How to check what is preventing sleep?

[Guest jenjenx]

My phone sometimes sleeps sometimes doesn't. When it sleeps I can only get the display back on when I press the power button. When it does not sleep I press any key and the display comes on.

Is there any app or any way of telling which app is preventing the blade from sleeping?

I've tried closing apps one at a time but couldn't find teh culprit.

[Guest oldfella]

settings, battery usage, partial wake