Hi @Mughrabi,
I'm sorry to hear that. Could you try to reboot your device to safe mode to see if this behavior may be caused by one of your downloaded apps as they will be temporarily disabled in this mode.
I'd also recommend that you test the light sensor in your Xperia M5 using diagnostics. To do so go to Settings > About phone > Diagnostics > Light sensor. How does the sensor react in this test?
Let me know how it goes!
