Sedentary time

Sedentary behaviour is defined by posture and energy use together: sitting, reclining or lying while awake at 1.5 METs or less [1]. Sleep is excluded, and so is standing. The WHO definition also applies to people who cannot stand, such as wheelchair users, and screen time is counted as sedentary only when it is done sitting, reclining or lying [1]. Sedentary time differs from physical inactivity, which means doing less than the recommended amount of moderate or vigorous activity; the two are linked separately to health in the exercise and longevity evidence.

Most studies measure sedentary time with questionnaires that ask how many hours a day a person sits, works at a desk or watches television. In a review of 47 studies of sedentary time and health, all but one relied on self-report, and the authors noted marked differences between studies in how sitting and activity were measured [2]. Newer studies use accelerometers, small motion sensors worn on the hip, thigh or wrist, which record time spent with little or no movement. A hip-worn sensor infers sitting from low movement rather than measuring posture directly, so device-measured and self-reported sedentary time are not directly interchangeable.

The largest single analysis combined 13 studies with sitting-time data on 1,005,791 adults, followed for 2 to 18.1 years, of whom 84,609 (8.4%) died [3]. The researchers reanalysed each study with the same four sitting-time bands and the same four activity levels, measured in MET-hours per week (hours of activity multiplied by their intensity), so that results could be compared directly. The reference group sat less than 4 hours a day and was in the most active quarter, with more than 35.5 MET-hours a week.

Compared with that group, the two least active quarters had 12% to 59% higher mortality, depending on sitting time [3]. Results are given as hazard ratios (the rate of death in one group relative to another; 1.59 means 59% higher). The least active people who sat less than 4 hours a day still had 27% higher mortality (hazard ratio 1.27, 95% CI 1.22 to 1.31), which indicates that low activity carried more risk than long sitting. In the most active quarter, sitting more than 8 hours a day was not associated with higher mortality (1.04, 0.99 to 1.10). The authors estimated that this level of activity corresponds to about 60 to 75 minutes a day of moderate-intensity activity such as brisk walking, well above the minimum in most guidelines [3].

An earlier systematic review reached a similar conclusion. Across 13 studies of all-cause mortality, high sedentary time was associated with a hazard ratio of 1.24 (95% CI 1.09 to 1.41), with 1.18 for cardiovascular death, 1.17 for cancer death and 1.91 for new type 2 diabetes, and the associations were generally stronger in people with low activity levels [2].

A second pooled analysis used only studies in which participants wore an accelerometer [4]. It combined individual data from eight studies with 36,383 adults (mean age 62.6 years, 72.8% women), followed for a median of 5.8 years, during which 2,149 died. Compared with the least sedentary quarter, mortality was 1.28, 1.71 and 2.63 times higher in the second, third and most sedentary quarters.

The same analysis found that light-intensity activity, such as slow walking, cooking or household tasks, was strongly associated with lower mortality. Compared with the quarter doing the least light activity, the next quarter had 40% lower mortality (hazard ratio 0.60, 95% CI 0.54 to 0.68) and the top quarter 62% lower (0.38, 0.28 to 0.51) [4]. Total activity of any intensity showed the steepest gradient, and the curves flattened at higher activity levels. Because a motion sensor records every waking minute as either still or moving, less sedentary time and more light activity largely describe the same change.

Some research examines how sedentary time is accumulated, not only how much there is. In a US study of 7,985 Black and White adults aged 45 and over who wore a hip accelerometer, 340 participants died over a median of 4.0 years [7]. In models that included moderate-to-vigorous activity, the quarter with the most total sedentary time had 2.63 times the mortality of the quarter with the least (95% CI 1.60 to 4.30). The quarter whose sedentary periods were longest on average (more than 12.4 minutes per bout) had 1.96 times the mortality of the quarter with the shortest periods (1.31 to 2.93).

