Why not all plans for a four-day working week would be a win for health

The right to request a short working week, with four longer “shifts” and three days off is being proposed as part of new flexible working legislation^[1] in the UK. Also known as working “compressed hours”, this schedule can sound attractive^[2], with reports claiming improved efficiency and productivity^[3]. And, of course, no pay cut for workers.

It could result in fewer commutes, which saves time for workers and can be more environmentally friendly. And it could provide more flexibility for workers with childcare or care for other dependants, for example.

But there could be negative consequences to squeezing typical workloads into fewer days. Under these plans, there is no suggestion that by compressing the working week, people will work fewer hours.

Compressed hours mean that, instead of working 7.5 hours a day for five days, you would work 9.4 hours per day for four days – putting in almost two hours more work every working day. There is strong evidence that longer work hours^[4] result in more errors^[5] and accidents^[6]. Long work hours are also linked to poorer decision-making^[7] and make it more likely people will have an accident on their drive home^[8].

For example, it has long been understood that working longer shifts increases the risk of workplace accident and injuries^[9]. The risk of a workplace accident is on average 13% higher for a ten-hour shift than an eight-hour shift.

Accident risk remains more or less constant for the first eight or so hours of work but then rises rapidly, so that the risk of an accident in the tenth hour of work is 90% higher than in the first eight hours^[10].

To function effectively and safely at work relies on sufficient sleep, ideally at the right time of day and in a regular pattern. This is based on fundamental physiological factors that cannot be changed by training, motivation or professionalism.

These factors that determine our ability to function are driven by time of day, how long we have been awake and accumulated sleep debt^[11]. For example, humans are sleepier during the night than the day, and it can take between two and four hours after waking to achieve full alertness.

What’s more, our ability to function decreases rapidly after we have been awake for 16 hours^[12], and especially so at night.

But what are the health consequences of a compressed hours schedule? It is already commonplace for people to have shorter periods of sleep during the working week and then try to catch up with sleep at the weekend, with mixed results.

If people work compressed hours, then on working days they have to fit in two extra hours of work but still carry out all the other activities in their daily lives. They still need to wash, eat, communicate, provide care for children and others.

So there’s a real chance that compressed hours then also lead to “compressed sleep” and accentuate irregular patterns of rest or chronic sleep debt. Irregular or insufficient sleep is increasingly associated with a higher risk^[13] of diabetes, cardiovascular disease, obesity, certain cancers and dementia – the leading causes of mortality in wealthy nations. In 2017, the economic cost^[14] of insufficient sleep in the UK alone was estimated as US$50 billion (£40 billion), up to 1.86% of GDP.

Read more: The science behind why you love a weekend lie-in^[15]

The negative effect of chronic sleep loss^[16] accumulates more rapidly than experts previously realised. This knock-on effect is most severe during night shifts, especially when those shifts are long^[17]. There are good reasons why the UK regulator, the Health and Safety Executive^[18], supports the EU working time directive^[19], which imposes constraints on the length, timing and number of shifts.

If the concept of fewer but longer work shifts is accepted, what happens next? Why not propose three 12.5-hour workdays a week, or two 18.75-hour workdays? Why not work 24 hours a day and then work only eight days a month?