There are three main divisions of the autonomic nervous system (ANS):
- Sympathetic - governs your stressed state which includes fight/flight/freeze reactions.
- Parasympathetic - governs your "at rest" or restorative operations.
- Enteric: governs your gastrointestinal system--not really relevant to the discussion but included for completeness.
When your sympathetic nervous system is dominant, your body automatically makes the following adjustments:
- Dilates the pupil (and inhibits sleep)
- Increases heart rate and force of contraction
- Dilates blood vessels in the heart and brain
- Constricts blood vessels in skeletal muscle and gastrointestinal organs
- Activates sweat secretion
- Inhibits the normal processing of food in the digestive tract
The fact that blood, which carries oxygen and nutrients is constricted to skeletal muscle while you are in a stressed condition explains a great deal about why your performance is impeded. The body is designed to handle short durations of your sympathetic nervous system being dominant, which would coincide with training stress or the stress of a physical job. In fact, if you had no stress you could never get stronger due to the body never receiving any demands that it needs to be stronger.
When the parasympathetic nervous system is dominant, the following changes occur:
- Relaxes the pupil and allows for normal sleep
- Relaxes the blood flow to the heart and brain
- Increases the blood flow to skeletal muscle and the digestive system
- Lowers the heart rate and production force
- Stops sweat secretion
- Re-establishes normal digestive processes
These are all the restorative functions that are necessary to allow your body to repair itself in a stronger state, clear out built up waste products, etc. Your body is designed to have a bit more time in the parasympathetic state particularly during sleep. Sleep is probably one of the most restorative things your body can do.
What does all this mean?
Basically, when you are in a stressed state, your sympathetic nervous system is in control. Practically this means:
- More food goes to fat because it's easier to process
- Muscles are are unable to repair themselves, or perform to their full potential
- You can't sleep well, which has a host of other health related problems.
- Left unchecked, it can escalate to nervousness, paranoia, depression, etc.
When you are in a relaxed state, the parasympathetic nervous system is in control. Practically this means:
- More fat is burned
- Muscle is rebuilt
- You can sleep well, which prevents and corrects a number of health problems.
The important take away is to learn how to get your body to go into the parasympathetic mode as much as possible. S.A.D. (Seasonal Affective Disorder) is something that has to be managed. You may also adjust your expectations for when S.A.D. is in effect and only push yourself when it is done for the year. If it can be managed with light therapy, timed doses of melatonin, etc. then by all means do so.
If you have a generally stressful job, try to find ways to manage the stress naturally if possible.