The scientific case for the Sunnah of 'khaylula'(short post-lunch nap):

 
Is There An Inborn Tendency To Nap?
Research indicates that the human body is inclined to rest in the middle of the afternoon as well as at night, even after adequate nocturnal sleep. A heavy lunch does not make you sleepy, it simply unmasks the physiological sleepiness that’s already in your body. The “post-lunch dip” in alertness occurs whether or not food is consumed. Our natural sleep pattern is biphasic: We have a significant drop in body core temperature and alertness at night, and a similar but smaller drop in the middle of the day. It’s then that you need a nap the most.
  
The ability to focus and concentrate typically starts to slide before noon time. Most people are more easily distracted from noon to 4 p.m., according to recent research led by Robert Matchock, an associate professor of psychology at Pennsylvania State University.
Alertness tends to slump after eating a meal, Dr. Matchock found. Sleepiness also tends to peak around 2 p.m., making that a good time for a nap, says Martin Moore-Ede, chairman and chief executive of Circadian, a Stoneham, Mass., training and consulting firm.

Timing Is Everything!
A nap should be about 15 to 30 minutes in duration. If you nap longer than thirty minutes, your body falls into deep sleep, which is difficult to wake from and when you do, you’ll feel groggy
If you are severely sleep-deprived and must nap longer than 30 minutes, then you shouldn’t sleep less than an hour and a half, which is a complete sleep cycle. WARNING: Don’t expect to be fully alert until at least an hour after that nap.
A 15 minute nap is enough time to turn the nervous system off and can recharge the whole system!
If you are going to nap in the middle of the day, BE CONSISTENT and make a habit of napping everyday. An irregular napping schedule might disrupt your internal body clock. Napping only on weekends is like dieting or exercising only on weekends to make up for a week of overeating.
Brief naps taken daily are healthier than sleeping in or taking very long naps on the weekend. Be careful about taking that long nap on a lazy Sunday afternoon- you may find it hard to get to sleep that night and getting up on Monday to start the new week.
Late afternoon napping isn’t healthy- it delays your falling asleep time in the evening and begins to shift your biological clock.