PHILADELPHIA -- Who wouldn't wake up to a ringing phone in the middle of the night.

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But it's before you even try to go to sleep that has researchers attention. A study funded in part by cell phone makers themselves finds cell phones and sleep may not mix.

Doctors asked men and women between the ages of 18 and 45 to talk on cell phones before bed. Some were exposed to radiation similar to a wireless signal. Others talked on "phoney" phones that released no radiation.

Those exposed to radiation reported headaches, took longer to fall asleep and didn't sleep well through the night.

Scientists believe cell phones radiation may activate the brain's stress system.

"Even having the phone in the room if it was turned on made a huge difference in the type of sleep you got. It prevented, it seemed to prevent deep sleep," said Dr. Joanne Getsy, director of the Drexel University Sleep Center.

Getsy said we all need deep sleep to feel rested, and it's crucial for teens, who produce hormones while sleeping.

"And so, when you cheat kids out of deep sleep, you are cheating them out of a period of time when their growth hormone is being secreted, and we don't know what effect that will have long term," Getsy said.

She has begun telling patients like Stephen Donnelly to turn their cells off and get them out of the bedroom.

"I'll do anything to get a better night's sleep," said Donnelly, who has had trouble sleeping for a long time.

He's working hard to fix it: no more phone by the bed; no more talking before bedtime; and he said it's working.

"I seem to sleep practically all through the night a lot of times," Donnelly said.

That's good news. But he's still concerned about how cell phones may be changing his brain.

"It is scary because, if that's what is happening at night, what is happening during the day when you're on it all day?" Donnelly asked.

"How do we not have an affect from that?" Getsy asked. "How is that not bad for us, if you're tossing and turning when you should be sawing logs?"

Some more of Getsy's tips are get the television out of your bedroom, no caffeine after lunch, and take time to unwind before you turn in, NBC 10's Denise Nakano reported.