We've known for a long while that too much sitting isn't good for you. Numerous studies have shown just how damaging long hours of sedentary behavior can be.
But as recent research reveals, it's not just the total amount of time you spend sitting each day that we should be looking at.
A first-of-its-kind study published in PLOS Medicine shows that the way we break up and accumulate periods of prolonged rest throughout the day may have a huge impact on health.
Most striking of all, researchers found that for every extra hour of prolonged sitting in the day, people in the study faced a significantly higher risk of cancer and cancer mortality.
"What our data shows is that sitting for more than 30 minutes at a time is particularly linked to a higher risk of cancer," says epidemiologist and senior author Frederick Ho from the University of Glasgow.
"The good news is that breaking up your sitting time with something as simple as a short walk could be protective."
In the research, the researchers analyzed data from more than 91,000 people who took part in the UK Biobank study – a long-term research project tracking the health of about half a million people in the UK.
In the cohort studied here, participants had an average age of 56 years and did not have a history of cancer.
Each wore an activity monitor with an accelerometer on their wrist for seven days continuously, which tracked their level of movement, registering it as either sedentary behavior or light, moderate, or vigorous physical activity. Researchers then kept tabs on their health status for approximately 12 years afterward.
Over the follow-up period, the researchers found that overall levels of sedentary behavior were associated with worse health outcomes – including a greater risk of being diagnosed with and dying from cancer – after accounting for age, sex, ethnicity, socioeconomic wealth, education levels, and dietary and lifestyle factors.
But when the researchers examined patterns of sedentary time, they discovered that not all time spent sitting down is equal.
Specifically, two distinct types of sedentary activity were measured: prolonged sedentary behavior (bouts of sitting lasting 30 minutes or longer, where 90 percent of the time was sedentary), and interrupted sedentary behaviors (sedentary periods lasting less than 30 minutes, or longer periods sufficiently broken up by non-sedentary activity).
According to the data, there's a big difference between these two relative kinds of rest.
The results indicated that for every additional hour per day spent in prolonged sedentary behavior, participants in the study had a 9 percent higher risk of dying from any cancer.
Every extra prolonged hour was also linked to increased cancer incidence (3 percent higher), with incidence of obesity-related and type 2 diabetes-related cancers both going up by 5 percent.
While those relative risk estimates may seem modest, it's the cumulative time that matters here, with each hour adding to cancer risk.
In contrast, interrupted sitting was associated with significantly better health outcomes.
Each hour of interrupted sedentary behavior was linked to an 18 percent lower risk of dying from cancer and a 6 percent reduction in overall cancer incidence.
The incidence of obesity-related and type 2 diabetes-related cancers was also reduced by 9 percent and 10 percent, respectively, among those who moved intermittently.
"Our findings suggest that the health effects of sedentary behavior may depend not only on total sedentary time, but also on whether that time is accumulated in prolonged bouts or interrupted by activity," the researchers explain in their paper.
"This pattern is biologically plausible: Experimental studies have shown that interrupting prolonged sitting with short bouts of activity can improve metabolic responses compared with uninterrupted sitting."
Beyond comparing the two types of sedentary behavior, the results also found new evidence showing how important exercise is to offset these kinds of health hazards.
The modeling suggests that replacing just one hour a day of sedentary behavior with physical activity can yield marked health improvements tied to lower cancer outcomes.
In the study, replacing one hour of prolonged sedentary behavior a day with light physical activity corresponded to a 12 percent reduced risk of cancer mortality and a 6 percent reduction in cancer incidence.
"Current health guidelines focus heavily on moderate or vigorous exercise, but our findings show that light movement shouldn't be ignored," Ho says.
Of course, it's worth noting that due to the nature of this observational research, the findings can only show associations in the data – and do not indicate that changing your sedentary behavior in these ways will necessarily reduce your own personal cancer risks.
But the findings run parallel with a host of related research, including studies demonstrating the link between sedentary behavior and chronic inflammation, and research showing how the effects of physical activity could help with cancer prevention efforts.
Related: Study Reveals Optimal Number of Daily Steps to Offset Sitting Down
In this context, it's worth recognizing that even very small amounts of daily activity that break up long bouts of sitting seem to be beneficial to our health, and we should try not to lose sight of that.
"To our knowledge, this is the first study that showed the differential associations of interrupted and prolonged sedentary behavior with cancer outcomes," the researchers write.
"Given the high prevalence of sedentary behavior, even modest associations could still have important public health implications."
The findings are reported in PLOS Medicine.
This article was fact-checked by Clare Watson and edited by Clare Watson. While we pride ourselves on our process, we are only human. If you spot a mistake, please let us know.