This is an excerpt from Physiology of Yoga, The by Andrew McGonigle & Matthew Peter Huy.
A review by Li and Goldsmith (2012) looked at the effects of yoga on anxiety and stress. The review included 35 studies and concluded that while yoga can relieve stress and anxiety, further investigation into this relationship using large, well-defined populations, adequate controls, randomization, and long duration should be explored before recommending yoga as a treatment option. In a systematic review of randomized controlled trials on the effects of yoga on stress measures and mood, the 25 studies that were included provided preliminary evidence to suggest that yoga leads to better regulation of the SNS and the HPA axis, as well as a decrease in depressive and anxious symptoms in a range of populations (Pascoe and Bauer 2015). A systematic review by Sharma and Haider (2013) focusing on the effect that yoga has on anxiety concluded that out of a total of 27 studies that met their inclusion criteria, 19 studies demonstrated a significant reduction in anxiety.
A review on neuroimaging in yoga practitioners showed decreased blood flow in the amygdala and increased activity in the prefrontal cortex, suggesting that practitioners do notice negative stimuli but are less affected by it (Desai, Tailor, and Bhatt 2015). Partaking in meditation and yoga practice is associated with smaller right amygdala volume (Gotink et al. 2018) and stress reduction has been associated with less amygdala volume (Holzel et al. 2010). Streeter and colleagues (2010) suggested that yoga increases PNS activity and neurotransmitter (GABA) levels in the thalamus, and that these increases are correlated with reduced anxiety and improved mood.