A microsleep is a sudden temporary episode of sleep or drowsiness which may last for a few seconds where an individual fails to respond to some arbitrary sensory input and becomes unconscious.International Classification of Sleep Disorders, , page 343Poudel, G. R., Innes, C. R., Bones, P. J., Watts, R., & Jones, R. D. (2012) Losing the struggle to stay awake: Divergent thalamic and cortical activity during microsleeps. Human Brain Mapping: 00:000-000 Episodes of microsleep occur when an individual loses and regains awareness after a brief lapse in consciousness, often without warning, or when there are sudden shifts between states of wakefulness and sleep. In behavioural terms, microsleeps may manifest as droopy eyes, slow eyelid-closure, and head nodding. In electrical terms, microsleeps are often classified as a shift in electroencephalography (EEG) during which 4–7 Hz (theta wave) activity replaces the waking 8–13 Hz (alpha wave) background rhythm.
Definition Some experts define microsleep according to behavioral criteria (head nods, drooping eyelids, etc.), while others rely on EEG markers. Since there are many ways to detect microsleeps in a variety of contexts there is little agreement on how best to identify and classify microsleep episodes.
Causes Microsleeps frequently occur as a result of sleep deprivation. However, individuals who are not sleep-deprived or tired can also experience microsleeps during monotonous tasks.Chou, Y. H., Chuang, C. C., Zao, J. K., Ko, L. W., & Lin, C. T. (2011, August). An fMRI study of abrupt-awake episodes during behavioral microsleeps. In Engineering in Medicine and Biology Society, EMBC, 2011 Annual International Conference of the IEEE (pp. 5060-5063). IEEE.
Effects Microsleep is extremely dangerous when it occurs in situations that demand constant alertness, such as driving a motor vehicle or working with heavy machinery. People who experience microsleeps often remain unaware of them, instead believing themselves to have been awake the whole time, or to have temporarily lost focus.
Background and significance
With over 1,550 fatalities and 40,000 nonfatal injuries occurring annually in the United States alone as a result of drowsy driving, sleep loss has become a public health problem.Insufficient Sleep Is a Public Health Epidemic. https://www.cdc.gov/features/dssleep/ When experiencing microsleeps while driving an automobile, from the perspective of the driver, they are driving a car, and then suddenly realize that several seconds have passed by unnoticed. It is not obvious to the driver that they were asleep during those missing seconds, although this is in fact what happened. The sleeping driver is at very high risk for having a collision during a microsleep episode.
Historically, many accidents and catastrophes have resulted from microsleep episodes in these circumstances. For example, a microsleep episode is claimed to have been one factor contributing to the Waterfall rail accident in 2003; the driver had a heart attack, and the guard who should have reacted to the train's increasing speed is said by his defender to have microslept, thus causing him to be held unaccountable. A possible microsleep was recorded as part of the narrative verdict in the inquest into the 2016 Croydon tram derailment.
