For decades, we have been told that going to bed early is the key to success and that one routine fits everyone. Sleep need does vary a little, but the 7–9 hour range for adults still holds. What's most personal is timing.
The Concept of Bio-Individuality#
Your chronotype decides when your body wants to sleep and when you are sharpest, and it is partly genetic. Sleep need varies less than people think. True natural short sleepers exist but are rare. Most adults need at least 7 hours (the consensus of the American Academy of Sleep Medicine and the Sleep Research Society), and feeling fine on 6 is more often accumulated sleep debt. You can estimate yours with the free sleep debt calculator.
So the useful question is less "how many hours?" and more "which hours?" A Wolf pushed onto a Lion's schedule can spend eight hours in bed and still feel off. (New to the animals? Start with what a chronotype is.)
Why ARC is Different#
ARC does not provide generic tips that you could find in a magazine. It starts from your chronotype, bedtime and caffeine sensitivity, then checks itself: after about ten days of one-tap energy check-ins it measures when your afternoon dip really lands and corrects the window you picked. Five-day experiments then test one habit at a time and tell you honestly when it made no difference. Stop trying to follow someone else's routine and start learning from your own days.