While significant research has been done into the physiological mechanisms that underlie sleep and the sleep/wake cycle, the available data regarding the nature of drowsiness is far more limited. An objective measurement of drowsiness would have clinical utility, and a precise definition of the drowsy state could offer insights into the nature and purpose of sleep. Studies utilizing fMRI have demonstrated increased area of central nervous system involvement with tasks of increasing complexity. Preliminary data from studies of magnetoencephalography (MEG) with a receptive language task have demonstrated a progressive increase in global coherence of activity between MEG sensors with increasing drowsiness. The relationship between global coherence and the level of drowsiness suggests that the former may serve as an objective measurement of the latter. If true, the relationship suggests the hypothesis that drowsiness may be defined as a progressive loss of cortical network processing efficiency, requiring the recruitment of greater amounts of cortical tissue to perform the same task.