Sedentary lifestyle: what the WHO guidelines and accelerometer data say

Most people who look up a sedentary lifestyle expect a number: how many hours of sitting is too many. There is no such number in the WHO guidelines, and the reason is worth knowing. What the guidelines do contain is a floor for activity, and a large accelerometer study shows where the steepest part of the curve sits.

This article goes through those two sources. It is informational material, not medical advice. If you feel unusually tired, short of breath or unwell, that is a question for a doctor.

Sedentary lifestyle: what the WHO guidelines actually say

The 2020 WHO guidelines on physical activity and sedentary behaviour update the 2010 version. They were built by an expert group that reviewed the evidence on activity and a set of health outcomes across population groups (Bull et al., 2020). The numbers a reader usually wants:

• Adults: 150 to 300 minutes of moderate-intensity aerobic activity a week, or 75 to 150 minutes of vigorous-intensity activity, or some equivalent combination of the two.
• Children and adolescents: an average of 60 minutes a day of moderate-to-vigorous aerobic activity across the week.
• All age groups: regular muscle-strengthening activity.
• Everyone: reduce sedentary behaviour.

The guidelines also say, for the first time, something specific for pregnant and postpartum women and for people living with chronic conditions or disability. The abstract does not give what those recommendations are, so this article does not either.

Two sentences carry most of the weight. The guidelines reaffirm that some physical activity is better than none, and that more physical activity is better for optimal health outcomes. Read together, they set a direction, not a pass mark.

Why there is no "norm" for sitting

The last item in the list above is where people expect a figure and do not find one. The abstract says reducing sedentary behaviour is recommended across all age groups and abilities, "although evidence was insufficient to quantify a sedentary behaviour threshold."

That is a plain statement of what is known. WHO does not say that four hours of sitting is fine and eight is harmful, because the evidence it reviewed did not support drawing that line. Any text that gives you a precise limit in hours is going further than this guideline does.

So the guideline gives two separate things. One is a weekly amount of activity, with numbers. The other is "sit less", with no cut-off. You can meet the weekly minutes and still sit a lot for the rest of the day, and the guideline lists them as different recommendations.

What accelerometers showed

The BMJ study measured activity with devices, not questionnaires. It pooled eight prospective cohorts in which activity and sitting time were measured by accelerometers, then followed participants for deaths from any cause (Ekelund et al., 2019).

The sample: 36,383 people, mean age 62.6 years, 72.8% women. The median follow-up was 5.8 years, with a range from 3.0 to 14.5. During that time 2,149 people died, which is 5.9%. Within each study the participants were split into four equal groups, called quarters, from least to most active, and the groups were compared with the least active one.

Total activity

The hazard ratio for death, with the least active quarter set at 1.00:

The authors describe it as a non-linear dose-response. The largest single step is the first one, from the least active quarter to the next: from 1.00 to 0.48. The step from the second to the third quarter is smaller (0.48 to 0.34), and the one from the third to the fourth smaller again (0.34 to 0.27). The abstract does not say how many minutes or steps separate the quarters, so this article cannot translate "second quarter" into a daily amount.

This is the same shape as the WHO line that some activity is better than none. The steepest part of the curve is at the bottom, among the least active people.

Light and moderate-to-vigorous activity

The paper also split activity by intensity. The ratios for the four quarters:

• Light activity: 1.00, 0.60, 0.44, 0.38.
• Moderate-to-vigorous activity: 1.00, 0.64, 0.55, 0.52.

Both fall across the quarters. The authors' summary is that any physical activity, regardless of intensity, was associated with a lower risk of mortality. The quarters are defined separately for each intensity, so the two lists are not a head-to-head comparison.

Sedentary time

For sedentary time the quarters run the other way, from least to most sedentary:

The risk rises with each quarter and the highest is the most sedentary group, at 2.63. The authors conclude that higher levels of total activity at any intensity, and less sedentary time, are associated with substantially reduced risk of premature mortality, with a non-linear pattern in middle-aged and older adults.

Each study defined its quarters from its own participants, so "most sedentary" does not mean the same number of hours in every cohort.

How far to trust the numbers

• These are observational cohorts. The authors report an association. They do not show that more movement causes the lower mortality. People who are ill already may move less, which would push the same numbers in the same direction. The abstract does not say how the analysis handled that.
• The participants are mostly older. The mean age was 62.6 and the authors' conclusion is about middle-aged and older adults. The numbers say little about a 25-year-old.
• 72.8% were women. The sample is not balanced.
• The outcome is death from any cause. Not fatigue, not mood, not sleep. The abstract does not look at how people felt.
• Follow-up varied. The median was 5.8 years, with a range from 3.0 to 14.5.
• A hazard ratio of 0.27 is a relative number. It compares groups. It is not a promise about any one person, and it says nothing about how many extra years anyone gets.
• This is two sources. One is a guideline, one is a pooled study. They point the same way, but this is not a map of the whole literature.

What nobody knows yet

• Where the sitting threshold is. WHO says the evidence was insufficient to quantify one.
• How the quarters in the BMJ study translate to minutes or steps. The abstract reports quarters only.
• What the specific recommendations for pregnant and postpartum women and for chronic conditions say. They are in the guideline, and this article reads only the abstract.
• Whether the gradient is the same for young adults. The cohorts were mostly older.

What you can look at in your own data

The app reads steps and active energy from Apple Health and sets them against your own sleep quality, looking for patterns in your own history. That shows whether your more active days line up with how you sleep.

It does not measure sedentary time, so it cannot tell you how long you sat. It sets no activity goals and gives no norms, including the WHO numbers above. It does not diagnose. On sleep, see sleep debt: how to actually think about it. On exercise from the other side, see can exercise make you sick?. On weather and energy, see heat, humidity, and fatigue.