Science supports several possibilities for how yoga helps with depression. Studies have found that it reduces levels of cortisol (a stress hormone that’s also secreted by the adrenals), which is often elevated in people with the disease.
And a study in India found that a yoga program that included asana, Pranayama, and meditation raised levels of serotonin and lowered levels of monoamine oxidase—two neurochemicals involved in depression.
Yoga is known to induce the relaxation response—to lower the activity of the sympathetic nervous system’s “fight or flight” mechanism and increase the work of the more restorative parasympathetic system; this characteristic could help with depression. But if that were the whole story, then postures that seem to rev up the sympathetic side—such as backbends and Sun Salutations—as well as rapid breathing techniques might be counterproductive to fighting stress and depression. The reality is that some yoga practices stimulate the nervous system and some are relaxing. It is the combination that in some complex way is beneficial.
One of the fruits of yoga practice is the realization of interconnections. Our bodies, minds, and emotions interact in complex ways that science is only just beginning to understand. In this dense web of interconnections, nothing we do has a single effect. In Urdhva Dhanurasana, you bring more oxygen into the bottom of the lungs (an area that usually gets less than the upper regions), your blood pressure and heart rate rise, pressure increases in the head and neck, and you stretch the muscles and organs in the front of the body as you compress those in back, where the adrenals are located.
From Patanjali’s perspective, the most reliable knowledge is derived from direct experience. The irony is that when we try to explain yoga in scientific terms when the science just isn’t there, we risk undermining our attempts to persuade others of yoga’s benefits.