Sept. 5 (Bloomberg) -- Sleeping badly? Check under your head. Pillows that change the spine's natural curvature may lead to back ache and restless sleep, a study in Japan found.

Even a pillow that's 2 centimeters (1 inch) too high can reduce sleep quality by causing shoulder and back strain, a study of 22 women found, while a pillow that gives correct neck support can yield a more refreshing slumber.

``People can remove the cause of what may be preventing them from improving their sleep by using a properly measured pillow,'' said Noriko Matsuura, a scientist at Tokyo's Research Institute on Sleep and Society, who led the study.

Researchers are trying to find ways to reduce daytime sleepiness and avoid fatigue-related accidents. The U.S. Academy of Sleep Medicine estimates 80,000 people fall asleep at the wheel every day, contributing to more than 250,000 sleep-related motor vehicle accidents each year.

Matsuura gave healthy women aged 50 to 59 years a properly fitting pillow and one that was 2 centimeters too high, and tracked the quality of their sleep for two weeks.

Those using an ill-fitting pillow were 2.8 times more likely to suffer back pain and 1.5 times more likely to experience tiredness than the women who slept on a pillow of correct height, the study found.

The results were presented at the Fifth Congress of the World Federation of Sleep Research and Sleep Medicine Societies, which is being held in Cairns, Australia, from Sept. 2 to 6.

To contact the reporter on this story: Kanoko Matsuyama in Tokyo at at kmatsuyama2@bloomberg.net

Last Updated: September 4, 2007 21:29 EDT