n4 screen goes dark what gives?

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http://forums.androidcentral.com – If I'm running an app and I leave it running without interaction for like 5-10 seconds, my screen goes dark as it goes to sleep mode? I just noticed this recently within the last few days. I set my sleep settings to 30 minute last night but noticed that didn't help this. The strange thing is its only some apps. For example I iuse vanguard and it does it. Chrome doesn't do it. Strangest thing. I never noticed it before. I haven't done much to my phone except installed a few new apps here and there.help? (HowTos)