Long before that
specific book came out. If you look at how they arrived at the recommendation here, a lot of it was based on discussions and voting by 15 experts chosen by the US Department of Health.
Actual studies are much less conclusive (see the one we are commenting on for example) and the evidence for 8 hours of sleep is shaky and the difference from 6 to 8 is rarely especially big even when it is supported with anything under 6 and over 8 having negative effect.
Check this systematic review (first result of my search[0]) which seems extremely sympathetic to 8 hours, yet the actual results are:
>compared with 7 h of sleep per day, a 1-h decrease in sleep duration was associated with 6% increased risk of all-cause mortality and a 1-h increase in sleep duration was associated with a 13% increased risk of all-cause mortality (N = 241 107 adults in 43 articles)
So, going from 7 to 8 hours increases risk more than going from 7 to 6!
Further
>Itani et al. (2017) reported that short sleep duration (>Lo et al. (2016) reported that extreme sleep durations (both short and long) were associated with cognitive decline compared with the reference sleep of 7 to 8 h/day (N = 97 264 adults in 18 articles)
But again it's only at the extremes that there is decline, which seems to be more like 3+ hours less than 8 a day.
>Health-related quality of life
No systematic review was identified that examined the association between sleep duration and health-related quality of life.
etc. If you go through actual studies and systematic reviews it is really hard to make much of a case that 6 hours is much worse (if at all) than 8, yet alone anything more than that.
0. https://cdnsciencepub.com/doi/full/10.1139/apnm-2020-0034