Why it is said not to leave your mobile on, on your bed when asleep.

Hmm. True, many people have heard this as a rumor but not heard it personally. Cell phones should be kept off or placed in Airplane mode to avoid brain damages and whatever.
Read OT below...
Cell phones pump out electromagnetic radiation whenever they’re on – which means sleeping with one nearby boosts your exposure all night long. What to do? Put the phone on “airplane mode” (which shuts down the transceiver) or turn it off. If you need to be available for calls, place the phone several feet away from your bed.
Mobiles emit harmful radiation due to transmission of signal around 900MHz. This radiation may damage your brain. The impact of mobiles on people worries experts. When you regularly sleep with your mobiles kept so close to you, you could become hyper sensitive which will create troubles falling asleep. The
result: insomnia and other sleeping disorders.
Experts are worried about the impact mobiles have on people. Keeping them so close to you makes you super-sensitive, so you will probably have troubles falling asleep. This can
cause insomnia and other sleeping disorders.
According to Dr Charles Czeisler, the main problem associated with mobiles is their strong artificial light that affects your system, copying daylight. The light stimulates the
retinal cells – the area around the side of your eye, which send messages to the brain. Cells
sensitive to light send your body information about the time of the day, explains Dr Meadows. This may not just result in insomnia, but also cause dizziness and headache.
Most experts advise you to take the safe route and limiting unnecessary extended exposure, such as sleeping with a cell phone next to your head. You must avoid sleeping near a cell phone that is on. Alternatively, you could put the phone on “airplane mode” which shuts down the transceiver or turn it off. If you need
to be available for calls, place the phone several feet away from your bed.