In the modern world, the sympathetic nervous system is related to activities that require your mental or physical alertness such as work and exercise. The parasympathetic is related to more relaxing activities such as sleeping, eating, and watching TV. We never have only one system working because both are necessary in order for the body to function. Proper balance between the two is the key to good health.
People who predominantly activate their sympathetic (Type A personality) tend to suffer from stress related symptoms such as heart problems, high blood pressure and insomnia. People who predominantly activate their parasympathetic (Type B personality) may experience depression, lack of motivation/ambition, and a weakened immune system.
With the constant pressures of today's society, it is often challenging to maintain proper balance between these systems. Your job may keep you in sympathetic mode. When you are sleeping, you are obviously in a parasympathetic mode. You must consciously balance your activities so that when one system becomes over stimulated, you switch to an activity that stimulates the other system.
For example, at work, it is important that you take breaks throughout the day and relax. On the weekends, you should avoid sleeping or zoning out in front of the TV the entire day (Sorry guys, but spending the entire Sunday watching sports is a very unhealthy habit). Instead try to do something relaxing, but active, such as playing recreational sports, going for a hike, fixing things around the house or even romping with your kids.