Use of the BIS Monitor to Detect Onset of Naturally Occurring Sleep

Abstract

Objectives.Inadvertent sleep episodes are a recognizedcomplication of sleep deprivation. Although such events can be lifethreatening, no system currently exists to detect and prevent sleeponset. Because sleep shares electroencephalographic similarities withthe anesthetized state, we hypothesized that the BIS monitor, acurrently available EEG-based monitor of anesthetic depth, would detectthe onset of physiologic sleep. To test our hypothesis, we monitoredvolunteers during the transition from waking to sleep. Methods.Non-medicated volunteers were asked to lie down in a dark room for30-minutes and fall asleep while attached to a BIS monitor locatedoutside the room. A laptop computer was used to generate an audio toneinside the room. Speaker volume was adjusted to the lowest leveldetectable by the awake subject. Testing was begun by activating acomputer to play a tone at random intervals. The subject was instructedto click a mouse connected to the computer upon hearing the tone toverify wakefulness. The session was terminated upon loss of response tothree consecutive tones or after 30 minutes. Subjects were questionedafterwards regarding their perceptions of sleep during testing.Results.11 out of 28 self-described good sleepers could notsleep under testing conditions. BIS values for the remaining 17 fellfrom 96.4 ± 2.1 to 86.5 ± 0.79 (p< 0.01) uponsleep onset. All subjects responded to audio stimuli at BIS values>90, and were asleep either by subjective or objective report at BISvalues <80. Three subjects retained the mouse-click response despitelow BIS scores and subjective descriptions of sleep. Conclusion.Although variability in the BIS value marking sleep onset was noted,the BIS monitor detected all episodes of sleep onset in our testingregimen. We conclude that a threshold BIS value can be defined to allowthe BIS monitor to detect sleep onset.