I've read before that sleep may serve a valuable function for your brain, that it actually benefits from down time to process new information, etc. So, it may be the case that the extra time could come with a trade off of a lesser functioning brain. Perhaps, as you hinted at, a relaxing non-sleep practice like mediation could offer some similar benefits (though it seems a bit different, and then you are still having to spend that time at rest).

Sleep has a pretty substantial role in our culture, it's a bonding activity both intimately (say with a lover) or socially (such as having a sleepover with friends growing up). Of course, we would develop new rituals no doubt.

It also forces us to physically rest, and break up our work schedules, which some people may need a forcing function to do that (given how many unused vacation days there are in the U.S. on average, for example, some people may need to have a forced resting time).

Physically, my suspicion (though not at all vetted) is that sleep hours probably decrease your longevity less than waking hours (all other things being equal with an otherwise healthy lifestyle), so I wonder if you'd really get more time, or just die much younger on average?