That's a well known rule of thumb: If you can fall asleep, but wake up groggy, decrease your total napping time by one or two minutes until you reach the spot w

That's a well known rule of thumb: If you can fall asleep, but wake up groggy, decrease your total napping time by one or two minutes until you reach the spot where it works for you. 20-25 minutes is what you start off with, but the more you learn to fall asleep quickly, the further you have to chop away from that alarm clock to have it work.