dangerous to look at this article and cut back to six or seven hours of sleep,” she said. “We know as a society we’re sleep deprived.”

Until more research is conducted on the potential risks of excessive sleeping, experts will not change their standard sleep recommendations.

“The consensus among clinicians in the sleep field is you need more than eight hours of sleep a night,” Weber said.

Benca said people should develop regular sleep schedules and sleep enough to make them feel awake and alert during the day.

“If you’re waking up at the right time, you shouldn’t even need an alarm clock,” she said.

Students often fall short of the recommended eight hours of sleep, but they require a full night’s sleep.

“Sleep needs appear to increase in puberty and decrease in the late 20s,” he said. “The younger you are, the more sleep you need.”

While such sleep recommendations may seem unattainable for students, Weber said getting enough sleep is worthwhile because doing so helps students perform better in classes.

“Try an extra hour for a week,” Weber said. “I think it will make a big difference.”