A newly published study said GSM 900 MHz mobile-phone signals cause changes to brain activity that linger after exposure to handset radiation, an effect observed in a sleep study that scientists said was unlikely the result of thermal effects on the brain.

The study, led by Dr. Peter Achermann at the University of Zurich’s Institute of Pharmacology and Toxicology and published in the Journal of Sleep Research, revealed blood flow increased in areas of the brain on the side nearest the phone and the effect lasted for more than half an hour afterward.

Scientists also discovered that the brain’s electrical activity—brain waves—was not only affected immediately after the exposure, but also lasted through much of the next night’s sleep.

The scientists said the cause of brain changes seemed to be the pulse modulation of the phone signal, not its carrier frequency.

However, Achermann cautioned that “it would be premature to draw conclusions about health consequences of these findings. Fields such as those emitted by mobile phones might even be used for diagnostic or therapeutic purposes.”