Many of us enjoy an occasional bedtime chat with a loved one who is far away. But as more and more people trade in their landlines for mobile phones, they may find that these late-night conversations are no longer a good idea. According to recent studies, cell phone signals can alter brain waves—and the consequences will keep you up at night.

Neuroscientist Rodney Croft and his colleagues at Swinburne University of Technology in Australia strapped a Nokia 6110 cell phone to the heads of 120 men and women and then monitored their brain waves. When the researchers switched on the phone without the subjects’ knowledge, they saw a sudden power boost in the volunteers’ alpha brain waves. Alpha waves normally surge as the mind shuts out the external world and spins internal thoughts. Croft believes the heightened alpha waves reflect the mind concentrating to overcome the electrical interference in brain circuits caused by the pulsed microwave radiation from cell phones.

In a different study, sleep researchers at Loughborough University in England found that after a 30-minute exposure to cell phone signals in talk mode, people took nearly twice as long to fall asleep as they did when the phone had been off or in standby mode. The scientists think the effect probably reflects the time it takes the brain to relax after being agitated by the phone’s electrical field.

James Horne, one of the study’s authors, cautions that the effects are harmless and less disruptive to sleep than half a cup of coffee. Still, he wonders, “With different doses, durations or other devices, would there be greater effects?”