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Ever feel guilty for using your vacation days — even when you’ve earned them? You’re not alone. “People assume this is the way Americans have always been, like it’s just who we are, it’s part of our work ethic,” Katie Denis, senior program director for Project: Time Off, told Fox News. “We’ve always had a strong work ethic, but we have not always been work martyrs.”

Americans took an average of about 20 vacation days a year from the 1970s to 2000. That number started to decline and fell to 16.2 days in 2015, according to research from Project: Time Off.

Leaving vacation days on the table, which 55 percent of people do, can take a toll on your health, both mentally and physically, Cynthia Thaik, MD, a holistic cardiologist and health practitioner in Los Angeles, told Fox News.

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One well-known study from State University of New York found men between the ages of 35 and 57 who were at risk for heart disease and didn’t take a week-long vacation every year were about 30 percent more likely to die of heart disease. Both Thaik and Denis said a study with women would have similar results.

What’s more, not taking vacation leads to increased stress, blood pressure and cortisol levels, which can increase inflammation and decrease immunity, Thaik said. “There’s also lack of mental focus and clarity, and increased depression associated with not vacationing,” she said. So although it may seem counterintuitive, vacations could actually help you be more productive because you’ll be less likely to call in sick and experience job burnout, Denis said.

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There’s no hard-and-fast rule for how often you need to take a vacation, but Thaik said you’ll see a benefit from as little as one week off each year. Follow your gut to decide when you’re due. “The thing we hear again and again is feeling stress is stress,” Denis said. “There is some good stress, but we’re talking about when it’s gotten so out of hand that you feel you don’t have control over your life anymore and it’s starting to have negative effects on your body.”

Take that feeling as a sign you need to get away — and keep in mind it doesn’t need to include expensive airfare to a swanky beach resort. “We say we’re agnostic about how people use their time,” Denis said. “There’s really no one way to vacation.”

Maybe you need to leave your hometown to really disconnect, or maybe simply putting up your email away message and spending a leisurely week at home will do the trick. “A vacation is something that gives you a break and a breather, and the rest that you need to come back and be recharged, be happy, be productive, and be less stressed out,” Denis said.

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Try to take it even further and cut back on technology (assuming the thought of ignoring your iPhone for seven days doesn’t just add to your stress), Thaik suggested. “The key is that you really decompress and get away from the mental stress of your work and the pressures and demands of that work.”

You’ll come back to the office with a boost to your health, focus, and happiness — and maybe even a subtle tan.

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