The familiar advice that adults need exactly eight hours of sleep every night may be too simplistic, according to a new review published in Brain Medicine. Researchers suggest that sleep needs can vary between individuals and that when people sleep may be just as important as how long they sleep.

Sleep needs can differ from person to person

The review challenges the idea that there is one ideal sleep duration for every adult. Studies of pre-industrial societies have found that people in Tanzania, Namibia and Bolivia slept between 5.7 and 7.1 hours a night, while another analysis estimated average sleep in pre-industrial societies at about 6.4 hours.

However, researchers caution that naturally occurring sleep duration does not automatically prove that shorter sleep is healthier. Differences in environment, season, lifestyle and how sleep is measured can all influence the findings.

New research points to a wider range

Evidence from modern populations also suggests that the healthiest amount may be below eight hours for some people. One recent study using brain and body scans and molecular measures estimated an optimal range of 6.4 to 7.8 hours, depending on a person’s sex and the organ being examined.

At the same time, the findings do not suggest that everyone should reduce their sleep. Sleep requirements vary with age and between individuals. The National Sleep Foundation recommends seven to nine hours for adults aged 26 to 64, while noting that six to 10 hours may be appropriate for some adults.

Sleep timing matters too

Researchers say the number of hours is only part of the picture. Modern lifestyles, artificial lighting and night-shift work can delay the body’s natural sleep-wake cycle.

Irregular sleep is also associated with poorer health. People who sleep less during the working week may benefit from catching up on some sleep at weekends, although maintaining a regular routine remains important.

There is no single magic number

The latest evidence suggests that the eight-hour target should be viewed as a useful general benchmark rather than a universal rule. Individual sleep needs, consistency and timing all matter.

The broader message is not to chase a particular number but to get sufficient, regular sleep and avoid becoming overly concerned about the occasional poor night’s rest.