The sleep guideline says seven or more hours. It has never said eight.

The formal recommendation from the two bodies that speak for American sleep medicine is that adults should sleep 7 or more hours per night on a regular basis to promote optimal health [s1]. It is not eight hours, it is not a range with an upper bound for the general population, and the number eight does not appear in the recommendation at all.

That distinction matters more than it sounds. "Seven or more" is a floor. "Eight hours" is a target, and a target implies both that less is a failure and that more is unnecessary — neither of which the consensus statement says.

What the panel actually decided, and how

The recommendation was produced by a panel of 15 experts in sleep medicine and research, working through a modified RAND Appropriateness Method, and it applies specifically to adults aged 18 to 60 years [s1]. The panel searched the literature with no date restrictions, which returned a total of 5,314 scientific articles, and graded what it found using the Oxford grading system [s1]. It concentrated on nine health categories where the evidence relating sleep duration to health was strongest: general health, cardiovascular health, metabolic health, mental health, immunologic function, human performance, cancer, pain, and mortality [s1]. The process ran for 12 months and ended with a consensus meeting held in February 2015 in Chicago, Illinois [s1].

The word doing the work there is consensus. Multiple rounds of evidence review, discussion, and voting were conducted to arrive at the final recommendation [s1]. This is a structured way of converting a large and inconsistent observational literature into a single actionable number when no trial exists that could produce one — and the panel published a separate paper describing the methodology, background literature, voting process and voting results, along with the assumptions and challenges it ran into along the way [s2]. Publishing the voting process is unusual and creditable. It is also an admission: the recommendation is the output of a deliberative procedure, not of an experiment in which people were randomised to different sleep durations and followed for decades. No such experiment exists.

The evidence behind the floor

What the panel found on the short side is a long list of associations. Sleeping less than 7 hours per night on a regular basis is associated with adverse health outcomes including weight gain and obesity, diabetes, hypertension, heart disease and stroke, depression, and increased risk of death [s1]. The statement adds impaired immune function, increased pain, impaired performance, increased errors, and greater risk of accidents to that list [s1].

Those are associations, and the statement presents them as associations. The performance and accident items have experimental backing that the disease outcomes do not; you can restrict somebody's sleep in a laboratory for a week and measure their reaction time, but you cannot do it for thirty years and count their heart attacks.

The upper end is where the honest uncertainty sits

The consensus statement's third bullet is the one almost never quoted. Sleeping more than 9 hours per night on a regular basis may be appropriate for young adults, individuals recovering from sleep debt, and individuals with illnesses [s1]. For everyone else, the statement says plainly that it is uncertain whether sleeping more than 9 hours per night is associated with health risk [s1].

That is a guideline body declining to answer a question. The long-sleep literature is full of associations with poor outcomes, and it is chronically confounded — people sleep longer when they are ill, and illness is what kills them. The panel did not resolve this, and said so.

The statement also notes that individual variability in sleep need is influenced by genetic, behavioral, medical and environmental factors, and that a clearer understanding of the biological mechanisms underlying sleep need continues to require further scientific investigation [s1]. In other words, the correct amount of sleep for a particular person is not something the guideline claims to know.

Children get ranges, not floors

The same organisation took a different approach for younger populations, and the contrast is instructive. In 2016 the American Academy of Sleep Medicine published age-banded ranges with both a lower and an upper bound: infants 4 months to 12 months, 12 to 16 hours per 24 hours including naps; children 1 to 2 years, 11 to 14 hours; children 3 to 5 years, 10 to 13 hours; children 6 to 12 years, 9 to 12 hours; and teenagers 13 to 18 years, 8 to 10 hours [s3].

Regularly sleeping the recommended number of hours in childhood is associated with better attention, behavior, learning, memory, emotional regulation, quality of life, and mental and physical health [s3]. Regularly sleeping fewer hours than recommended is associated with attention, behavior and learning problems, and with increased risk of accidents, injuries, hypertension, obesity, diabetes and depression [s3]. Note that the teenage band — 8 to 10 hours — is the only place in either statement where eight hours is named as a recommended quantity, and it applies to 13-to-18-year-olds, not to adults.

What a reader can take from this

The defensible version of the popular advice is narrower than the popular advice. A regular schedule of fewer than seven hours is associated, across a large observational literature, with a range of bad outcomes. Whether the right number above that floor is seven, eight or nine for any given adult is not something the consensus statement attempts to specify, and the statement's own language about individual variability suggests the panel did not think it could be specified from the evidence available [s1].

The consensus statement's final bullet is a piece of advice this newspaper will not improve on: people concerned they are sleeping too little or too much should consult their healthcare provider [s1]. What it does not tell anyone to do is chase a round number that its authors never wrote down.

Sources

• Recommended Amount of Sleep for a Healthy Adult: A Joint Consensus Statement of the American Academy of Sleep Medicine and Sleep Research Society — Journal of Clinical Sleep Medicine, 2015-06-15
• Joint Consensus Statement of the AASM and SRS on the Recommended Amount of Sleep for a Healthy Adult: Methodology and Discussion — Journal of Clinical Sleep Medicine, 2015-08-15
• Recommended Amount of Sleep for Pediatric Populations: A Consensus Statement of the American Academy of Sleep Medicine — Journal of Clinical Sleep Medicine, 2016-06-15