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This is not a sleeping nap, but it will give you the similar feeling of renewal that you get after a short nap. In a nutshell a meditation nap is conscious quiet rest time. Here's how you do it.

Allot approximately 10-20 minutes.
Start by doing a couple stretches, whatever feels good.
Next take 10 deep breaths with eyes closed and sitting upright.
Take the rest of the time to empty your mind of everything. A good way to do this is to mentally repeat one word or phrase such as "empty the mind" or "peace" or "om" or whatever works for you. As thoughts come into your mind, notice them like clouds going by in the sky, but don't grab hold. Just keep coming back to your word.

At the end of your allotted time open your eyes, take another couple deep breaths and stretches. Optional: Lightly place your hands over your eyes during part of the time. This helps eye strain tremendously. You can also try giving yourself a face and head massage. Here is some additional info on meditation naps.