Until recently, four-day week evidence leaned heavily on firms reporting about themselves. A study published in Nature Human Behaviour in 2025 strengthened the base: 2,896 employees in 141 organisations across six countries, compared with twelve control companies, followed through a structured trial. Its central finding is the one leaders should pin above their desks: gains tracked how much hours actually fell.
Dose matters
Average working time fell from about 39.1 to 34.5 hours a week. Burnout fell and job satisfaction, mental health and physical health improved relative to controls. People whose hours fell more gained more, with effects linked through sleep, fatigue and self-rated ability to work. Around a quarter of participants reported little change in hours, and their results underline the point: a relabelled week buys little.
This is why Day5Group insists on counting actual hours, including after-hours messages and quiet overtime. Token versions, where Friday becomes unofficial catch-up, predictably produce token benefits while still costing goodwill.
What it does not prove
Participating firms volunteered, most evidence remains survey-based, and objective output per hour across whole cohorts is still thin. The study strengthens the wellbeing case and the maintenance case for output; it does not license miracle productivity claims. Twelve-month follow-up in the wider programme kept gains above baseline, which argues against pure novelty, but independent data beyond two years remain limited.
Design accordingly: make the hour reduction real, redesign work so it stays real, survey at baseline, midpoint and end with a validated short scale, and publish your own twelve-month follow-up. That publication is how the whole field, your firm included, gets better evidence.
Sources
- Fan, Schor, Kelly and Gu (2025), Nature Human Behaviour; reported by Becker’s Hospital Review and PsyPost (2025).
- Autonomy / Boston College pilot syntheses (2022-25) on realised hours reductions.
- Day5Group research brief (2026), evidence-quality and measurement sections.
Day5Group is a consultancy that assists in the transition to a 4 day work week. More about Day5Group · All blog articles