We all have a biological clock, located in our brain, whose function is to alert us throughout the day. Research has enlightened us to the fact that there is a natural dip in the alerting power of the clock. This dip occurs in the mid-afternoon. Therefore, an afternoon nap or "siesta" will increase our overall alertness if we are not quite getting all the sleep we need in the evening.
Clipping is a handy way to collect important slides you want to go back to later.