Do you use all of your paid vacation time? If you're like most Americans, the answer is no. In the annual Alamo Rent A Car Family Vacation Survey released earlier this month, only 47 percent of the 2,100 respondents say they take all the vacation time they're entitled to. This is the first time that number has fallen below the halfway mark. With Americans already entitled to less vacation time than employees in most Western countries, that's bad news.

Even more depressing is why we aren't taking vacations: Our colleagues make us feel bad when we do. The study showed "vacation shaming" on the rise, with 49 percent saying their co-workers make them feel guilty for taking time off and letting others pick up the slack. And things are likely to get worse as more Millennials join the work force. More than two thirds of Millennials say they've felt guilty about going on vacation, and 40 percent say that guilt has made them feel bad enough to stop them from going. That makes them more than twice as likely to skip vacations out of guilt than their older colleagues. Just to make matters worse, Millennials are also twice as likely as their older peers to deliberately shame someone else out of taking a vacation. Peers aren't the only problem. Nearly half the respondents said they feel they must justify taking vacation time to their employers--even though they've earned it.

And it gets worse. When we do take vacation time, only 18 percent of us are using all that time for an actual vacation. Most of us spend at least some of our vacation days running errands or staying home and getting stuff done around the house. When we do manage to go on vacation, we aren't really gone from work. Nearly two thirds of respondents say they pressure themselves into working while on vacation, either because they don't want to come home to a mountain of work or because they like feeling needed. Disturbingly, 23 percent say they work or at least "check in" while on vacation because their employer expects them to.

The problem with all this is that skipping your vacation makes no sense at all. Neither does pressuring other people to skip theirs. Here's just some of what can happen if you do:

1. You'll be less productive.

You should consider this before you pressure employees out of taking their vacation time, or pressure them into working while they do. The human brain is not designed to work all the time, which is why taking at least one day completely work-free every week is important for both brain health and productivity. Similarly, taking time away from work, at least a week, at least once or twice a year, will drive better productivity the rest of the time. It's especially important for people whose jobs depend on their creativity or strategic skills, since most people do their best thinking about the big picture of their jobs when they get away from the office for a while.

2. You'll be no fun to be around.

A burned-out, exhausted, grumpy person should be no one's idea of a good co-worker or employee. That's why I'm mystified by bosses and colleagues who deliberately discourage others from taking the vacation time they're entitled to. Do these people really enjoy having their miserable co-workers souring the air at every meeting? Apparently they do.

3. Your relationships will suffer.

If exhausted colleagues are tough to work with, ones who are going through a breakup, divorce, or rocky patch in their home lives are even tougher, and surveys show their work performance suffers. Every good relationship needs the partners to fully focus on each other from time to time, and vacations are often the only time to do that, which is why research shows that people who take vacations have better relationships than those who don't.

4. You could die.

No, seriously. Skipping vacations can be very bad for your health. People who take at least one vacation a year are likelier to rate their health as "good" or "very good" than those who don't. And taking more than one vacation a year has been shown to reduce the risk of heart disease by 30 percent in men and 50 percent in women.

So start planning that summer getaway. It could be a real lifesaver.