1.1 Overview

Sleep deprivation has become one of the most significant causes of error and accident throughout our society.  The Department of Transportation estimates that 200,000 traffic accidents each year may be fatigue- or sleep-related.  Annual sleep-related accidents in transportation alone claim over 5,000 lives, cause hundreds of thousands of injuries, with accumulated cost for health care, death, lost productivity, and damage to property in the billions [U.S. Dept. of Health and Human Services, 1992].

Pilots say their inconsistent schedules often force them to snooze in the cockpit in order to get enough sleep.  Industry insiders report that flight attendants need to periodically check to ensure that  the crew is awake.  The National Transportation Safety Board (NTSB) cited pilot fatigue as either the cause or a contributing factor in 69 plane accidents from 1983 through 1986  [Stanford Sleep Disorders Clinic and Research Center, 1991].

Recent analysis of spectacular accidents and catastrophes suggest that sleepiness may have played an important role in such events, including the Three Mile Island nuclear disaster and the Exxon Valdez accident [Mitler, et.al., 1988].   Such accidents endangered large segments of the population and the environment.  The presidential Commission on the Space Shuttle Challenger accident ruled that ground crew fatigue was a contributing cause of the 1986 disaster.  In the near catastrophic launch of the shuttle Columbia only three weeks before, operator fatigue was reported as one of the major factors contributing to this incident [Stanford Sleep Disorders Clinic and Research Center, 1991].  Thus the importance of human vigilance or attention is critical to the performance of individuals in various types of occupations.

Each human being requires a specific amount of sleep in each 24-hour period to maintain a functional level of alertness.  If an individual obtains less sleep, he/she will be less alert the following day.  Moreover, sleep loss accumulates from one night to the next as a "sleep debt."  Therefore, sleep of durations that represent only a modest loss of sleep on a single night may produce a serious sleep debt when sustained over several nights.  The more sleep lost each day, the greater the sleep debt and the larger the impairment.  Because individuals often do not recognize that they are sleepy, they seldom guard against inappropriate sleep episodes.  Much like the intoxicated driver, sleepy drivers do not realize that they are incapable of adequate performance and may deny drowsiness and impairment [U.S. Dept. of Health and Human Services, 1992].

The effects of sleep loss can be amplified by the bi-modal circadian rhythm.  One evidence is the temporal patterns of accidents attributed to "falling asleep" or due to inadvertent lapses in operator attention.  Studies of single vehicle truck accidents in Israel [Lavie et al., 1986], Texas [Langlois et al., 1985], and New York [G.W. Duff, unpublished observations] all reveal two distinct peaks in the occurrence times of these accidents over 24 hour periods.  One peak occurs in the early morning hours from 1 a.m. to 7 a.m. and another lower peak occurs during the mid afternoon from 1 p.m. to 4 p.m.

Another factor contributing to accident occurrences and greater risk of accidents is the increasing level of automation.  For example, drivers using cruise control and pilots using automatic flight control systems are more susceptible to drowsiness due to the removal of stimulating influences.  The Exxon Valdez was on automatic pilot during the critical minutes leading to its grounding as it hit Bligh Reef at 12:04 a.m. [Stanford Sleep Disorders Clinic and Research Center, 1991].  The NTSB's investigation of the accident indicated that the third mate was asleep on his feet and failed to respond to the warning light and alarm identifying the reef [U.S. Dept. of Health and Human Services, 1992].  Although automation has provided tremendous benefits, it tends to limit operator activity to vigilant monitoring of the system, which over a period of time can reduce the awareness level of an individual and impair their ability to react properly to external stimulus.  In addition, vigilance is most degraded by sleep loss and fatigue.

If a system could be developed to detect drowsiness in individuals prior to their reaching a state in which they are incapable of safely performing their duties, a considerable number of accidents could be prevented and many lives could be saved.  Several studies have indicated that drowsiness in an individual can be detected from particular characteristics of their electroencephalogram (EEG) [Torsvall et al. 1987, 1989; Akerstedt et al., 1990, 1991;  Wierwille et. al., 1992;  Dingus et. al., 1987].  There is also considerable evidence that indicates a strong correlation between the EEG waveform and degraded vigilance [Fruhstorfer et al., 1977; O’Hanlon and Beatty, 1977; O’Hanlon and Kelley, 1977].  More importantly, this correlation can be established from ambulatory EEG recordings.

The purpose of this research is to develop signal processing methods that can be used to detect and track subtle changes in the EEG signal characteristics of an individual that precede performance lapses and the onset of sleep.  The approach taken in this work is to first identify measurable characteristics in the EEG signal that correlate with drowsiness or a loss of vigilance in an individual performing a given task.  Then, this information is used to construct a "drowsiness" measure that tracks the state of alertness or vigilance of an individual.  Signal processing methods that extract this information from the EEG data are developed.  The measure can then be used to warn an individual if they are getting dangerously close to a low state of alertness.  Of course, the value of the measure that corresponds to a dangerous state will be application specific.  Also, because we are detecting the decreases in alertness and vigilance that precede the onset of sleep, an individual can be given sufficient warning time to react properly and take steps to avoid incipient sleep.

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