Phone Is Restarted During The Night

Apr 24, 2016
I bought Xiaomi Redmi Note 3 Pro 3GB/32GB from ********** (T*p-0ne) and let sent it to Europe. I dont know if there was anything did with ROM - but I have installed Google Apps. After OTA update I have MIUI version MIUI 7 Global 6.4.14 Beta, android security patch level 2016-04-01, Build number LMY47V. I am using 2 SIM cards.
1. Sim card from foreign (calls, roaming)
2. Sim card from this country where I am living - internet
I observered that every morning I look at smartphone and he is asking about PINs. I assume that restart occured during the night.
How to solve it? Or is it a bug ?