ABSTRACT
In a variety of species, including humans, sleep onset is typically characterized by gradual changes in cortical electroencephalographic (EEG) activity and progressive loss of postural muscle tone (1-6). However, these changes at sleep onset contrast markedly with those normally observed at spontaneous awakening from sleep when abrupt activation of the EEG and postural muscle tone occur (1-3,7,8). Awakenings are also normally accompanied by large brief changes in autonomic output producing transient surges in heart rate (HR), blood pressure (BP), and ventilation (V˙ ), which then decline in subsequent wakefulness (8-12). An example of the effects of awakening on HR and V˙ is shown in Figure 1. Since these cardiorespiratory responses occur even during normal spontaneous awakenings and without any overt behavioral activity (e.g., gross body movements or locomotion), it is apparent that the magnitude of cardiorespiratory changes at wake onset exceed obvious physiological need and are not a simple reversal of the changes that occur at sleep onset.
