Alcohol triggers the sympathetic nervous system: High frequency energy is suppressed

Thus, both arousal and sympathetic activity were increased by alcohol consumption after 20 min.

Ironically we are taught to think that alcohol relaxes the body

 Alcohol has a depressive effect on the central nervous system because it binds to certain GABA receptors. That's why most people feel relaxed and even tired after drinking. While alcohol doesn't increase GABA, it does increase the amount of serotonin and dopamine released in the brain.

 - but the opposite is true!

 So heart rate variability or HRV is actually the ratio of parasympathetic to sympathetic or High frequency to low frequency energy

 This suggests that alcohol, in a dosage-dependent manner, suppresses the high-frequency component of HRV that is an indicator of parasympathetic nerve activity during sleep.

So what happens then is that the REM sleep or dreaming in the first half of the night is wiped out - and then in the 2nd half of the night - the sympathetic nervous system is still too aroused so the deep sleep is lost.

 heart rate variability (HRV): the high frequency, respiration-linked component (HC, >0.15 Hz) and the low frequency component (LC, <0.15 Hz).

This is why the qigong master says to only drink a half glass of wine - if you have alcohol. He says alcohol decreases the qi energy. To build up qi energy means to build up the parasympathetic nervous system. Combining energy drinks and alcohol can cause instant death

 the decrease of heart rate after peak exercise, also called Heart Rate Recovery (HRR), can be used as a marker of parasympathetic activity

 and

  In most subjects, the high‐frequency (HF) component peaks at the respiration frequency, reflecting parasympathetic activity (Task Force of the European Society of Cardiology and the North American Society of Pacing and Electrophysiology, 1996).

So now we know - the Breathing synchronizes with the Parasympathetic nervous system as the peak amplitude energy! Hence the secret of the Lungs as the source of Yang qi to create yuan qi.

 Caffeine increases vagal autonomic nerve activity in resting subjects (Hibino et al., 1997; Yeragani et al., 2005). The intake of caffeine before exercise has also been associated with exaggerated vagal withdrawal during post‐exercise recovery because of higher baseline level of vagal activity before exercise(Yeragani et al., 2005) There is also evidence that caffeine can improve physical performance (Doherty, 1998; Laurent et al., 2000; Cox et al., 2002). Our study showed a decreased PLF/PHF and a tendency to increased PHF, most likely reflecting an increased vagal modulation, when subjects were resting after consuming energy drinks.

 So ironically caffeine actually increases the vagus nerve - as higher frequency energy of the heart.

in the nervous system (2), including its ability to increase rates of dopamine release in the anterior cingulate gyrus (3).

 So caffeine increases the dopamine which is the vagus nerve 

But caffeine at the same time increases cortisol which is a sympathetic nervous system reaction:

 Caffeine in dietary doses increases both adrenocorticotropin (ACTH) and cortisol secretion in humans (15).

 But ironically at higher doses - the body develops a tolerance to the cortisol reaction:

Five days of moderate caffeine intake, at 300 mg/day, resulted in incomplete tolerance formation. Challenge doses given in the morning and afternoon caused a significant elevation of cortisol at 1:00 PM, lasting for approximately 6 hours. In contrast, a high daily dose of 600 mg produced a more complete tolerance to caffeine effects on the hypothalamic-pituitary-adrenocortical axis (HPAC), with cortisol rising above the PP week control level only for the hours immediately after the 1:00 PM caffeine dose.

 reports of greater numbers of hypothalamic corticotropin-releasing factor neurons in spontaneously hypertensive rats and in brain tissue taken from deceased hypertensive patients (38). The greater HPAC responses seen in hypertension risk groups are accompanied by greater cortisol responses to caffeine.

 given the possibility that caffeine intake could counteract the depth of slow-wave sleep in the early part of the sleep cycle, a time of minimal cortisol secretion now thought to be important for memory consolidation.

Acute effects of melatonin administration on cardiovascular autonomic ...

https://www.ncbi.nlm.nih.gov/pubmed/11320384

by K Nishiyama - ‎2001 - ‎Cited by 100 - ‎Related articles

BACKGROUND: Previous studies have suggested that melatonin, a major pineal hormone, possibly modulates the autonomic nervous system in animals. The aim of this ... CONCLUSIONS: These findings indicate that melatonin administration increased cardiac vagal tone in the supine position in awake men. Melatonin ...

green tea inhibits cortisol production  of caffeine

 In conclusion, we provide evidence that aqueous extracts of tea (Camellia sinensis) are able to inhibit cortisol formation by the enzyme 11β-HSD1

 There are four primary polyphenols in fresh tea leaves – epicatechin (EC), epigallocatechin gallate (EGCG), epicatechin gallate (ECG), and epigallocatechin (EGC). A previous Japanese study (published in the journal Nutrition) concluded that these green tea extracts may, in fact, help offset the various signs associated with stressful lives, including physical and mental fatigue.

 green tea study