The participants wore accelerometers for seven days and nights and were followed up over an average of seven years.
Phillips and his team directly compared the health outcomes of the participants based on sleep duration and sleep regularity. Unsurprisingly, they found the biggest health effects among those who had very short sleep or very irregular sleep patterns.
But there were two surprising findings for sleep regularity. Firstly, there was a graded response.
So, even if their sleep was slightly irregular, there was a meaningful increase in the risk of dying during the follow-up period. “Whereas for sleep duration, it was really those down below six hours where we saw this effect,” Phillips explains.
Secondly, they saw twice the mortality rate in those with irregular sleep patterns compared to those who had short sleep.
“There’s this longstanding assumption that sleep duration is the most important health index, but our findings suggest sleep regularity is even more important: that how regular you are is actually affecting your longevity basically,” he says. “It’s standing these assumptions on the head.”
The researchers controlled for multiple possible confounders including age, sex, ethnicity, and sociodemographic, lifestyle, and health factors.
How well they were able to control for these factors remains a question, says Ron Grunstein, a professor of Sleep Medicine and Senior Specialist Physician at the University of Sydney and Royal Prince Alfred Hospital.
“But, in the end, it’s hard to argue with the findings,” Grunstein adds.
Based on the findings, Phillips and his coauthors recommend that people try to keep their sleep and wake times within the same one-hour windows each day. They assure that the odd variation – a late night here, a particularly early start there or a weekend lie-in – is nothing to worry about: it’s what we do week-on-week that matters.
But why does sleep regularity have such a significant effect?
It’s likely because of how changes in our sleep patterns tinker with our circadian clock, Phillips hazards.
We have circadian clocks in every cell in our body, which regulate every biological function from metabolic health to mental health, but they evolved under very stable environmental conditions, such as where the sun rose and set at the same times each day.
“It would only change by minutes a day,” he says. “But now we are waking up, and we’re sleeping at all different times, maybe hours apart. And when we’re awake, we’re invariably using electric lighting. So, we’re sending this very confusing signal to what’s a pretty delicate and incredibly important biological system.”
When our sleep patterns are consistent, our exposure to light tends to be consistent, adds Jill Dorrian, the dean of research for the University of South Australia’s Justice and Society unit, and a professor of Psychology specialising in sleep research.
“It means that the circadian master clock is getting a consistent, strong, daily, synchronising signal,” says Dorrian, who was not involved with the study.
“When sleep is irregular, patterns of light exposure change. Further, timing of other behaviours such as eating and exercising also change. This means that the different clocks in our bodies may be getting intermittent and/or inconsistent time signals and may become misaligned.”
This is important because circadian misalignment is associated with increased risk of chronic illness, including gastrointestinal, cardiovascular, and psychological disorders.
“Our circadian systems (governing our near 24-hour rhythms) in physiology, thought, and behaviour are a critical and under-recognised piece of our overall health,” says Dorrian.
Getting enough sleep is important in and of itself for cellular level repair, cellular metabolism and supporting brain function. Generally (about one per cent of the population can function well on short amounts of sleep), short-sleepers accrue sleep debt, which has its own health impacts, but if they have a stable sleep pattern each day, they at least maintain a consistent signal to the circadian clock.
“That means the entire organisation of clocks in the body will be more stable and probably contribute to better health.”
While the findings have implications for all of us, the impacts of sleep irregularity may have the greatest effect on shift workers, whose sleep patterns often vary.
Researchers, including Phillips and Dorrian, are investigating how deliberately timing aspects of behaviour, including when shift workers sleep, eat, exercise, and are exposed to light, might minimise circadian misalignment.
But, understanding the importance of sleep regularity can benefit us all, Phillips says:
“By being as consistent as you can, you can hopefully extend your life, extend your health, improve your health.”
Make the most of your health, relationships, fitness and nutrition with our Live Well newsletter. Get it in your inbox every Monday.