To get fit, you work out regularly. But if you don’t give your body time to rest after strenuous exercise, you’re only upping the chance that you’ll get sick. Lack of sleep and overworked muscles have the same effect on your health, producing an “acidic environment in your body, which creates a breeding ground for viruses and germs to take over,” says Dr. Eliaz. The solution? Give your muscles some down time, and make sure you get 7 to 9 hours of solid sleep a night.