If you've ever felt sick when you're stressed or had trouble sleeping when something is on your mind, you know that the thoughts in your head can affect the way your body feels.

This physiological/psychological link is powerful. For example, depression—a condition we usually tag as a mental health issue—can manifest itself in physical symptoms, such as loss of appetite, headaches, or stomach pain. Even more benign conditions such as stress can cause everything from labored breathing to a weakened immune system.

Your brain can help you heal.
The good news is that a positive outlook can make you feel better physically. We have our brains to thank for this! Scientists have found that our brains are responsible for producing various substances that regulate physical health.

For example, when your brain senses pain, it produces endorphins, which are amazingly powerful painkillers. Your brain also makes two other superstar substances: interferon, which can combat infections and viruses, and gamma globulin, which helps strengthen your immune system. Your brain knows what's going on with your body, and when to beef up production of these natural chemicals.

Harness the power of positive thinking.
Here's what is really intriguing: Your brain can't make these and other illness-fighting substances without positive thoughts. So if you're trying to recover from an illness and you have a positive outlook, your brain is more likely to produce the right chemicals to get you on the road to recovery.

By contrast, negative expectations and a defeatist attitude can prevent your brain from making enough of these key substances. Of course, no magic pill exists: Many factors go into healing, and positive thinking is just one.

See the serotonin connection.
Researchers also have spent a lot of time studying serotonin—a neurotransmitter known to regulate mood and keep you in a state of emotional well-being. Lack of serotonin is a contributing factor to depression. Many foods (like bananas) contain serotonin, but you can't get serotonin directly from eating the foods that contain it. Your brain has to produce serotonin as part of a larger chain reaction involving insulin.

In her book, The Serotonin Solution, Judith Wurtman, Ph.D., visiting scientist at the Massachusetts Institute of Technology, argues that eating the right amount of carbohydrates at key points during the day (such as mid-afternoon when your mood tends to dip the lowest) can cause your brain to make more serotonin. Ideal carb-based snacks are pretzels, cereal, mashed potatoes, or a piece of bread. If you eat a low-fat carb snack of about 150 calories when you feel the blahs coming on, Wurtman says your mood can improve within about 30 minutes.