I've been reading a lot lately about the autonomic nervous system (i.e. Sympathetic and Parasympathetic systems), and have noticed many interesting tie-ins with paleo theories.
The Parasympathetic system modulates:
- sexual arousal;
- lacrimation (tears);
- digestion; and
- defecation 
It is also associated with a "plan and decide" response that engages the prefrontal cortex, in contrast to the sympathetic nervous system, which is associated with a "fight or flight" response that shunts energy away from the prefrontal cortex and other "rest and digest" activities.
SHOULD WE VIEW HEALTH ISSUES MORE THROUGH AN AUTONOMIC NERVOUS SYSTEM LENS?
GERD and Heart Palpitations
I've seen a number of paleo folks ask questions about issues that appear to be related to autonomic nervous system imbalance, primarily sympathetic system dominance. One example is GERD and heart palpitations, which I believe to often be related to each other via vagus nerve interactions . Solid advice is often dispensed related to electrolyte balance, etc., but nobody mentions the ANS. Why? See Footnote 4 for more information on the ANS and heart issues.
Another common topic is constipation and digestive troubles. Often the advice is to increase carb intake, eat fermented foods, decrease FODMAPS, etc. This is all great advice, but what about the ANS? Should we also advise interventions that focus on restoring ANS balance? For example, for me, nothing gets the bowels moving like a nice long walk. Also, from a GERD perspective, research is emerging that indicates that GERD may be a function of ANS imbalance. Refer to the following paper, which also discusses Heart Rate Variability, which I will introduce below.
Sex and Testosterone
Further, what of all the questions about sex and testosterone? Yes, adequate saturated fats/cholesterol, weight lifting and sleep is important, but why don't we move higher upstream, and start with the brain and autonomic nervous system? You can squat twice a weak and follow it up with steak and eggs, but if you're chronically stressed and screwing up your ANS, I doubt testosterone levels will be optimized.
Everyone's favorite low carb surgeon, Dr. Kruse, uses sweating as a proxy for leptin sensitivity. If sweating is driven by the parasympathetic system, it would stand to reason that sweating dysfunction would also indicate ANS dysfunction, specifically sympathetic nervous system dominance. Is it possible that the "leptin reset" is just as much an "autonomic nervous system" reset?
Still on Dr. Kruse, what about his favorite health intervention, cold therapy? The mainstream use of cold therapy is post-exercise recovery. This makes sense, as cold water immersion appears to stimulate the parasympathetic system pretty effectively. Some studies have also indicated that cold water immersion improves heart rate variability, introduced next. Is it possible that cold therapy is as simple as balancing the ANS, rather than all of the complicating craziness that Dr. Kruse espouses?
Heart Rate Variability ("HRV"), which I've seen mentioned occasionally by Art Devany and Robb Wolf, is an objective physiological measure that seems to correlates well with a number of interesting outcomes, such as:
-Willpower  (high HRV is positively associated with willpower; more accurately, the ability to engage the prefrontal cortex to "plan and decide" in alignment with our long term goals, rather than immediate desires)
-Heart rhythm irregularities  (low HRV correlated with heart rhythm irregularities)
-Heart Rate Recovery ("HRR")  (low HRV correlates with low HRR). Incidentally, Heart Rate Recovery (specifically, lack thereof), is associated with increased incidence of cardiac disease, especially following cardiac surgical procedures 
Sympathetic system dominance (i.e. chronic stress; sound familiar?) tends to depress HRV. Interestingly, in the weeks and months following 9/11, researchers noticed a pervasive decrease in HRV in the Americans they studied .
So how do we increase Heart Rate Variability? Not so coincidentally, paleo principles help. Among them:
-Intermittent Fasting 
-Art Devany/Mark Sisson style workouts (i.e. lots of low intensity movement, with intermittent high intensity workouts comprising sprints/kettlebells/weight lifting) 
-Diaphragmatic/belly breathing at a rate of 4-6 breaths per minute 
So this leads me back to my question- does Paleo pay enough attention to the Autonomic Nervous System? As Paleo continues to move mainstream, we should continue to look for objective ways to quantify and scientifically corroborate the health benefits we claim, as Mat Lalonde has prominently urged. Perhaps Heart Rate Variability and discussions of the Autonomic Nervous System should play a more prominent role in our methodology.
Selfishly, I hope this question will spur some dialogue on the topic, as I find it incredibly interesting and you all are a heck of a lot smarter than I!
 The Dorsal nucleus of the vagus nerve sends parasympathetic output to the viscera, especially the intestines; the Nucleus ambiguus of the vagus nerve sends parasympathetic output to the heart (see http://en.wikipedia.org/wiki/Vagus_nerve)
 See Wait: The Art and Science of Delay (http://www.amazon.com/Wait-The-Art-Science-Delay/dp/1610390040) and The Willpower Instinct (http://www.amazon.com/Willpower-Instinct-Self-Control-Works-Matters/dp/1583334386/ref=sr_1_1?s=books&ie=UTF8&qid=1344101881&sr=1-1&keywords=the+willpower+instinct)
"Vikman et al. calculated the approximate entropy (ApEn) and short term scaling exponent, α1, of heart rate variability (HRV) over 20-min periods and found that a reduced complexity of HRV dynamics and altered fractal properties usually precede the onset of [Paraoxysmal Atrial Fibrillation] as indicated by decreasing value for ApEn and α1."
"Our data show that among ARVC/D patients HRV analysis might be helpful in identifying those with increased risk of major arrhythmic events."
MY NOTE: This study examined patients with existing heart issues; they were not individuals. Extrapolate to healthy individuals only as you feel comfortable.
 Don't have time to source this one, but pubmed has some good studies in this area.
 Supra at Note 3 (The Willpower Instinct).
"Researchers at Yale University School of Medicine found that during the week after September 11, 2001, patients heart rate variability decreased significantly. We were a nation overwhelmed, and its not surprising that rates of drinking, smoking, and drug use increased for months following the attacks of of 9/11."
 I haven't fully vetted this; it's my speculation. Here is my circumstantial evidence:
- Type 2 Diabetics have been shown to have low HRV
- Carbs stimulate the sympathetic nervous system, sometimes chronically (especially if you get addicted to them), thereby depressing the parasympathetic system.
- Those with high willpower instincts (see Notes above) have been shown to have high HRV. Anecdotally, I've noticed that my willpower and cognitive instincts are highest on a high fat, medium protein, low carb diet. Higher carbs tend to make me more "jumpy" and impulsive, which are characteristic of the SNS/"Fight or flight"/low HRV package, versus the PSNS/"Pause and plan"/High HRV package.
 I need to vet this out with the research more fully; any help would be appreciated. I do remember Devany making a post about weight lifting and "fractal" heartbeats, but I couldn't find it for this post, and can't remember if he mentioned HRV specifically.
 Supra at Note 3.
"Slow your breathing down to four to six breaths per minute. That's ten to fifteen seconds per breath--slower than you normally breathe, but not difficult with a little bit of practice and patience. Slowing the breath down activates the prefrontal cortex and increases heart rate variability."