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We all know we need to be more active, but there is increasing evidence that we need to spend less time sitting down as well.
Research has suggested that remaining seated for too long is bad for your health, regardless of how much exercise you do.
Studies have linked excessive sitting with being overweight and obese, type 2 diabetes, some types of cancer, and premature death.
Prolonged sitting is thought to slow the metabolism, which affects the body's ability to regulate blood sugar, blood pressure and break down body fat.
Many adults in the UK spend more than seven hours a day sitting or lying, and this typically increases with age to 10 hours or more.
This includes watching TV, using a computer, reading, doing homework, travelling by car, bus or train ‐ behaviours referred to as sedentary ‐ but does not include sleeping.
Experts believe there is something specific about the act of sitting or lying for too long that is bad for our health.
One of the largest pieces of research to date on the subject ‐ involving almost 800,000 people ‐ found that, compared with those who sat the least, people who sat the longest had a:
The strength of the evidence is such that the government issued new recommendations in 2011 on minimising sitting for different age groups.
The Start Active, Stay Active report (PDF, 1.34Mb) recommends breaking up long periods of sitting time with "shorter bouts of activity for just one to two minutes".
A panel of leading experts (PDF, 964kb) who reviewed the evidence on sitting for the report recommended taking "an active break from sitting every 30 minutes".
The advice applies to everyone, even people who exercise regularly, because too much sitting is now recognised as an independent risk factor for ill health.
Professor Stuart Biddle, who led the national guidelines on reducing sitting, now at Victoria University, Australia, says people who take regular exercise may still be broadly sedentary.
"If someone goes to the gym or walks for 30 to 45 minutes a day, but sits down the rest of the time, then they are still described as having a 'sedentary lifestyle'.
"All-day movement is now seen as being just as important for the maintenance of good health as traditional exercise."
The advice is clear: to reduce our risk of ill health from inactivity, we are advised to exercise regularly ‐ at least 150 minutes a week‐ as well as reduce time spent sitting or lying.
However, there is currently not enough evidence to set a time limit on how much time people should sit each day.
"At the moment, we don't know if a one size fits all approach is appropriate," says Professor David Dunstan of the Baker IDI Heart and Diabetes Institute, Melbourne, Australia.
"For example, it is unclear whether the advice for someone who is overweight or obese may need to be different for someone who is leaner."
Nevertheless, some countries, such as Australia, the US and Finland, have made recommendations for how long children should sit, typically one to two hours a day.
The link between illness and sitting first emerged in the 1950s, when researchers found London bus drivers were twice as likely to have heart attacks as their bus conductor colleagues.
There has been an explosion of research on the ills of sitting in the past few years, prompted by our increasingly sedentary lifestyles.
It is thought excessive sitting slows the metabolism ‐ which affects our ability to regulate blood sugar and blood pressure, and metabolise fat ‐ and may cause weaker muscles and bones.
"Essentially, the body is 'shutting down' while sitting and there is little muscle activity," says Professor Biddle.
Current thinking is still shaped by research on astronauts in the early 70s, which found life in zero gravity was linked with accelerated bone and muscle loss and ageing.
"Sitting for an extended period of time is thought to simulate, albeit to a lesser degree, the effects of weightlessness on astronauts," says Professor Biddle.
Most of the evidence is based on observational studies, which have only shown an association between sitting and ill health but not a direct cause.
Other limitations with current research are that many studies rely on self-reported sitting time and don't always account for other contributing factors, such as smoking, alcohol and diet.
"With the current body of evidence, we don't have a definitive answer to what's happening," says Professor Dunstan. "We're now expanding on what's seen in observational research in the lab."
The research on NASA astronauts suggests that on their return from space, even light walking was effective in overcoming the negative effects of weightlessness.
"Breaking up sitting time engages your muscles and bones, and gives all our bodily functions a boost ‐ a bit like revving a car's engine," says Professor Dunstan.
The recommendations apply to all age groups and should be considered in tandem with age-specific advice for increasing physical activity.
Under-5s
In children under five, the advice is to limit the time they spend watching TV, travelling by car, bus or train, or being strapped into a buggy.
"There is emerging evidence that sedentary behaviour in the early years is associated with overweight and obesity, as well as lower cognitive development," says the Start Active, Stay Active report.
While this may be a challenge for busy parents, the advice reflects growing awareness that early life experiences and habits impact upon our health as adults.
"There is a need to establish healthy patterns of behaviour during the early years in order to protect against possible health detriments in the future," says the report.
Tips to reduce sitting time:
Children and young people
Research suggests that children and young people in households with multiple TVs and computers tend to sit more.
For children aged 5 to 18 years, reducing sitting time includes anything that involves moving in and around the home, classroom or community.
Tips to reduce sitting time:
Parents could lead by example by also reducing their TV time and other sitting-based tasks.
Adults
Adults aged 19 to 64 are advised to try to sit down less throughout the day, including at work, when travelling and at home.
Tips to reduce sitting time:
Older adults
Some older adults (aged 65 and over) are known to spend 10 hours or more each day sitting or lying down, making them the most sedentary population group.
"It could be partly due to reduced functionality or ill health, but there are also social norms expecting those in later years to 'slow down' and rest," says Professor Biddle. "That's not helpful."
Older adults should aim to minimise the time they spend in extended periods of sitting each day.
"Sitting needs breaking up," says Professor Biddle. "Long periods of TV should be avoided, and you should try to do activities that involve light movement and being 'on your feet' as much as possible.
"Do some tasks standing, like having coffee and chats, or even writing a letter ‐ Ernest Hemingway wrote his novels standing."
Tips to reduce sitting time:
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