It's common sense that 8 straight hours per day is not ideal in general, nevermind for everyone. If you check out your sleep rhythms (and with exercise and without stimulants like alcohol, coffee, nicotine, etc.) the ideal is usually something like:

1) sleep 5.5ish hours. 2) wake up at like 3am, work for an hour or two 3) sleep for another 1.5ish hours 4) wake up at dawn and somewhere in the early afternoon take a brief nap

The knowledge that 8 hours straight isn't ideal isn't the issue, it's fitting it into your daily schedule that's the problem, for the same reason everyone knows that 5 small meals per day is much better for you than 2.5 huge meals.

So you try to compromise, which usually means adjusting your sleeping block to hit your REM rhythms well (i.e. 6.5 or 9 hours, give or take each way).