No one really knows how much sleep we need, or for that matter, why we sleep at all.
There are numerous approaches to sleep reduction, and while most of the posts here refer to polyphasic sleep (sleeping in many short intervals through out the day instead of one prolonged nightly chunk), some people do well with, say, one 4-hour period of “anchor” sleep at night and 2 half-hour naps during the day.
The best reference I’ve seen is “Why we nap” by Dr. Claudio Stampi. It’s written for academia (and it’s god-awfully expensive… you might be able to get a copy via interlibrary loan) and is an impressive compilation covering years of cross-disciplinary research, including the military’s investigation into sleep reduction in sustained/emergency operations.
About a year ago I reduced my sleep to 6 hours per night, and I feel at least as alert and rejuvenated as I did on 8. I plan to try a further reduction to 5 hours per night later this summer. Curious: when I was sleeping 8 hours per night I would, on occasion, take naps - usually lasting 30-45 minutes. But these days, if I take a nap, I fall asleep immediately and wake up in 10 minutes feeling quite refreshed. Pretty cool.
The “tricks” that worked best for me when I was reducing my sleep time were to turn on very bright reading lights as soon as I got up, keeping them on until the sunlight took over, and to reduce my sleep gradually, no more than 15 minutes per week.
Here’s a link to the transcript of a “Scientific American Frontiers” episode done 10 years ago. It follows one of Dr. Stampi’s experiments recreating Leonardo’s 15-mins-round-the-clock sleep schedule
This link is to an article by Stampi that references his work with around the world solo sailors. It’s chatty, without much “how-to” info, but interesting, nonetheless