New Random reboot of device

May 20, 2013
My MI2s with miuiandroid_lang_aries-3.6.28, during the night sometimes reboots itself I think.
I find the device on PIN request screen, with a battery drain bigger than normal.
It happened to me 4/5 times since May when I bought the device. (also in older roms).
Does someone know what could be the problem ?