The title explains my problem but here're the details. Whenever I turn off the screen there's no problem, power butto works but when I try to turn on the screen it doesn't work -sometimes- and sometimes it works. When it doesn't work I just use my USB cable and connect to computer so screen turns on. I realised it caused by stamina mode as when I charge the phone it always works flawlesly (cause charging turns off stamina mode). I contacted with Sony and they told me to use PC Companion and make a reset SUPRISE! I didn't do it, yet. And I couldn't find stamina mode app in Settings>Apps>All is there a different name? Maybe deleting cache and data helps..Any suggestions?