Possible Mechanisms
One of the hallmark indices of physical or psychological stress is an elevated cortisol level, which has been implicated in depression of immune system function and subsequent increased incidence of ill health (Sapolsky, 1998). Kimura, Ohno, Kumano & Kimura (2000) found that blood serum cortisol levels decreased in yoga instructors while they practiced physical yoga exercise, breathing exercises and meditation in comparison to being at rest. This is evidence of relaxation in general and reduction of stress. Furthermore, frontal alpha waves became more strongly activated, which suggests 'mental relaxation'. Kimura, et al. cite studies that report a correlation between increased alpha waves and enhanced natural killer cell activity, which implies greater activation of the immune system. Several other experiments confirm the increase in alpha wave activity during yoga, which is associated with the relaxation response (Yardi, 2001). Further studies of this sort perhaps with separation of the physical yoga exercises from relaxation and mediation would be more informative.
However, there is some indication that aspects of yoga may indeed reduce either physical or psychological stress or both and promote health. The effects of yoga and meditation in particular were examined in relation to melatonin levels. Melatonin has been linked to immunoaugmentation, anti-cancer, anti-aging, and anti-stress effects and is an effective antioxidant and free-radical scavenger (Tooley, Armstrong, Norman & Sali, 2000). Likewise, meditation has been implicated in the same four areas, and therefore, Tooley, et al. designed an experiment to determine if yoga and meditation could influence melatonin levels and thus elucidate a biological pathway through which the benefits of yoga and meditation are reached. The experiment involved 11 meditators and 7 yoga practitioners whose levels of melatonin were measured on a control night of rest and an experimental night during which they meditated or practice yoga. The yoga regimen also incorporated mediation, but for half the amount of time as mediation alone. Melatonin is typically released only during nocturnal hours in mammals and remains relatively stable around midnight in humans. Therefore, the experiment was conducted around midnight. Tooley, et al. found that melatonin levels increased significantly following practice of meditation and yoga for at least one hour. The beneficial, health-promoting effects of melatonin may therefore be increased and prolonged in those who regularly meditated or practice yoga.
