How many vacation days do you have left this year? If it’s more than you can take in the two months remaining before year’s end, you’re hardly alone. Americans leave 429 million vacation days unused every year. In fact, as a nation we’re taking a week’s less vacation annually than we did 15 years ago. That’s the sobering research out of Project: Time Off, a group that is trying to persuade Americans that sufficient time away from work is vital to their health. Should you happen to use that time off to travel, you’ll likely boost your health and happiness even more. Here are five reasons why: 1. Planning a vacation increases your sense of well-being. A 2002 study by professors at the University of Surrey in the United Kingdom found that we are happiest when we have a trip coming up. People who have a vacation on the horizon have a greater sense of well-being, and feel better about their health (as well as their family life and economic situation), than people who don’t. In fact,