I rely on a specific app's push notifications, but if my phone has been sitting on the table for a while I don't receive the notifications when they come in.
Whenever I pick up the phone and press the power button to turn the screen on, then I suddenly receive all the notifications from this app and from other apps too, including gmail. I turn on the phone and suddenly get the push notifications from the app and also push notifications from gmail about new emails.
If I have recently used the phone, then I will receive notifications at any time.
So it seems like the phone is going into some kind of sleep mode after some number of minutes.
What exactly is this sleep mode and how many minutes does it take to turn on? And how can I disable this sleep mode so that I always receive push notifications immediately?