A 2013 systematic review on the use of yoga for depression found moderate evidence of short-term benefit over "usual care" and limited evidence compared to relaxation and aerobic exercise. Only 3 of 12 randomised controlled trials had a low risk of bias. The diversity of the studies precluded analysis of long-term effects. A 2015 systematic review on the effect of yoga on mood and the brain concluded that "yoga is associated with better regulation of the sympathetic nervous system and hypothalamic-pituitary-adrenal system, as well as a decrease in depressive and anxious symptoms in a range of populations." A systematic review in 2017 found some evidence of benefit in major depressive disorder, examining outcomes primarily of improvements in remission rates and severity of depression (and secondarily of anxiety and adverse events), but considered that better randomised controlled trials were required.