Thus, microsleeps are often examined in the context of driver drowsiness detection and prevention of work-related injuries and public safety incidents (e.g. truck crashes, locomotive crashes, airplane crashes, etc.). Some statistics are below: 44% of drivers during late-night driving become dangerously sleepy.Åkerstedt, T., Hallvig, D., Anund, A., Fors, C., Schwarz, J., & Kecklund, G. (2013). "Having to stop driving at night because of dangerous sleepiness–awareness, physiology and behaviour." Journal of Sleep Research. Extremely fatiguing work protocols increase accident probability from near 0% to 35%.Sirois, B., Trutschel, U., Edwards, D., Sommer, D., & Golz, M. (2010, January). "Predicting Accident Probability from Frequency of Microsleep Events." In World Congress on Medical Physics and Biomedical Engineering, September 7–12, 2009, Munich, Germany (pp. 2284–2286). Springer Berlin Heidelberg. Chronic microsleeps not only increase probability for injury but also decrease worker productivity and increase likelihood for absenteeism from work.Swanson, L. M., ARNEDT, J., Rosekind, M. R., Belenky, G., Balkin, T. J., & Drake, C. (2011). "Sleep disorders and work performance: findings from the 2008 National Sleep Foundation Sleep in America poll." Journal of Sleep Research, 20(3), 487-494. According to one Centers for Disease Control and Prevention (CDC) study, among 74,571 adult respondents in 12 U.S. states, 35.3% reported <7 hours of sleep during a typical 24-hour period, 48.0% reported snoring, 37.9% reported unintentionally falling asleep during the day at least once in the preceding month, and 4.7% reported nodding off or falling asleep while driving at least once in the preceding month. The National Highway Traffic Safety Administration estimates that 2.5% of fatal crashes and 2% of injury crashes involve drowsy driving.National Highway Traffic Safety Administration. Traffic Safety Facts Crash Stats: Drowsy Driving. Washington, DC: DOT; 2011. DOT HS 811 4492011. Fatigue is associated with 250 fatalities in air carrier accidents in the last 16 years. Microsleep episodes are not dangerous in and of themselves. The risk comes from the potential that they carry to cause incidents resulting from lack of awareness. If an individual has occurrences of microsleep in an environment that is free from potential environmental risk and its associated consequences, then episodes of microsleep should be non-problematic.
Neural correlates Generally, microsleeps are characterized by a decrease in activity in wakefulness-related regions of the brain and an increase in activity in sleep-related regions of the brain. Looking at neural correlates of microsleeps is difficult because microsleeps can also be triggered by monotonous tasks (e.g. such as driving or dozing off in class). Therefore, it is important to examine neural correlates of microsleep events with respect to experimental set-ups (e.g. simulated driving set-up, reaction time set-up, etc.). Individual variability in brain structure also makes it difficult to diagnose microsleep events objectively.
In one study, neural activity underlying microsleeps was investigated by simultaneously measuring eye video, response behavior, EEG, and fMRI in normally-rested individuals engaged in a sensory-motor task. Twenty participants tracked a visual stimulus with a joystick for 50 minutes in 2 dimensions (up/down/right/left) on a computer screen. Participants performed this task in an fMRI scanner such that joystick response, right eye-video, EEG (60 EEG electrodes), and fMRI data were recorded simultaneously. Most participants had frequent microsleeps (>35) in a continuous visuomotor task (tracking visual stimulus on a screen), corresponding with decreased activity in arousal-related brain regions over time (thalamus, midbrain, and the posterior cingulate cortex).
Another study examined the activation patterns of 5 people who woke up from microsleeps in a simulated driving experiment. It was found that upon awakening the visual area, frontal cortex, limbic lobe were activated (in the intense activation phase) and the frontal cortex, temporal cortex, primary motor area, and insula were activated (in the post abrupt awakening phase). Therefore, the study concluded that decision-making was not activated immediately upon waking up from a microsleep episode, likely increasing risk of injury in intense decision-making tasks like driving or surgery.
The transition from wakefulness to sleep is regulated by a variety of chemicals. Adenosine likely causes the 'feeling sleepy' side of microsleeps, while dopamine likely reduces microsleep events by promoting wakefulness. It has been shown that microsleeps correlate with spontaneous pontine-geniculate-occipital (PGO waves) waves, which suppress visual processing in the basal ganglia. When this pathway is not activated, cells in the superior colliculus (which causes release of dopamine) cannot be dis-inhibited via the basal ganglia, leading to poor processing ability and microsleep onset.Silkis, I. G. (2010). Analysis of the effects of neuromodulators on the generation of spontaneous pontine-geniculate-occipital (PGO) waves. Neurochemical Journal, 4(3), 170-177.
Detection methods and classifications There are currently many ways to detect microsleeps; however, there is a lack of general consensus as to the best way to identify and classify microsleeps. The simplest methods to detect these events seem to be through psychological tests, speech tests, and behavioral tests (e.g. yawn test and eye-video test). More complex and expensive ways to detect microsleeps include EEG, fMRI, EOG, and PSG tied to various software platforms. When multiple tests are used in parallel, detection of microsleeps most likely will become more accurate.