Napping Made Easy! How Long is Too Long?

When you're up at 3 AM like we are, an afternoon nap not only sounds great most days, but can be a must.  But how long SHOULD you nap?  10 minutes?  Half an hour?  Longer?  I consider myself an expert napper, able to fall asleep almost anywhere!  But here is a scientific guide to proper napping.  Please Nap Resonsibly.