Scientists at the Karolinska Institute and Uppsala University and Wayne State University in Detroit studied 35 men and 36 women, The Independent reported. Half were exposed to radiation like that emitted by cell phones, while the others thought they were being exposed to it.

Those who were exposed to the radiation took longer to get into deep sleep. They also spent less time in the deepest part of sleep.

The findings bear out those of a large study of Belgian teens that found those who used their cell phones after they actually went to bed were more likely to complain of tiredness.

The U.S.-Swedish study was funded by the Mobile Manufacturers Association, which called the results "inconclusive."

Progress in Electromagnetics Research Symposium at the Massachusetts Institute of Technology published the study.