Earlier quoted context omitted.
I can never take short naps. I always end up wanting to sleep the rest of the day.
Have a cup of coffee just before you take a nap. The caffeine will kick in at around the same time your 20-min alarm goes off.
So if it's "kicked in" when the 20 min alarm goes off that means the last 5 min of that 20 will have been restless because the caffeine was already working.
Caffeine is the great antagonist of sleep quality anyway. Just drop it for a month, see how that feels. Then see how much it fucks up your sleeping quality as you take even two cups of strong coffee (which is great fun because the caffeine will actually work full power, it's amazing, but your sleep after that night will suffer).
This "coffee power nap" thing is a very nice story, but it's based on nothing. People that have success with it are finding the benefits of having a nap despite their caffeine usage.