We live in an age of increased workplace stress (not to mention societal stress in the form of newspaper headlines), and burnout has now been recognized in the International Classification of Diseases as an occupational phenomenon.
For each individual business, it will depend on whether a 4-day work week means reducing hours, or simply compacting the same hours into less time. Some research has shown that workplaces can maintain the same level of productivity with staff working 32 hours a week as they do with teams working a full 40 hours.
Employers have the duty to protect their workers and provide a healthy and safe work environment. Health & safety is about both physical and mental health.