There's a lot of talk about vacation when summer comes, but that hardly means everyone is able to take time off. Two issues prevent employees in the U.S. from taking necessary vacation time: The environment at some companies discourages it, and many employers do not provide paid vacation time.

The issue of vacation time is no small thing. Mandatory paid vacation will be a subject 2016 presidential candidates will have to address, thanks to Bernie Sanders. The Independent, self-proclaimed democratic socialist told reporters in Washington last week that part of his "Family Values Plan" campaign will be a national mandate of two weeks paid vacation time for every employee in the U.S. (He is also pushing for 12 weeks of paid maternity leave and paid sick leave.)

Politics aside, not taking a vacation hurts employees and hurts the company. Skipping vacation time as a way to climb the corporate ladder faster has been found to be ineffective; a new study has found that employees who take a vacation are more likely to get promoted and get a raise.

Shawn Achor, the bestselling author of The Happiness Advantage and Before Happiness and founder of consulting firm GoodThink, writes in Harvard Business Review about how people who take time off excel at work. Research from Achor and the U.S. Travel Association's Project: Time Off initiative has revealed that Americans are using less vacation time today compared to any point in the last 40 years. That's unfortunate given that vacation helps the entire company--from the employee's mental state and productivity, to the company's bottom line.

Project: Time Off's study found that employees who take all of their vacation time increase their chances of getting promoted and getting a raise by 6.5 percent, compared with people who leave 11 or more days of paid vacation unused. 

But the benefits are not solely for the workers. If your employees take time off, they will perform better. In his book The Happiness Advantage, Achor mentions research that found when "the brain can think positively, productivity improves by 31 percent, sales increase by 37 percent, and creativity and revenues can triple."

After decades of research on the topic, Achor concludes "the greatest competitive advantage in the modern economy is a positive and engaged brain." But to be truly engaged, your brain needs a break, Achor says. If you offer paid vacation, but your environment doesn't support people actually using it, you're actually taking away benefits you promised them.

Achor found that four out of 10 employees say that they can't use paid vacation because they have too much work. But as a leader, you know that happy employees are more productive and collaborative, Achor says. So why would you hamstring your workers' happiness and hurt your bottom line?

Here's to encouraging your employees to take time off and to a more productive and happy workforce that will help your company perform better. Not only does employee retention increase if you encourage them to use all of their vacation time, employees who don't use it can suffer from burnout.

Lastly, if you pay your employees for vacation time they didn't take, you have a big interest in making sure employees use all of their time off before they quit or retire. According to a study conducted by economic analysis firm Oxford Economics (commissioned by U.S. Travel Association), companies on average owe each employee $1,898 in accrued paid time off. The larger the company, the greater the cost per employee; the average for companies with 500 employees or more is $2,609 per employee.