It's December. A lot of U.S. workers will end this year without having taken all of their vacation days.

Explanations usually go something like this:

"I'm too busy."

"My boss never takes any time and he'll think I'm a slacker if I do."

"I feel guilty leaving colleagues in the lurch."

But at some companies such self-denial is not acceptable.

They actually require everyone -- including the bosses -- to take their vacation time.

Balsamiq, which makes tools to create easy-to-use software, technically has an "unlimited vacation" policy but found its U.S. employees weren't taking enough time off. So Giacomo Guilizzoni, the founder, changed that.

"We expect people to take at least 20 days of vacation, in addition to 9 national holidays," said Guillizzoni, who along with half the company is based in Italy, where vacation days are "written into the contract, and pretty much sacred."

There are no formal consequences if someone doesn't toe the line. But the mandatory minimum policy does more than alleviate guilt associated with taking time off. "It instills an opposite kind of guilt: 'If I don't take my 20 days I will set a bad example for my team,'" he said.

App maker Travis CI has been experimenting with mandatory vacation for the past year, with largely positive results.

"The entire team is much more attuned now to encourage each other to spend time away from work [and] take a proper vacation (not just an extended weekend)," said CEO Mathias Meyer.

Related: Netflix CEO: I take 6 weeks of vacation each year

Meyer noted that as of Dec. 1, some employees still hadn't taken all their time yet, and are doing so now. But next year he wants his teams to have "more regular conversations about planning out vacation time to make sure that time off is a regular thing rather than something that's only happening at the end of the year."

Also experimenting with a partial form of mandatory vacation is jobs site Anthology (formerly Poachable). On top of major holidays, employees get two weeks of paid vacation plus a bank of 5 extra days.

The rule is that if you haven't taken any vacation in the past three months, you must take at least one of your 5 extra days. And by "take off" they mean scram.

"If we see you online, we won't reply to you. It's serious," said CEO Tom Leung. Apparently, they won't let you into the office either.

Leung, who had been told by an employee once that that employee felt guilty taking time when Leung worked so hard, equates his new policy to the FAA "time out" rule for airline pilots when they've flown too many hours at a stretch.

Another jobs site, Authentic Jobs, which only has three employees, is also trying its hand at mandatory vacation. The minimum required number of days that must be taken: 15 on top of the 12 company holidays.

Founder Cameron Moll was inspired to institute the policy when one of his employees was running himself into the ground. Not taking time off contributed to the stress, Moll said.

At Groove, which makes simplified support software for small businesses, CEO Alex Turnbull wrote a one-sentence vacation policy: "You must take time off."

There's no minimum or maximum at the 10-person company, where everyone works remotely.

But Turnbull sets the tone by taking time off and letting his team know it -- typically two weeks of uninterrupted vacation time per year plus another week away that's more of a working vacation.

"That builds a culture where people know that time off is encouraged, and ... they don't feel scared or guilty to do the same," he said.

And the business is better off when they do.

"I can usually feel when someone is burning out. Productivity suffers, enthusiasm drops and the whole team is impacted as a result," Turnbull said. "Employees not being able to recharge isn't just a personal problem, it's a company problem."