In the office or at home—sitting down is ingrained in most people's days. But staying sedentary for too long can increase the risk of serious health conditions like cardiovascular disease and type 2 diabetes, due in part to vascular dysfunction. My father died at the age of 56 from a coronary heart attack on an flight to South Africa, so since the age of 17 I have been very tuned in to the risks of heart disease. And prolonged sitting is a MASSIVE causal risk.

In 1953, epidemiologist Jeremy Morris discovered that London bus drivers were more than twice as likely as bus conductors to develop coronary heart disease. Demographically (in age, sex, and income range), the two groups of workers were the same, so why was there such a significant difference? Morris found that bus conductors, who were required to be on their feet and regularly climb the steps of London's iconic double-decker buses, had lower rates of heart disease compared to the drivers who remained seated for long stretches. This landmark study laid the groundwork for research on the links between physical activity and coronary health.

Since the Covid-19 pandemic, there has been a significant shift towards working from home, increasing our collective sitting time. Without the trips to the water cooler and the trot from meeting room to meeting room, it's easy to spend hours sitting behind a desk without getting up. Office culture had already changed our working lives so much by the 1980s that some researchers joked that our species had become Homo sedens – the "seated man" – rather than Homo sapiens. Prolonged sitting is a form of sedentary behaviour, characterised by a highly reduced energy expenditure in a seated or reclined position. Typical sedentary behaviours include television-watching, gaming, driving, and desk-bound work. Sedentary behaviours are associated with a higher risk of cardiovascular disease, type 2 diabetes, and premature mortality. The time we spend sitting down has been identified as an independent risk factor in a range of health conditions. In 2020, the World Health Organisation suggested measures to reduce sedentary behaviour.

Since 2010, researchers have pointed out that sedentary behaviour is distinct from a lack of physical activity. You can get sufficient exercise per day and still sit for too long. However, the risks of sedentary behaviour are heightened for those who do not exercise enough.

The primary hypothesis is increased vascular dysfunction, particularly in the legs. The vascular system is responsible for keeping blood and lymph fluid moving through the blood vessels. David Dunstan, a physiologist at Deakin University's Institute for Physical Activity and Nutrition, in Melbourne, Australia, has extensively researched the effects of prolonged sitting and possible interventions. "What characterizes sitting is a reduction in muscular activity," says Dunstan. "If I'm on a chair, the chair is taking all the responsibility there." The combined effect of reduced muscular activity, lower metabolic demand, and gravitational forces decreases peripheral blood flow to the leg muscles, which can lead to blood pooling in the calves. The biomechanics of sitting, with the legs usually bent, can also reduce blood flow.

Researchers suggest that 120-180 minutes of prolonged sitting is the threshold of too long spent seated. But in theory we need to keep moving every 45 minutes, even if that means just standing up for 5 – 10 minutes and walking around for 2 -3 minutes every hour. Reduced muscular activity of the leg muscles reduces their metabolic demand. Metabolic demand is the primary determinant of blood flow, so blood flow in the legs is also reduced. The biomechanics of sitting, with the legs usually bent, can cause blood to pool in the calves – one study of 21 young healthy volunteers saw their calves increase in circumference by nearly 1cm (0.4in) over the course of two hours. This can also reduce blood flow. Normal blood flow provides friction, known as arterial shear stress, against the endothelial cells which line the walls of blood vessels. The endothelium responds to this force and secretes vasodilators, like adenosine, prostacyclin, and nitric oxide, which keep the vessels sufficiently dilated and maintain the ability of the vascular system to regulate itself – known as homeostasis.

Reduced blood flow, however, reduces shear stress, and the endothelium produces vasoconstrictors like endothelin-1 which cause the blood vessels to narrow. In a vicious cycle, vasoconstriction further reduces blood flow, and blood pressure rises to keep the blood moving. High blood pressure, or hypertension, is one of the predominant risk factors for cardiovascular disease.

While the underlying mechanisms are hypothetical, recent studies support the theory. A study with 16 young, healthy men found that sitting for three-hour periods increased blood pooling in the legs, peripheral vascular resistance, diastolic blood pressure, and leg circumference. Another study finds that blood pressure increases with time spent sitting uninterrupted. Researchers generally agree that 120-180 minutes of uninterrupted sitting is probably the threshold at which you have probably spent too long in a seated position, but vascular dysfunction generally increases with time spent sitting.

Prolonged sitting also contributes to reduced muscle strength, lower bone density, and increased total and visceral fat in adipose tissue. Additionally, prolonged sitting is associated with physical discomfort, stress at work, higher depression, and can even lead to pressure ulcers.
Dunstan, who specializes in researching type 2 diabetes, also notes that sedentary behaviour increases post-meal, or post-prandial, rises in blood glucose and insulin. Impaired insulin sensitivity and impaired vascular function both contribute to a higher risk of cardiovascular disease and type 2 diabetes.

"I think people are becoming more sedentary because it's what society has encouraged," says Benjamin Gardner, a social psychologist specialising in habitual behaviour at the University of Surrey. "It's not that anyone's deliberately pushing it. It's just as things get more efficient, we don't have to move around so much." Some researchers believe we need more social factors to make it easier for people to break up the time they spend sitting. Interventions include height-adjustable workstations, sit/stand chairs, treadmill workstations, and fidgeting the legs which enhances blood flow.

Yo-Yo WELLNESS is a Smart Sit-Stand desk which will allows the user to set heights and goals and will double the health benefits of a sit-stand desk. Wearable technology may also help nudge us into action. In a promising new study, wearable devices called accelerometers provided 24-hour data on individual behaviour patterns, including sitting, standing, sleeping, and exercising. This potentially allows for tailored optimal sitting and standing times, with devices then sending automatic reminders whenever we sit for too long. Above all, Gardner and colleagues encourage moving between sitting and standing positions more frequently. The premise of breaking up sedentary time by just standing up is simple but has significant health benefits, particularly for low-activity individuals. For many, sedentary behaviour can seem like an unavoidable consequence of modern life and work. But even small changes to your routine – be it stretching more, fidgeting, or standing up to make a cup of tea – can help break your sitting habit.

For more detailed insights and references from medical research, check out the original article on BBC: Why you are probably sitting down for too long.