Your autonomic nervous system-the part of your nervous system responsible for involuntary/vital functions such as the beating of your heart, digestion, elimination and so on- is divided into two branches: The sympathetic and the parasympathetic branches. The sympathetic branch is responsible for "fight or flight" or exerting responses. Activation of the sympathetic nervous system increases your heartbeat and constricts your blood vessels thus making your blood flow faster. Your muscles are tensed, reflexes are sped up and digestion and elimination are slowed or halted completely. In essence, all of the primeval responses related to defending your life or running from danger are activated. Conversely, the parasympathetic branch is responsible for "rest and recovery" or relaxation responses. Activation of the parasympathetic nervous system decreases your heartbeat, relaxes your blood vessels and your muscles thus allowing blood to bring nutrients and carry waste away from your cells. Your digestive and elimination systems are stimulated and your breathing slowed down. Basically all the normal bodily responses associated with recovering, resting and "essential activities" you do when your life was not in danger.
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.
It is also vital that you are in the right mode when performing an activity. It is not in your best interest to be in a stressed state when eating or sleeping. Those are "parasympathetic activities", and you should be in a "parasympathetic mode" when engaging in them. On the other hand, if you overly tired, you won't have a productive time at work or will probably not have the best workout.