By Karim Hammam
Growing scientific evidence suggests the long-standing recommendation of eight hours of sleep per night may not be ideal for everyone, according to a new narrative review published in Brain Medicine. The eight-hour concept originated in the mid-19th century alongside the idea of dividing the day into eight hours each for work, recreation, and sleep.
Hunter-gatherer studies show shorter natural sleep
Researchers studying hunter-gatherer communities in Tanzania, Namibia, and Bolivia found these populations sleep between 5.7 and 7.1 hours nightly without artificial light or modern distractions. They wake near dawn naturally and do not compensate with long daytime naps. A recent analysis estimated the average natural sleep duration in pre-industrial societies at 6.4 hours.
Large-scale studies link optimal sleep to 6.4-7.8 hours
Studies in industrialized societies using brain and body scans, along with molecular measures, indicate the optimal sleep duration for organ aging ranges from 6.4 to 7.8 hours, varying by sex and organ studied. Multiple large studies show both insufficient and excessive sleep correlate with poorer health outcomes, including heart disease, stroke, cognitive decline, and obesity. Scientists continue to debate whether long sleep causes health issues or signals underlying problems.
Recommendations now reflect individual variation
The U.S. National Sleep Foundation advises adults aged 26 to 64 to aim for seven to nine hours, but acknowledges six to ten hours may be appropriate for some individuals. Young adults typically sleep longer than healthy older adults when given sufficient opportunity, and needs vary widely even among people of the same age.
Timing matters as much as duration
Modern sleep patterns are characterized not by reduced total sleep but by a delay relative to the natural light-dark cycle. Artificial light shifts the body clock, leading to shorter weekday sleep and longer catch-up sleep on days off. This misalignment affects sleep quality and health regardless of total hours.
The review was authored by Derk-Jan Dijk, Professor of Sleep and Physiology and Director of the Surrey Sleep Research Centre, and Anne Skeldon, Professor of Mathematics at the University of Surrey. The article was originally published by The Conversation under a Creative Commons licence.
