Not safe at all. But almost on one can leave it…
It is reported that over using cellular phone may lead to hypomnesis, insomnia and headache, especially for teenagers.
And another report I have seen before said if you put an egg between two cellular phones, and keep them connecting, the egg will be cooked after 65 minutes… although it is just spread on internet, and have not been proved officially, however, for your health, try to avoid long-time talking by cellular phone and leave it far from your head when sleeping.