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What is Fatigue?

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What is Fatigue?

Fatigue is the state of being unable to reliably to perform duties due to lack of sleep, prolonged duty, or other operational and personal factors.

When we are fatigued, we are impaired. The more fatigue, the worse the impairment. Our reactions are slowed, we experience lapses of attention, we become impulsive, we lose situational awareness, and become prone to distraction.

Drivers of Fatigue

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Drivers of Fatigue

There are three main drivers of fatigue. Sleep debt builds up when our duty schedule, or activities during our time off, reduce the time we have available for sleep. Being on duty at night or in a time zone to which our body has not yet acclimated is fatiguing because we are working against our biology. And long duty shifts contribute to fatigue as our natural daily sleep drive ramps up the longer we stay awake.

[Image: Drivers of Fatigue]

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Any one of these factors can increase fatigue risk, and when multiple factors are present, the fatigue effects are amplified.

Sleep Debt

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Sleep Debt

The average person needs about 8 hours of sleep per day—some need a little more and some a little less. When go day after day without getting the sleep that we need we build up a sleep debt.

[Image: Sleep Debt]

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This chart is from a study that was conducted to examine how sleep debt arises. Participants were assigned to one of four sleep restriction conditions:

1. 8h: One group (represented by the green dots) was given eight hours’ time in bed each day.
2. 6h: Another group (represented by the light blue dots) received six hours’ time in bed.
3. 4h: A third group (represented by the dark blue dots) received four hours’ time in bed.
4. 0h: The fourth group (black dots) was subjected to total sleep deprivation for three straight days.

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The vertical axis represents the degree of alertness deficit measured each day.

Sleep Debt Study Findings

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Sleep Debt Study Findings

An important finding from this study is that each day we go with inadequate sleep, we become more and more impaired. This is illustrated by the upward sloping trajectory of the lines that fit the data.

[Image: Sleep Debt Study Findings]

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Notice that by the middle of Week 2 of this study, the group that received at most six hours of sleep per night showed an alertness deficit comparable to that of the group that got no sleep whatsoever on the first night. This finding is relevant to aviation operations, in which peak periods often restrict sleep opportunities and it is common to get six hours of sleep per night for a week or more.

The only way to pay off a sleep debt is by getting the base amount of sleep that we need on a daily basis plus some extra recovery sleep. If we make it a priority to get adequate sleep each day, we can avoid getting into sleep debt.

Extended Waking Time

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Extended Waking Time

Sleeping eight hours per day means that we are awake for 16 hours. When we extend the time that we are awake beyond 16 hours, we begin to experience alertness deficits. The longer we extend the time we are awake the worse these deficits become.

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One study compared the impact of extended wakefulness to that of alcohol. After subjects had been kept awake for 22 hours, their alertness deficits were comparable to having a blood alcohol concentration (BAC) of 0.08—the legal limit in every state. In other words, by staying awake for six hours longer than a standard 16-hour period of wakefulness, we may become as impaired as if we were legally drunk.

Video - Trucking Fatigue Meter

Video - Trucking Fatigue Meter

Video

Time of Day Effect

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Time of Day Effect

Humans are not a nocturnal species. Owls, bats, and mice are nocturnal, but humans are not. This is not a choice—it is a biological fact we cannot change. By our very nature, we are alert during the day and sleepy at night. This means that when we work at night, we must take extra precautions to ensure that we able to sustain reliable alertness throughout the duty period.

[Image: Time of Day Effect]

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The driver of our daytime alertness and nighttime sleepiness is called the circadian rhythm. It is generated by a part of the brain called the suprachiasmatic nuclei. These send out a master timing signal that influences every part of our biology.

The circadian rhythm is not exactly the same in everyone. Some people tend to be early birds and prefer to wake up early and go to bed early, while others are more night owls and prefer to go to bed later and wake up later.

Window of Circadian Low (WOCL)

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Window of Circadian Low (WOCL)

The time when our bodies are programmed to sleep is called the window of circadian low, or the WOCL [pronounced: “WAH-kul”] The FAA defines the WOCL as the period of maximum sleepiness that occurs between 2am and 6am during a physiological night.

[Image: Window of Circadian Low (WOCL)]

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However, the WOCL can actually happen earlier for early birds and later for night owls. This is an important consideration when assessing fatigue risk. Early bird should be on the lookout for elevated fatigue risk when their duty day gets extended into late evening hours. Night owls, on the other hand, are more susceptible to fatigue risk during early starts.

Circadian Rhythm and Light

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Circadian Rhythm and Light

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Our circadian rhythm is synchronized to the daylight and dark cycles of our local environment through special receptors in our eyes, which are a type of ganglion cells. Different from the rods and cones that enable us to see color and motion, these photosensitive ganglion cells measure the intensity and timing of light in our surroundings. They are sensitive to light in the blue part of the visual spectrum. This information is transmitted to the brain and helps maintain a regular circadian rhythm.

