The twenty minute nap really does help, but the window is strict. Wake up between fifteen and twenty minutes and you'll have avoided deep sleep entirely, coming out of it feeling genuinely refreshed.
Cross thirty minutes and your body starts sliding into deep sleep, and waking up from that produces the heavy, foggy feeling known as sleep inertia. Some days that leaves you feeling worse than before the nap.
What works for me is setting an alarm before lying down, and napping in a dim room rather than total darkness with a steady sound playing. Waking up before the mind fully shuts down makes a real difference in the middle of the day.