As body, breath and mind settle, a distinctive slowing down of the overall metabolism begins to take effect. One needs to sleep less, eat less, breathe less. In fact, spontaneous slowing of breath is probably the most easily observed physical barometer of depth of samadhi. Normal adults at sea level breathe about fifteen times per minute. During seated meditation, at a middle level of shamatha, the breathing rate may drop to only two or three breaths a minute. Because shamatha practice produces such conspicuous changes in the body's function, there has recently been a good deal of physiological research on meditators. A few results of this research will be summarized here.

    Meditators' brainwaves are usually highly synchronized; typically this takes the form of increase in the alpha rhythm whose frequency ranges from eight to twelve cycles per second. This enhanced alpha production in meditators continues even when their eyes are open. In non-meditators, opening of the eyes normally stops production of alpha waves. Electromyography reveals deep muscle relaxation in spite of the upright, unsupported posture. Skin conductivity (GSR) decreases, probably indicating less sweating and hence decreased sympathetic activity. This too implies relaxation.

A group of meditators and a group of non-meditators were asked to sit quietly with electrodes attached to monitor brainwaves. A click sound was repeatedly presented to both groups. At first, both groups showed momentary "blocking" of alpha. This was as expected, for such blocking is part of the normal orienting response to a new stimulus. After several clicks, the non-meditator group no longer showed this blocking. This also is normal. They had accommodated to the stimulus: it was no longer new and fresh. But the Zen practitioners continued to momentarily block alpha with every click as if each time they were hearing the click for the first time. This fits nicely with the Zen ideal of "living in the moment." In India, a similar click experiment was done with some yogis. They showed no alpha blocking. Apparently, withdrawn in trance, they did not hear the sound.