Sleep
MTDC and University of Montana researchers have been conducting human factors studies on wildland firefighters for several decades. Incident management team (IMT) members asked MTDC to study human factors associated with their duties. In 2006, we began studying stress and cardiovascular risk in incident management team members. Those studies, which continued in 2007 and 2008, showed that sleep deprivation contributed to fatigue, stress, and impaired performance of team members. This report reviews some basic information concerning sleep and sleep deprivation, information that could be useful for incident management team members and wildland firefighters, as well as other agency personnel. Other information on studies of stress in incident management team members can be found in “Wildland Firefighter Health & Safety Report” Nos. 11 and 12.
“The most common source of stress I observe is probably directly attributed to lack of sleep…just as I observed in the military there are some folks who can function quite well on limited sleep, while others simply cannot.” (Public information officer for a type I incident management team)
Sleeping in fire camp can be a challenge. Noise from generators, vehicles, and other firefighters all contribute to sleeplessness. Because almost all wildland firefighters need to sleep either in fire camps or in spike camps, they sleep in tents, on the ground, and in hot, smoky, and dusty conditions. Shift work interferes with sleep, especially for those on night shift.
Sleep log data were collected on members of five incident management teams at fire camps in California and Montana during 2008. Data for 140 team members (36 percent female, 64 percent male) indicated that they averaged 6.1 hours Featured Topic of sleep, ranging from 3.5 to 9.0 hours per night. On average, team members went to bed at 9:30 p.m. They reported being awakened an average of 2.2 times per night, awakening from zero to six times per night. When team members were asked to rate the quality of their sleep, the average was 6.6 on a 10-point scale. Nearly one-fourth (23.8 percent) reported feeling tired when they woke, while 53.6 percent felt somewhat rested, 20.2 percent felt rested, and 2.4 percent felt very rested.
What Is Sleep?
Sleep consists of stages of light and deep sleep (figure 1).
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| Figure 1—The stages of sleep. REM stands for rapid eye movement sleep |
We begin in light sleep, progress to deep sleep, and then to rapid eye movement (REM) sleep. Deep sleep is believed to be essential for recovery from fatigue and for restoration. When awakened from deep sleep, we do not adjust readily and often feel groggy and disoriented for several minutes. During REM sleep we breathe more rapidly, our eyes move rapidly, and the muscles in our limbs become temporarily paralyzed. Heart rate and blood pressure increase. As the night progresses, REM sleep becomes longer and the periods of deep sleep are shorter.
We dream during REM sleep, perhaps because our inhibitory neurons are relaxed and previously unconnected thoughts mix in our subconscious. If REM sleep is disrupted, we tend to catch up with more REM sleep the next night. Disruption of REM sleep in a controlled study led to irritability. Continued disruption of REM sleep can lead to neurotic behavior. While laboratory rats normally live 2 to 3 years, they survive only about 5 weeks when deprived of REM sleep. When rats are deprived of all sleep, they live only about 3 weeks.
How Much Sleep?—Those who sleep 6 hours or less per night are not as healthy as those who sleep 7 to 8 hours per night. Those who sleep 9 hours or more are slightly less healthy than those sleeping 7 to 8 hours. Sleep allows neurons to rest and recover. Proteins, including hormones such as growth hormone, are manufactured during sleep.
Too little sleep leaves us drowsy and unable to concentrate. Our memory suffers, as does our physical performance, and we aren’t as good at carrying out math calculations. If sleep deprivation continues, mood swings and hallucinations may occur. Sleep deprivation also impairs immune function, increasing the likelihood of upper respiratory and other infections.
Sleep Deprivation—Sleep deprivation can lead to impaired mental and physical performance, to accidents and injuries, increased blood pressure, strokes and heart attacks, and even obesity. Higher order cognitive tasks are affected early; tests show that speed deteriorates before accuracy is affected. Total sleep deprivation for 1 week has led to cognitive impairment when work requires multitasking. In driving, accidents increase as sleep duration is decreased. In tasks requiring judgment, risky behaviors emerge when sleep is limited to 5 hours per night.
Sleep deprivation could influence cognitive performance because of an increase in periods of microsleep, several second periods of sleep-like brain activity. Microsleep increases with sleep deprivation, impairing cognitive function and possibly leading to failures in performance. Other possible explanations for performance impairment include perceptual impairments, such as reduced vigilance.
Circadian Rhythms—Circadian rhythms are regular changes in mental and physical characteristics that occur in the course of the day. If you have ever worked a night shift, you have experienced the challenge of working during the low point of your circadian rhythms. It is extremely hard to concentrate, perform, or even stay awake during the low point of the rhythms (2 to 6 a.m.). Sleep, hormones, and body temperature respond to rhythms dictated by the brain (actually the hypothalamus). The brain sets the body’s clock based on light cues, causing us to sleep overnight. By morning, body temperature, cortisol level, and wakefulness all increase. Interrupting the circadian rhythms leads to sleep deprivation. It takes weeks to adjust to a new shift and the adjustment is never complete. Operations that require workers to maintain high levels of alertness over extended periods, such as firefighting, require managers to consider the consequences of sleep loss and the disruption of circadian rhythms.
Naps?—Naps can allow workers to recover. To avoid waking up groggy and exhausted, workers should nap for 20 to 30 minutes or for longer than 90 minutes. Figure 1 on page 2 [shown above] shows that a 1-hour nap places you in the middle of deep sleep, making it difficult to wake up. You will be disoriented and clumsy, might make poor decisions, and could be at risk of an injury. A 20-minute nap ends before you descend into deep sleep; a 90-minute nap catches you rising out of deep sleep. Naps restore alertness, enhance performance, and reduce the risk of mistakes.
Summary
Sleep deprivation can impair cognitive performance and decisionmaking. If prolonged, sleep deprivation can cause a number of physical problems, including a decline in immune function, hypertension, heart problems, obesity, and increased stress. Performance problems include slow reaction time, impaired vigilance, errors of omission, and impaired memory, reasoning, and complex decisionmaking. As sleep deprivation continues, problems may include irritability, neurotic behavior, anxiety, and depression.Behavioral alertness and a range of cognitive functions, including sustained attention, and working memory deteriorate when nightly sleep is limited to between 4 and 7 hours. Decisionmaking skills, such as the ability to assess risk, assimilate changing information, and revise strategies to solve problems based on new information are likely to suffer.
Fatigue and sleep loss compromise memory and attention functions including:
- Assessing the scope of a problem based on changing information
- Maintaining focus
- Avoiding risks
- Recognizing ineffective actions
- Changing behavior based on new information
A military review of sleep research illustrates the recommended recovery times following sleep loss. It provides the basis for the 2:1 work-to-rest ratio. It also demonstrates the variability in recovery needs because of individual and situational differences. While the average worker will require 12 hours of sleep or rest to recover from 24 hours of sleep loss, some will need as much as 15 hours of
sleep and some will need less than 12 hours. This variability should not be ignored when sleep, shift length, and work/rest cycles are considered.
Eight hours of rest between work periods is inadequate because only 50 to 75 percent of the rest period is devoted to sleep. Thus it is advisable to have longer rest periods (10 to 14 hours) so workers have time for adequate sleep.
You can learn more about sleep and sleep deprivation at the National Institute of Neurological Disorders and Stroke Web page.
Source: Wildland Firefighter Health and Safety Report, Summer 2008, No. 13.
Source: Wildland Firefighter Health and Safety Report, Summer 2008, No. 13.
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