BIELEFELD, Germany—Lasse Rheingans realized taking time to check Facebook or respond to reply-all emails distracted him from work goals and caused him to spend extra hours at the office rather than with his young daughters.

So when he acquired a small tech consulting firm here in late 2017, he introduced a radical idea: Reduce the workday to five hours, from the standard eight, while leaving worker salaries and vacation time at the same levels.

“They were not sure if I was kidding,” he says. “Some of them thought I was testing them. But yeah, I was being serious.”

At the firm he renamed Rheingans Digital Enabler, the 16 employees start work at 8 a.m. and may leave at 1 p.m. Mr. Rheingans, the firm's managing director, says employees can deliver the same output during a focused 25-hour week as in 40 hours interrupted with distractions.

“We have all experienced that: We sit in the office, out of energy, reading newspapers online or Facebook, just in need of the little pauses to recharge, but you don’t really recharge,” he says. “My idea is focusing on the first five hours and then just leave, and have a proper break.”