A The idea that a working week should occupy five days is younger than most people assume. Until the 1920s a six-day week was standard in British and American industry, and the change to five owed less to enlightenment than to arithmetic: Henry Ford, who cut his factories to forty hours in 1926, had calculated that rested workers made fewer expensive mistakes and that people with a free Saturday were more likely to buy motor cars. A century later, a similar argument is being made for removing a further day — and this time it is being tested at scale.

B Between 2015 and 2023, four-day-week trials were run in Iceland, New Zealand, Japan, Spain, Portugal and the United Kingdom, involving everything from software firms and charities to fish-and-chip shops and a local authority. The largest, coordinated by the campaign group 4 Day Week Global with researchers from several universities, followed sixty-one British organisations and around 2,900 employees over six months in 2022. The design was consistent: staff moved to roughly 80 per cent of their previous hours, kept 100 per cent of their pay, and committed to maintaining 100 per cent of output. That last condition is what distinguishes these experiments from a simple reduction in hours, and it is also what makes their results contested.

C The headline findings were striking. Of the sixty-one organisations in the UK trial, fifty-six chose to continue with the shorter week when the six months ended, and eighteen made the change permanent immediately. Revenue across participating firms was broadly flat when compared with the same period a year earlier, and slightly up when weighted for company size. Resignations fell by more than half. Days lost to sickness fell by about two-thirds. Employees reported lower rates of burnout, better sleep and — a finding that surprised the researchers — a marked reduction in the time women spent on domestic work relative to their partners, because the extra day was distributed more evenly than expected.

D How, though, did the work get done? The trials suggest that the answer lies less in working faster than in working differently. Participating organisations reported cutting the number and length of meetings sharply; several introduced a rule that any meeting must have an agenda circulated in advance or be cancelled. Others adopted “focus blocks” during which internal messaging was switched off entirely. One frequently cited figure from the British trial is that meeting time fell by roughly a quarter without any measurable loss of coordination. Sceptics point out that this is an admission of how much time was previously being wasted, rather than evidence that a four-day week is intrinsically superior; the same efficiencies, they argue, could be captured within five days.

E There are also questions about which organisations the results apply to. The firms that volunteered were self-selecting: small, mostly office-based, and often already flexible in their working practices. Manufacturing, hospitality, health care and retail were thinly represented, and these are precisely the sectors in which output is tied most directly to hours present. A hospital ward cannot cancel its meetings to free up capacity. When the Spanish government funded a manufacturing-focused trial, the reduction in hours had to be accompanied by additional hiring, which shifted the cost from the employee to the state and to the employer rather than eliminating it.

F Nor is the shorter week uniformly popular with those who work it. Some trials operated a “compressed” model, in which the same forty hours were fitted into four longer days. Employees in these schemes reported significantly worse outcomes than those on genuinely reduced hours: longer days made childcare arrangements harder, and the intensity of a ten-hour shift eroded much of the benefit of the extra day off. Where staff were expected to cover for absent colleagues on the fifth day, the “day off” frequently became a day of monitoring email. Researchers now distinguish carefully between reduced-hours models and compressed-hours models, and warn that conflating them makes the evidence look better than it is.

G The longer-term evidence remains thin. Most trials have run for six months, which is long enough to capture the enthusiasm of a new arrangement but not long enough to establish whether productivity gains persist once novelty fades, or whether firms quietly reabsorb the lost day through unpaid work at home. Iceland’s programme, which ran from 2015 to 2019 and eventually covered a large share of the public sector, is the main exception, and its results have held: by 2023 roughly half of Icelandic workers had negotiated shorter hours, and the country’s economic growth continued to outpace most of Europe. Whether an economy of 400,000 people, with an unusual industrial structure, tells us much about larger ones is a fair question.

H What can be said with reasonable confidence is narrower than the headlines suggest, and more interesting. In knowledge work, where output is not proportional to hours at a desk, a well-designed reduction in working time does not appear to reduce what gets produced, and it substantially improves how workers feel. Beyond that, the honest answer is that we do not yet know.

List of Headings (for Questions 14–19)

• i   Sectors where the model may not transfer
• ii   An unexpectedly recent invention
• iii   A version of the idea that workers disliked
• iv   The financial cost to governments
• v   What we can and cannot claim
• vi   Changes to daily working habits
• vii   The measured outcomes
• viii   Why long-run conclusions are difficult
• ix   How the experiments were set up