I'm pretty certain I'm not a drastically short sleeper. I'm almost always tired to some degree lately. My doctor has already given me a laundry list of things that I have to fix, most presumably fixable by regular sleep. Problem is I can't sleep normally. I refuse to take sleep aids cuz that's just masking the problem.
A few years ago I was chatting with a guy online who was polyphasic. With his guidance I tinkered with my sleep schedule to find out what a good amount of sleep is for me. Too much and I wake up tired. Too little and I wake up tired. I got to 5 hours sleep and would wake up feeling refreshed and not drowsy at all. The catch to this was that I had to take a 15-30min nap in the late afternoon to recharge. Good thing about the nap was I could WILD oftentimes during it. I stayed on that schedule for a while and then just fell off the wagon I guess. I'd love to get back on that schedule but I just can't sleep at all some nights anymore.

So in my opinion, if someone can get the whole 7-8 hours of sleep, I'd say do it. It's not so bad compared to the screwy stretches of sporadic sleep I'm getting lately.
Yes, Vexxie there's something wrong with you and you should look at nutrition before anything else. I'm a short sleeper and usually wake up refreshed, but I sleep only when I'm tired, during the warm months that's normally 3-4 hours and in the cold months 4-5 hours, but when I'm sick I almost hibernate in sleep. I have a cold right now, Tuesday I slept 9, Wednesday 9 hours and then went back to bed for another 5 hours-I've almost beaten it. I'm taking 6,000 mg of vitamin C and plenty of Vitamin D3 too. I still have it but the vitamins are curing it enough I don't have the cough or sore throat.
Johnb going back to your original example when I tried to cherry pick when I slept, as a natural short sleeper, I got by with two hours a day for two years. I was waking refreshed the whole time and am pretty sure I was not going into REM as soon as I fell asleep. I've seen HI going to sleep for all my life and did not see any or dream during that time. This was very cool as I tried this when my Dream Guide Death was coming to teach me in dreams every night. It was very good to have his visitations stop!
As for the advantages of short sleeping.
#1 When a small child I was forced to go to bed at 8 o'clock and not get up until 8 o'clock the next morning and was not allowed to have a light on and yes I was afraid of the dark, the closet monster, the under-the-bed monster and the at the window monster. Taught me early to face fear and beat the hell out of it.
#2 One is alone much more than other people, allowing one both time to think and study more than others- makes one a little weird as one has had time to become intellectual.
#3 Being alone with nothing to do that's active, as almost everyone else is sleeping, forcing one to know and live with oneself more than most people do.
I don't know if the extra time is worth it or not.