Marshmallow: Apps NOT battery optimized still seem to doze off I have a Nexus 6P. I have disabled battery optimization for a few apps and yet those apps still seem to doze off.

Specifically those apps are 1) a sip client (csipsimple and bria) and 2) pushover.

It would be great if those apps would never sleep :-)

Is this possible?

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Edit: I agree with CmosBattery below that this might not be the way to go. In fact, I have set battery optimization on all apps like it was before. As mentioned in my comment below, some apps, like Bria, suggest that we should do so (and maybe they shouldn't suggest this at all). After a few weeks with the suggested setting, I see no improvement in call reception and a reduction in battery life.

GCM allows developers to send high priority messages. High priority messages allow you to wake your app (haven't tested, but the Android documentation says that this is reliable). Could there be other ways?
Apps are designed how they are designed by each developer. Apps can acquire wakelocks and keep the cpu (for instance) from sleeping thereby allowing the app to stay active: http://developer.android.com/training/scheduling/wakelock.html

A great app will do this as little as possible especially as apps can set alarms to check the internet for updates or an app can wait to receive a push from an internet server.

Apps never sleeping is the wrong way to look at it and will do nothing for you but hasten the battery usage for typically no good reason.

If there's an app that stops working when it should stay working then the developer should be contacted so as to let them know you want it to continue working even after situations where it may have become disabled regardless of what happens.