There are psychophysiological markers for hypnagogia, hypno-pompia, and napping. In the case of meditation, a number of studies have identified markers such as reduced respiration rate and volume of air breathed, reduced oxygen consumption and carbon dioxide elimination, and reduced blood lactate. In these studies, heart rate and the skin's electrical conductance decreased, but the frequency of alpha (and sometimes that of theta) brain waves increased. All of this suggests reduced energy metabolism, autonomic nervous system arousal, cortical energy metabolism, autonomic nervous system arousal, and cortical arousal. Additional research indicated that reduced arousal during meditation is due to its rest and relaxation aspects rather than to the specific meditation practice employed. However, it is probably more accurate to speak of "meditative states of consciousness" than to hypothesize a single "meditative state," because different practices may emphasize rapid breathing and active movement rather than counting breaths, repeating phrases, focusing on a mandala, or witnessing one's thoughts.
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