I would like to recommending a simple kitchen timer. ya know the ones that are small & hand held. I prefer that to the App. for the timer, especially if i am really mentally tired or lazy. The App timer has been good for me if I am trying it as a napp AND i am mentally nuetral.

I like the timer because I have to turn it off manually which keeps me mentally engaged as opposed to accidently falling unconscious. My only point of contention here is that you have to be on your game and reset the timer or you will fall asleep. I know I have done that before too