Physical activity supports healthy bones, organs and mental health. Time and time again, research has proven the health benefits of exercise.

But you don't often hear about the effects of not exercising. When you abruptly reduce your physical activity, your body undergoes some biological changes — both quickly and in the long term.

While rest days offer everyone a chance to relax and recharge, permanent sedentary living can negatively impact your health in many ways. Here are some of the changes you could experience by not getting enough activity:

• Within the first few days: Your active heart rate may increase and you may lose some endurance.
• Within the first weeks: The body starts to undergo biological changes in muscle size that can lead to weight gain.
• Over the long-term: Physical inactivity can lead to greater risks for major health problems, from heart disease and diabetes to early death.

Effects of Not Exercising

When you exercise, you're essentially sending your muscles a message that they're needed in a new and different way, and the only way to satisfy that need is to grow bigger and stronger. If you've ever woken up sore the morning after a tough workout, you've experienced this firsthand. That soreness, called delayed onset muscle soreness (DOMS), occurs when the muscles heal and rebuild to prepare for future workouts.

However, if you skip those future workouts, the muscles don't need the increased size and strength after all. They've essentially gotten big for no reason, so they shrink back down to what they were before.

Your Body Responds

As muscle cells get smaller, fat cells tend to get bigger. After all, the muscles aren't working hard enough to burn away calories. This can lead to weight gain as soon as 14 days (or earlier) once you stop exercising, according to Men's Journal.

Over time, those extra pounds can cause many problems throughout your body. The increase forces your bones to carry extra weight and puts more pressure on your lungs and heart to supply blood and oxygen throughout your body.

Without exercise, you may also miss out on the mood-boosting benefit of endorphins! You might feel like you're in a fog or just otherwise tired, unmotivated, stressed or down.

Regular Exercise Can Prevent This

Clearly, physical inactivity can wreak havoc on your health — but you can reverse the effects by staying active! In fact, research that was published in the journal Circulation suggests that even if you're in your 50s or 60s and have been inactive all your life, you can reverse the negative effects of a sedentary lifestyle with just two years of regular exercise.

To reclaim what you might have lost due to inactivity, experts recommend starting out with 30 minutes of physical activity per day for five days a week. You can always work your way up to more challenging workouts if you wish — often, a brisk walk around the park or trying out a new dance class will do the trick. If you're desk-bound at work, a number of mobility workouts can help.

Switch Things Up

Keep in mind that it's also about building activity into your routine, which may take time to get used to at first. For example, try parking farther away when you go to the grocery store so that you get the benefit of a longer walk or opt for the stairs instead of the elevator when possible.

Whatever you do, know that your muscles will get the message after just one workout. But to glean all the health benefits of exercise, make sure to keep the momentum up so that your body doesn't go back to its baseline. Your health in the short and long term depends on it!