NASA did a study that basically states that if you are in a positions where you need to think a lot during the day (such as being in school) It is easier and healthier to sleep 6 hours at night and to take a 2 hour nap mid day. However it must be 2 hours or the length of 1 sleep cycle (sleep cycle averages 2 hours).
Also, the bit about doing a physical activity is totally true.
For me though, i realized I was wasting sooo much time laying in bed. I decided to work until I was tired. Same for when I would randomly wake up at 6am. I would get up and do work for 2 hours till I was tired again. Eventually it got to the point where I was tired during the day, would take only a 2 hour nap would still be a little sleepy, do some sit ups or something to get the adreniline going and when I would go to bed I would be totally fine and fall asleep right away. I've become way more efficient too.