Risk was highest in participants who were high on both measures: at least 12.5 hours of sedentary time a day and an average bout of 10 minutes or more [7]. Sedentary time accumulated in bouts shorter than 30 minutes was associated with a smaller increase in risk, and the authors suggested interrupting sitting every 30 minutes as a possible target for guidelines. The study was small, with wide confidence intervals, and participants may not represent the general US population.

Television viewing shows stronger associations with mortality than total sitting time. In the pooled analysis of one million adults, six studies with television data covered 465,450 people and 43,740 deaths [3]. Watching 3 or more hours a day was associated with higher mortality at every activity level except the most active quarter, and even in that quarter 5 or more hours a day carried 16% higher mortality (hazard ratio 1.16, 95% CI 1.05 to 1.28). High activity reduced the excess risk linked to television but did not remove it.

An earlier pooled analysis of prospective studies estimated the gradient per 2 hours of daily viewing: relative risks (risk in one group divided by risk in another) of 1.20 (95% CI 1.14 to 1.27) for type 2 diabetes, 1.15 (1.06 to 1.23) for fatal or non-fatal cardiovascular disease and 1.13 (1.07 to 1.18) for all-cause mortality [5]. In absolute terms, each 2 hours a day corresponded to about 176 extra cases of type 2 diabetes, 38 cardiovascular deaths and 104 deaths from any cause per 100,000 people per year. The diabetes and cardiovascular risks rose in a straight line, while the mortality risk appeared to rise only above about 3 hours a day. The mortality estimate came from three studies with 26,509 people.

For cancer, a review of 43 observational studies with 68,936 cancer cases compared the highest with the lowest sedentary groups [8]. Heavy television viewing was associated with 54% higher colon cancer risk (relative risk 1.54, 95% CI 1.19 to 1.98) and 66% higher endometrial cancer risk (1.66, 1.21 to 2.28), compared with 24% and 32% for total sitting time. Sedentary behaviour was not associated with cancers of the breast, rectum, ovary, prostate, stomach or oesophagus.

Experimental evidence on breaking up sitting comes from short laboratory trials. A meta-analysis (a pooled statistical analysis of several studies) of seven randomised crossover trials, each lasting one day, compared uninterrupted sitting with sitting broken by frequent short bouts of standing or light walking [6]. Standing breaks reduced the rise in blood sugar after meals (standardised effect size 0.31), and light walking breaks reduced it more (0.72) and also lowered insulin (0.83). Walking breaks lowered glucose and insulin more than standing breaks when the two were compared directly, and neither changed systolic blood pressure. These are short-term changes in blood markers, not disease outcomes.

For standing and mortality, the main source is the 1981 Canada Fitness Survey, in which 16,586 adults aged 18 to 90 reported how much of the day they spent standing and were followed for an average of 12.0 years, during which 1,785 died [9]. Across rising categories of standing time, the adjusted hazard ratios for all-cause mortality were 1.00, 0.79, 0.79, 0.73 and 0.67. Standing was not associated with cancer mortality, and the association with all-cause mortality was significant only among physically inactive participants (under 7.5 MET-hours a week).

None of these studies tested standing desks against disease or death, so evidence for standing desks is indirect. The trial data indicate that light walking breaks change blood sugar and insulin more than standing breaks, which links sitting research to the evidence on walking and daily steps.

The WHO 2020 guidelines strongly recommend that adults limit sedentary time, replace it with activity of any intensity, and, when sedentary time is high, aim for more than the recommended 150 to 300 minutes a week of moderate activity [1]. The guideline group found the evidence insufficient to set a daily limit on sedentary time.

The main limitation of the evidence is reverse causation: chronic illness, pain and low energy lead people to sit more, so some of the excess mortality among heavy sitters may reflect illness that was already present. Most studies measured sitting once, by self-report, and could not follow changes over time [2]. Device-measured studies have shorter follow-up, a median of 5.8 years in the pooled accelerometer analysis [4]. No long-term randomised trial has tested whether reducing sitting lowers mortality.