Android : Instance variable getting set to null if app is started after a day

I have been struggling with this problem for quite some time now.

Instance variables of my main activity are getting set to null if I start app after a day. So, for my users every other day app crashes once and then restarts just fine. I am assuming that my users pause the app and when they resume the next day, it crashes.

Basically, Oncreate is not called and thus my code does not reinitialize variables.

If android is doing cleanup, whole activity should be killed and thus Oncreate should be called.

Any ideas why this partial cleanup might be happening?

Thanks, Rony