Sleeping with your phone in your bed, or near to your head, could increase the risk of brain cancer, warns the California Department of Public Health.
Phones emit radio frequency energy, which some scientists believe could increase the risk of brain cancer, and tumours of the acoustic nerve and salivary glands.
Radio frequency energy has also been linked to lower sperm counts, and less mobile sperm.
Scientists haven’t revealed any definitive health effects of using mobile phones, but people wanting to reduce their risk anyway should avoid keeping their mobile phone nearby when sleeping, said the Department of Health.
“When a phone sends signals to a tower, the radio frequency energy goes from the phone’s antenna out in all directions, including into the head and body of the person using the phone,” it said.
“Cell phones work by sending and receiving signals to and from cell phone towers.
“Don’t sleep with your phone in your bed or near your head.
“Unless the phone is off, or in aeroplane mode, keep it at least a few feet away from your bed.”
People could also reduce their risk of brain cancer by carrying their phones in a bag or purse – not in their pockets.
Reduce or avoid using phones when it only has one or two bars of signal, the California Department of Public Health said. When the signal is weak, the phone sends out more radio frequency energy to boost its connection to signal towers, it said.
Using wireless headsets could lower the risk of cancer, as they emit much less energy than mobile phones, it added.
Cancer Research UK said it was “very unlikely” that sleeping with phones could lead to cancer, but more research is needed.
“It’s very unlikely that sleeping with mobile phones could increase the risk of cancer from the evidence we have so far,” said Carl Alexander from Cancer Research UK.
“But as mobile phones are still a fairly new technology research is continuing to check for any effects over time.”
Almost seven million people use mobile phones globally, according to the World Health Organization (WHO).
Research is ongoing to find whether mobile phone use plays any role in developing cancer.
But, because many cancers aren’t detectable until many years later, research can only assess those that become evident over shorter periods, said the WHO.
While no link has been scientifically confirmed, the increased use of mobile phones warrants further research, it added.