For years, public health advice about sleep has fixated on a single number: the seven to nine hours adults are told to aim for each night. A study published this week in the Journal of Circadian Medicine suggests that advice has been telling only half the story. Drawing on data from more than two hundred and ten thousand adults tracked for an average of eleven years, researchers at the Calderwood Institute for Population Health found that the consistency of a person's sleep schedule predicted mortality at least as strongly as its duration.
Participants who kept their bedtimes and wake times within a narrow window from one day to the next were roughly a third less likely to die during the follow-up period than those whose schedules swung widely, the researchers reported. The association held even after accounting for age, body weight, smoking, exercise and the presence of chronic illness at the start of the study.
"We have spent a long time asking people how long they sleep, and almost no time asking when they sleep," said Dr. Priya Venkataraman, the study's lead author and a chronobiologist at Calderwood. "What this tells us is that the body cares deeply about rhythm. A predictable schedule appears to be doing something protective that simply logging eight hours, in whatever pattern, does not capture."
To measure consistency, the team relied on wearable sensors worn for two weeks at enrollment rather than on self-reported questionnaires, which earlier sleep research has shown to be notoriously unreliable. From those readings they calculated a "regularity index," a score that rises as a person's day-to-day timing becomes more stable. Volunteers in the most regular fifth of the sample fared dramatically better than those in the least regular fifth, with the steepest differences appearing in deaths from cardiovascular and metabolic causes.
The biology behind the pattern is still being worked out, but Dr. Venkataraman's group points to the master clock in the brain's suprachiasmatic nucleus, which coordinates the timing of hormone release, body temperature and metabolism. When sleep wanders unpredictably, that clock and the peripheral clocks scattered through the liver, heart and gut can fall out of step with one another — a state researchers call internal desynchrony that has been tied in animal studies to inflammation and insulin resistance.
Not everyone is ready to rewrite the guidelines. Dr. Oluwaseun Bankole, a sleep physician at Thornfield University who was not involved in the work, cautioned that the study is observational and cannot prove that irregular sleep causes early death. People with undiagnosed illness, he noted, may sleep erratically precisely because they are sick. "It is a beautifully large data set, and the signal is hard to ignore," he said. "But the arrow could point in either direction, and untangling that will take controlled trials."
Still, the authors argue that the practical takeaway is low-risk and broadly available. Unlike many interventions, anchoring a consistent wake time costs nothing and carries no side effects. They are now planning a trial in which middle-aged adults with erratic schedules will be coached toward steadier routines to see whether markers of cardiovascular health improve. "If even a fraction of the benefit we observed turns out to be causal," Dr. Venkataraman said, "regularity may be the most underrated lever we have in preventive medicine."