IPhone 5 :: Crashes On Charge During Night

my I phone 5 has developed a problem in crashing during the night when it's on charge. It 1st happened Saturday night and my alarm didn't go off so was late for work. The only way I could power up the phone was to hold the power button with the menu and reboot and reset the phone. The phone switched on and was 100% charged so it's charging ok then crashing by the look of it. The phone was ok all of Saturday no problem at all so thought it might of just been a little glitch. On Saturday night I woke up during the night checked my phone to see what time it was and yes the phone was blank again and wouldn't power on as normal so had to do the reset method to power it up again. It was 100% charged so disconnected the charger and the phone stayed on for the rest of the night and alarm went off as normal. I decided to back it up and did a full factory reset on the phone through iTunes. Last night during the night the phone had done exactly the same. I did use the back up to restore the phone back to normal after the factory reset.

Info:
iPhone 5, iOS 7.1.2