This is a technical paper exploring what occurs in the brain during acupuncture and in the ensuing time period. It also looks at the difference in the brain between sham (superficial) acupuncture and verum or real acupuncture. They also correlate this activity with Heart Rate Variability, a measure of the autonomic nervous system balance. It has the added feature of citing my research study at least twice… The abstract can be found at the link, the entire paper can be downloaded here . Some excerpts of the paper follow and short discussion.

Pain. 2008 Jun;136(3):407-18.
Acupuncture modulates resting state connectivity in default and sensorimotor brain networks
Our results demonstrate for the first time that acupuncture can enhance the post-stimulation spatial extent of resting brain networks to include anti-nociceptive, memory, and affective brain regions. This modulation and sympathovagal response may relate to acupuncture analgesia and other potential therapeutic effects.
A review of the literature suggests that acupuncture influences a diverse set of bodily organs and functions including brain processing [25, 26] and autonomic nervous system (ANS) activity [1, 44].
It has long been known that the analgesic effects of acupuncture may actually peak long after cessation of active needle stimulation [58]. ..
Furthermore, recent reports have suggested that ANS(Autonomic Nervous System) response to acupuncture (measured by heart rate variability, HRV, [49]) may be linked to clinical response for different chronic pain populations [60]. As variable peripheral ANS response to acupuncture may influence post-stimulation brain activity, we also sought to explore the relationship between acupuncture induced ANS outflow to the heart as estimated by HRV and acupuncture induced changes in RSN (Resting state network) connectivity.
We also sought to explore the relationship between acupuncture-induced autonomic modulation and subsequent change in RSN connectivity. Increased DMN (Default Mode Network) connectivity with the hippocampal formation was anti-correlated with acupuncture-induced increase in sympathetic modulation, and correlated with parasympathetic modulation. ..
Our data contribute to these results by suggesting that when acupuncture induces a sympathovagal shift toward parasympathetic, the hippocampal formation has greater connectivity with the DefaultModeNetwork, a brain network thought to subserve self-referential cognition and autobiographical memory [14]. While pain is known to modulate autonomic outflow [18], the response is typically increased sympathetic tone. However, increased hippocampal/DMN connectivity following acupuncture was associated decreased sympathetic and increased parasympathetic modulation, suggesting that acupuncture modulation of DMN connectivity may not result from classic pain-like aspects of this complex intervention. Interestingly, other groups have found that acupuncture-induced decrease of LF/HF ratio, relating a shift in sympathovagal balance toward greater parasympathetic modulation, may relate to acupuncture therapeutic effects on chronic pain [60]. Our correlative results connecting HRV metrics with enhanced DMN connectivity should to be replicated in a clinical population to investigate this possible connection with clinical efficacy…

Not that I can speak to the entirety of this complicated and excellent study, but they are exploring why acupuncture would decrease the stress response, and why it might lead to prolonged pain relief in chronic pain patients. Since chronic pain is a syndrome where the suffering of the patient is not in direct correlation to the nociceptive stimulus, it is postulated in this paper that the acupuncture sets up a new memory of sorts which requires the default mode network connectivity with certain brain regions. They showed that the acupuncture increased the connectivity of the Default Mode Network with the hippocampus, which is connected with the Autonomic Nervous System, which caused a decrease in the stress response as demonstrated by HRV (Heart Rate Variability) analysis.
In sum, this paper may show why the stress response can be lowered by acupuncture, as my own research paper suggested.