Recent advances in understanding the brain circuitry underlying the waking and sleeping states have given rise to models that may explain these transitions. Similarly, during the sleep period, animals and people rapidly transition between rapid eye movement (REM) and non-REM (NREM) sleep states. We measure the changes in cortical activity and muscle tone, respectively, by recording the electroencephalogram (EEG) and electromyogram (EMG), and the actual transitions in electrophysiologically monitored state occur over just a few seconds ( Takahashi et al., 2010). These state transitions involve dramatic alterations in easily observed physiological variables, including eye closure, breathing, arousability, and muscle tone. Conversely, it is common experience that one can fall asleep over just a few seconds or minutes.
Because sleeping animals are inherently more vulnerable, it is necessary for an animal to be able to awaken quickly so it can flee or defend itself. We spend nearly one-third of our lives asleep, and many mammals, including small laboratory rodents, spend half or more of their existence in this state ( Savage and West, 2007 Siegel, 2009).
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