Xiaomi India — screen black auto in redmi 3s

Brightness of my phone (Redmi - redmi 3s) drops suddenly and suddenly gets higher without reason even the manual brightness is full. My 'auto brightness' option is also remains in off mode.
When my phone's brightness gets dim, i could not able to see anything in phone, suddenly i get stuck. And i get normal brightness after 1-2 hr/1day (Sometimes).
Earlier this problem occured sometime. But now this occures recently within 1-2 hr..
I totally got bothered with this problem... Please fix this problem as soon as possible