This video shows how Psychologist Matt Hall was tested on the effects of napping. The study shows that if the nap includes REM sleeping it will enhance the problem solving abilities. REM is the period of the sleep where dreams occur. The nap has to be done during the day time.
The test consisted on doing a test in the morning and in the afternoon after the nap. Only those that had REM sleep improved the performance. It is said that during REM period the information circulates freely between brain networks .
The doctor says that only naps that are longer than 60 minutes will have a REM sleep So it is believed it is a good idea to nap and you do not have to feel like you are wasting time, on the contrary you would be thinking of great ideas in your sleep.
SO after all when you feel that you need to sleep, do not put it off, even f you are at work you should talk to your boss about power napping. It is being implemented already in some companies and it is a great way of boosting your creativity.
For more sleep benefits read about the power of power nap.
Some researches encourage the 20 minute power nap and some other ones like this one encourage the longer sleep. I woudl suggest you test it, if you have teh time and decide which one suits you better,according to your lifestyle and daily schedule.