When you're not tired yet, but you know you will be in an hour or two, so you take a nap to prevent that sleepiness that prevents you from being productive later on.
"Do you find yourself tired throughout the day? Do you spend hours trying to study or work, but find yourself dozing off and regretting it later? Consider taking a prophynaptic. Because investing 20 minutes now will save you hours of non-productivity later!"