Frequently Asked Questions.
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The PVT is a validated computerized fatigue assessment tool that has become a gold standard measure of behavioral alertness and attention. The PVT was invented by Dr. David F. Dinges of the University of Pennsylvania to measures changes in: (1) psychomotor speed; (2) lapses of attention; and (3) instability of alertness. Over 150 peer reviewed scientific studies have contributed to the published literature on the PVT. The tool works because it requires no training, is not confounded by learning effects, and is unaffected by aptitude differences.
In a head-to-head comparison of various cognitive performance tests known to be sensitive to fatigue induced by sleep loss, investigators at the Walter Reed Army Institute of Research concluded that “the Psychomotor Vigilance Test was among the most sensitive to sleep restriction, was among the most reliable with no evidence of learning over repeated administrations, and possesses characteristics that make it among the most practical for use in the operational environment.”
The fatigue graph displayed in Fatigue Meter is an estimation of fatigue level. It should not be interpreted as an assessment of whether a person is fit or unfit for duty. A variety of factors can contribute to the risk of having an incident – fatigue is only one of the factors. High levels of fatigue do not necessarily mean that an individual will be involved in or cause a work related incident, but the overall risk is increased. Low fatigue estimates do not guarantee that an individual will not be involved in or cause a work related incident.
Fatigue is expressed in terms of “PVT lapses”, that is, the expected value of the number of lapses of attention that a given individual would get if a PVT was done at an equivalent level of fatigue. This calibration of fatigue expressed as PVT lapses is based on laboratory data on PVT performance relative to sleep timing and duration and time of day.1
1 Van Dongen, H. P. A., N. J. Price, et al. (2001). "Caffeine eliminates psychomotor vigilance deficits from sleep inertia." Sleep 24(7): 813-819.
There are a variety factors potentially contributing to fatigue that are not accounted for in performance estimates generated by Fatigue Meter, such as: hypoxia, time on task, visual fatigue, and cumulative duty time. Including these effects in fatigue models is a subject of ongoing scientific investigation, and when validated results become available, they will be included in the fatigue model used by Fatigue Meter.
Individuals can experience different levels of performance deficits due to fatigue.1 Some people are much more susceptible than others to performance deficits due to working at night or during extended periods of wakefulness. Performance estimates generated by Fatigue Meter are based on population averages. Future versions of Fatigue Meter will be able to tailor performance estimates to account for individual differences in susceptibility to fatigue.2
1 Van Dongen, H. P. A., M. D. Baynard, et al. (2004). "Systematic interindividual differences in neurobehavioral impairment from sleep loss: Evidence of trait-like differential vulnerability." Sleep 27(3): 423-433.
2 Van Dongen, H. P. A., C. G. Mott, et al. (2007). “Optimization of biomathematical model predictions for cognitive performance impairment in individuals: Accounting for unknown traits and uncertain states in homeostatic and circadian processes.” Sleep 30(9): 1125-1139.
Sleep inertia is the grogginess experienced immediately after waking. The performance effects of sleep inertia include impaired vigilance and cognitive deficits. The effects of sleep inertia usually dissipate quickly but residual effects can last up to two hours. Sleep inertia can be more intense when sleep is restricted or when waking up at a time during the biological night (for instance waking up during the WOCL)1. Caffeine is effective at eliminating vigilance impairment from sleep inertia2. Sleep inertia can be a problem when individuals need to be alert immediately after waking. For instance, one way to mitigate sleep inertia is to allow sufficient time to elapse after waking or having caffeine (or both) before returning to the operational environment. Our aviation specific version of the biomathematical fatigue model accounts for sleep inertia3.
1 Jewett, M. E., J. K. Wyatt, et al. (1999). "Time course of sleep inertia dissipation in human performance and alertness." Journal of Sleep Research 8: 1-8.
2 Van Dongen, H. P. A., N. J. Price, et al. (2001). “Caffeine eliminates psychomotor vigilance deficits from sleep inertia.” Sleep 24(7): 813-819.
3 Hilditch, C. J., Centofanti, S. A., Dorrian, J., & Banks, S. (2016). “A 30-minute, but not a 10-minute nighttime nap is associated with sleep inertia.” Sleep, 39(3), 675-685.
You may have heard the term WOCL (Window of Circadian Low) that refers to the part of your circadian rhythm that occurs during the later part of your usual sleep period and is associated with intense sleepiness and fatigue. Operations that involve working during the WOCL are especially susceptible to fatigue.
Some operations result in circadian misalignment—a mismatch between individuals’ internal circadian rhythm and the time that they are asleep or awake. Circadian misalignment occurs when working at night but also occurs when crossing time zones (this is what causes jet lag). When crossing time zones, the individual's internal "body clock" time becomes misaligned with the local clock time. An individual who travels to a new time zone and hasn't had adequate time to adjust will typically experience fatigue during daylight hours and be unable to sleep well during local nighttime.
The effects of jet-lag are temporary; eventually the individual’s internal body clock will synchronize with the local environment. This synchronization is a gradual process that occurs over days and is dependent on the number of time zones crossed. While the neurobiology that governs resynchronization is complex, one commonly used model is that the "body clock" adjusts towards the local time zone at a rate of 1/2 of the difference every 24 hours (Darwent, Dawson et al. 2010). This is the model that Fatigue Meter uses to account for performance deficits due to jet lag. So, for example, if an individual travels from his or her domicile (say at Z+0) to a location with time zone Z+10, their “body clock” is assumed to adjust to Z+5 after 24 hours, Z+7.5 after 48 hours, Z+8.8 after 72 hours, etc.
Ferrara, M., L. D. Gennaro, et al. (2000). "Time-course of sleep inertia upon awakening from nighttime sleep with different sleep homeostasis conditions." Aviation, Space, and Environmental Medicine 71(3): 225-229.
Fatigue estimates provided by Fatigue Meter are based on a validated biomathematical fatigue model.1
This model accounts for the two primary biological processes that govern human sleep/wake cycles and alertness levels: the homeostatic process and the circadian process. The homeostatic process represents the buildup of sleep pressure over time awake.
The circadian process ("circadian" means "about a day") makes an individual alert during their biological day and sleepy during their biological night.
1 McCauley, P., L. V. Kalachev, et al. (2013). “Dynamic Circadian Modulation in a Biomathematical Model for the Effects of Sleep and Sleep Loss on Waking Neurobehavioral Performance.” Sleep 36(12): 1987-1997.