Can anyone tell me why my Lumia 520 starts to beep every few minutes for no apparent reason when connected to mains during the night. The screen lights up following the beep. When I slide down the notifications there are none shown. Battery is about 70% but never full. WiFi and bluetooth are set. The mains connection seems fine and when I disconnect from mains the beeping stops. I have always charged my phone overnight at my bedside and for this reason am charging it overnight in my kitchen. Is there any option like flight mode that would only allow phone and text?