In one study, researchers had patients be treated by acupuncture for stress. They looked at the low-frequency–to–high-frequency (LF/HF) ratio of the heart rate variability (HRV). After a few weeks to a few months, they found some interesting results. They found from the patient that HRV increased over weeks to months during the course of acupuncture treatment for hypertension as evidenced by a decrease in their LF/HF and conclude that this would indicate a relative decrease in their physiologic stress. 
In another study, they looked at the effects of acupuncture for stress in a large urban college population. They had 111 participants and randomly assigned them into a treatment group with acupuncture for stress and a control group with acupuncture. They measured it by Cohen’s global measure of perceived stress scale (PSS-14) each week for 12 weeks. Afterward, they were able to conclude that acupuncture may be successful in decreasing the perception of stress in students and staff at a large urban university. It was also found that the effect persists for at least 3 months after the treatment.