Jetlag

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Jetlag

The specialized photo receptors that help maintain a regular circadian rhythm play an important role when we travel across time zones. The daylight and dark pattern in our new time zone will trigger circadian phase adjustments.

For example, when we travel east, our circadian rhythm must phase advance to become synchronized with the new time zone. The timing of light is very important. Light detected by our eye between around 4:00am and 9:00am in our origin time zone will trigger a phase advance. This shift means that we will go to bed earlier and wake up earlier.

Jetlag cont.

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Jetlag

When we travel west, our circadian rhythm must phase delay. This is triggered by our eye detecting light between around 10:00pm and 3:00am in our origin time zone. A phase delay means that we will go to bed later and wake up later.

[Image: Jetlag]

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A rule of thumb is that takes one day for each time zone crossed to fully synchronize our circadian rhythm to the new time zone.

Workload

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Workload

We have discussed operational factors that cause fatigue such as sleep debt, long days, night work and jet lag. Now let’s talk about workload.

Intuitively, fatigue should increase with heavier workload. Indeed, several scientific studies have shown that fatigue does build up more quickly under conditions of higher workload.

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This chart shows the results of one such study. Note that the difference is about one lapse on the PVT, which is relatively small and pales in comparison to the impact of the other factors that we have talked about.

Signs and Symptoms of Fatigue

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Signs and Symptoms of Fatigue

Here are some key indicators of fatigue:

- Impaired judgment
- Slower reaction time
- Lapses of attention
- Impulsivity
- Lower situational awareness
- Higher susceptibility to distractions

Summary

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Summary

In this introductory module we have learned about the basics of fatigue. We have learned that factors such as sleep debt, long days, night work and jet lag contribute to fatigue, and that when multiple factors are present, the fatigue effects are amplified.

We have learned that single most effective way to protect against fatigue is by making getting daily adequate sleep a priority. In the next module we will learn about how fatigue translates to operational risk, and what approaches have been developed to support the effective management of fatigue risk.

Quiz Question 1

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Q: What is the best way to avoid sleep debt?
- Make getting daily adequate sleep a priority (correct) (Correct! The single most effective way to protect against fatigue is by getting adequate sleep each day.)
- Caffeine (Incorrect. While caffeine is a stimulant that can upregulate you, its results are temporary and incapable of repaying a sleep debt.)
- Jumping jacks (Incorrect. Exercise, such as jumping jacks, can help reduce sleep inertia (that is, the tired feeling upon waking) by increasing heart rate and blood flow, but it cannot repay or prevent sleep debt.)
- Light (Incorrect. Light exposure can shift the circadian rhythm forwards or backwards, but it cannot prevent sleep debt.)

Quiz Question 2

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Q: After how many hours awake do alertness deficits become comparable to individuals with a blood alcohol content of 0.08?
- 10 hours (Incorrect. In the absence of a sleep debt or other fatiguing factors, 10 hours awake is not long enough to produce alertness deficits comparable to being legally drunk.)
- 17 hours (Incorrect. Fatigue deficits only begin to accelerate after being awake for 17 hours. At this level of extended wakefulness, deficits are only comparable to a 0.04 BAC.)
- 22 hours (correct) (Correct! By the time we are awake for 22 hours, we may experience alertness deficits comparable to being legally drunk.)
- 24 hours (Incorrect. After a full day of extended wakefulness, we may experience alertness deficits that are worse than being legally drunk.)

Quiz Question 3

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Q: Select all of the animals that are nocturnal:
- Owls (correct) (Correct. Owls are a perfect example of a nocturnal species, hence the term “night owl”.)
- Humans (Incorrect. Humans may pretend to be nocturnal by working at night, but we work against our biology when we do this.)
- Bats (correct) (Correct. Bats are naturally active at night, which is when they hunt and feed.)
- Mice (correct) (Correct. Mice are active at night, which allows them to avoid predators as well as bright light.)

Quiz Question 4

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Q: True or False: Light is the primary signal for our bodies to adapt to a new time zone.
- True (correct) (Correct! The specialized photoreceptors in our eyes that help maintain a regular circadian rhythm also play a key role in adapting us to new time zones. Light exposure in a new time zone triggers circadian phase adjustments.)
- False (Incorrect.)

Quiz Question 5

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Q: True or False: Workload is the most important factor when considering drivers of fatigue risk.
- True (Incorrect. Workload plays a relatively small role in driving fatigue risk.)
- False (correct) (Correct. Compared to sleep debt, long days, and night work, workload plays a much smaller role in driving fatigue risk.)