The detection of sleep onset: behavioral, physiological, and subjective convergence

In this report, a sleep deprivation/multiple arousal paradigm was used in which response time (RT) and respiratory and electroencephalographic (EEG) measures were combined with a continuous behavioral index of arousal (a deadman switch) and frequently repeated Stanford Sleepiness Scale ratings to examine the process of falling asleep. Sleep was defined behaviorally as failure to respond to the faint auditory RT cue. Although response rates decreased significantly as EEG stages passed from W through 1 to 2, responding continued in both light "sleep" stages. Respiratory, subjective, and DM changes were more pronounced between Stages W and 1 than between Stages 1 and 2. If the criterion for wakefulness is cognitive response to external stimulation, accurate distinctions between sleep and wakefulness can only be made in EEG Stages 3, 4, and rapid eye movement sleep. If EEG is the criterion, then the data suggest that cognitive response is possible during Stages 1 and 2 "sleep". The concept of a Sleep Onset Period, characterized by lengthening response times and intermittent response failure (thereby reflecting neither true sleep nor wakefulness), may provide a useful resolution to this definitional dilemma.