[Greece Best Islands Tinos Mykonos Santorini (18 of 33)]Harrison Jacobs/Business Insider It isn't always easy to carve out time for a vacation. The average American worker takes just eight days a year, according to 2015 figures from the US Department of Labor. That would have been unimaginable for a medieval peasant, who took anywhere from eight weeks to half the year off. The short breaks mean that Americans are donating an average of $604 in work time to their employers every year in unused vacation time that can't be rolled over or paid out, according to a 2017 estimate from the US Travel Association. Nearly half (49%) of American workers said they can't vacation because their workloads are too heavy, according to a 2016 survey from NPR, Harvard, and the Robert Wood Johnson Foundation. And 42% of those survey respondents said they felt they couldn't take all their paid vacation days because there weren't enough people to cover for them. But there are some signs Americans are starting to change their ways: a 2018 report from the US Travel Association suggests that after two decades of declines in the number of vacation days workers took per year, employees who work more than 35 hours a week and receive paid time off are now starting to both earn and use up more allotted holiday time than they did a few years back. In 2017, vacationers spent an an extra half day away from their desks on average than they did in 2016, according to the report. Although it can feel tough to get away from the office, we're likely not doing ourselves any favors by working ourselves into the ground. There are real, important benefits to a restful vacation. Here's what vacation can do for your body and your brain, according to research. THE OPPOSITE OF VACATION — OVERWORK — CAN LEAD TO HEALTH PROBLEMS. RESEARCHERS STUDIED THE WORK HABITS OF MORE THAN 600,000 PEOPLE IN THE US, UK, AND AUSTRALIA, AND FOUND THAT PEOPLE WHO WORK MORE THAN 55 HOURS A WEEK ARE 33% MORE LIKELY TO SUFFER A STROKE AND HAVE A 13% GREATER RISK OF HEART ATTACK THAN THOSE WHO WORK 35-40 HOURS WEEKLY. Dragon Images/Shutterstock Source: _The Lancet_ ECONOMISTS HAVE EVEN CALCULATED THE MOST PRODUCTIVE AND EFFICIENT SCHEDULES FOR FACTORY WORKERS, AND FOUND THAT WORKING MUCH MORE THAN 48 HOURS A WEEK TENDS TO MAKE PRODUCTIVITY PLUMMET. Harrison Jacobs/Business Insider Economist John Pencavel from Stanford says that calculation would vary by a bit for other professions, though. CLINICAL PSYCHOLOGIST DEBORAH MULHERN TOLD ABC THAT WHEN YOU DON'T UNWIND AND GET AWAY FROM "EXTERNAL STRESSES" (LIKE A HEAVY WORKLOAD), IT GETS HARDER FOR YOUR BODY TO RELAX. Nattakorn_Maneerat/Shutterstock "What neuroscience is showing is that we require down time in order for our bodies to go through the process of restoration," she told ABC News. "Without time and opportunity to do this, the neural connections that produce feelings of calm and peacefulness become weaker, making it actually more difficult to shift into less-stressed modes." SEE THE REST OF THE STORY AT BUSINESS INSIDER SEE ALSO: * We took a scientific look at whether weed or alcohol is worse for you — and there appears to be a winner * Gun control really works — here's the science to prove it * 11 foods with a bad reputation that you can feel good about eating — including coffee, butter, and cheese SEE ALSO: A HARVARD DOCTOR SAYS IT'S HARDER THAN EVER TO LOSE WEIGHT RIGHT NOW — BUT THERE ARE 5 WAYS TO DO IT WELL