For years, eight hours has been treated as the default target for adult sleep. New research, however, suggests that the ideal amount may be more flexible, with individual needs varying more than the old rule implies.
A narrative review in Brain Medicine highlights evidence from studies of hunter-gatherer communities and large modern health datasets. In several pre-industrial settings, adults were found to sleep roughly 5.7 to 7.1 hours a night, while a recent analysis estimated an average of about 6.4 hours. These findings suggest that natural sleep duration can be shorter than the familiar eight-hour benchmark.
At the same time, researchers caution that naturally shorter sleep does not automatically define the healthiest pattern for everyone. Large-scale studies in industrialized societies show that both too little and too much sleep are associated with poorer outcomes, including heart, brain, and metabolic health indicators. One recent study using scans and molecular measures estimated an optimal range of about 6.4 to 7.8 hours, depending on sex and organ health.
The review also emphasizes that timing matters as much as duration. Artificial light and modern routines often delay sleep relative to the natural light-dark cycle, while irregular schedules can disrupt the body clock. Even so, catching up on sleep after a short week may be better than losing sleep without recovery.
The broader message is clear: sleep is not a one-size-fits-all formula. Current guidance from the National Sleep Foundation already reflects that adults may need anywhere from seven to nine hours, with some people doing well outside that range. As sleep science advances, future recommendations may become more personalized, helping people align rest with their own biology and daily rhythm.
