Introduction and background

Lack of sleep is a major contributor to fatigue which, in turn, can profoundly influence flight safety. Insufficient sleep can lead to poor concentration, increased irritability and lethargy. Most people have experienced these symptoms, but despite their experience, many continue to operate with a sleep decrement and therefore put others at risk.

A common misperception is that nocturnal sleep onset is simply a habit pattern based on external cues. However, when no direct time indicators (e.g., watch, radio, newspapers) or indirect time indicators (e.g., lighting, noise, temperature, vibrations) are present, a person will maintain a consistent sleep-wake cycle. The persistence of the sleep-wake cycle reflects the existence of an internal system, the biological clock, that can function autonomously when no time indicators are available.

This Briefing Note provides an overview of:

• What is happening in the brain and the body during sleep
• Why humans sleep
• Sleep problems.

Understanding Sleep

Sleep stages

Sleep is a temporary state in which the senses are disengaged. A variety of techniques are used to study sleep. These include the monitoring of electrical activity in the brain (electroencephalograph, EEG), muscles (electromyograph, EMG) and eye movements (electroocculograph, E0G). Research has demonstrated that sleep is divided into five stages: Stage N1, Stage N2, Stage N3, Stage N4 and rapid eye movement. Stages N1 to N4 are called non-REM sleep because the eyes show very little or no movement.

When falling asleep, a person enters Stage N1. This stage is the transition between waking and deep sleep. Brain activity slows down, and an EEG will show a shift from fast alpha waves to slower theta waves, an indication that brain activity is slowing. During this time, a person will no longer respond to cues such as a visual alarm or tones. The eyes close and start to roll slowly and involuntarily. During Stage N1, a person may experience sudden muscle contractions. In Stage N1, an individual may appear to be awake. This is referred to as microsleep, or as a microlapse, that often occurs when driving, flying, watching TV or during a meeting. When sleep lasts for more than 10 minutes, the memory of the few minutes before sleep is lost.

Sleep then progresses through Stages N2, N3 and N4. In Stage N2, brain activity shows a stereotypical pattern of activity called sleep spindles and K-complexes. Stages N3 and N4 are called slow-wave sleep since EEG readings of brain activity show longer and slower delta waves. The frequency of these slow waves distinguishes Stage N3 from Stage N4. In Stage N3, the slow waves account for 20 percent to 50 percent of brain activity; in Stage N4 the slow waves account for more than 50% of brain activity. It becomes more and more difficult to awaken people as they move from Stage N1 sleep to Stage N4 sleep. A person waking up out of slow-wave sleep may feel disoriented and confused for several minutes and is said to be in a state of sleep inertia.

The fifth stage of sleep, REM, is different from the other stages of sleep in a number of ways. The eyes move rapidly from side to side, brain activity becomes very fast and de-synchronous, and muscles are completely relaxed. REM sleep is also the stage during which most dreaming occurs and when dreams are most vivid. The muscle relaxation prevents a person from moving around and acting out any dreams.

Sleep architecture

During the night, an individual passes in and out of the five stages many times. This occurs in a predictable pattern called a sleep cycle (Figure 1). Each sleep cycle typically lasts 90 to 120 minutes. Sleep begins in Stage N1 which lasts about 5 minutes, and then progresses to Stages N2, N3 and N4. They are followed by REM sleep, which concludes the first cycle. During the first half of the night, there is much more non-REM sleep. Nearly all deep sleep is obtained in the first four hours. As shown in Figure 1, REM sleep becomes longer and longer in the later hours of the sleep cycle. The last two sleep cycles are generally made up of REM sleep and Stages N1 and N2. During sleep, brief awakenings occur from time to time. The total duration of these awakenings throughout the night is typically about 10 minutes. They are common for adults and are considered normal.

Sleep benefits

Research has shown that sleep provides several benefits. REM sleep is said to be involved in the restoration of:

• Attention
• Learning
• Memory in ATC
• Emotional balance and mood mechanisms.

Deep, slow-wave sleep appears to allow reconstruction, or maintenance, of physical integrity through nocturnal increases in protein synthesis and cell division.

The timing of sleep

Two components control the timing of sleep:

1. Circadian component - Sleep onset, due to circadian rhythms (the biological clock), is a periodic phenomenon that occurs at almost the same time every day. Sleep generally starts at the end of the day when body temperature begins to drop, and it continues for 6 to 8 hours. The spontaneous awakenings occur during the part of the thermal cycle when body temperature is rising, 2 to 3 hours after the thermal minimum. This temperature increase takes place, on average, around 5 am.
2. Homeostatic component - Homeostasis[1] refers to maintaining equilibrium in physiological and metabolic functions. Sleepiness increases with the length of time a person stays awake. The longer an individual stays awake, the higher the tendency to fall asleep.

The interaction of these two components determines the optimum time for sleep. The circadian component limits the ability to sleep at unusual times. How long a person stays asleep depends on when in the circadian cycle the individual falls asleep. Therefore, even when strong sleepiness results from sleep deprivation, the homeostatic component may not be powerful enough to lead to good sleep if the timing of a sleep attempt goes against the circadian component.

Individual differences

Average sleep duration is between 7 and 8 hours. The widely recommended minimum and maximum sleep duration limits are 4 and 11 hours, respectively.

A small percentage of the population (less than 5%) is either short sleepers or long sleepers. Short sleepers generally sleep less than 5.5 hours, and long sleepers more than 9.5 hours. The difference between short and long sleepers is primarily explained by differences in Stage N2 and REM sleep. Long sleepers have more awakenings, more light sleep and more REM sleep stages, but almost the same amount of deep, slow-wave sleep (Stages N3 and N4) as short sleepers.

Awareness of how sleep is affected by age can help an individual understand why he or she feels tired during certain times of the day or has sleep difficulties. As a person ages, many changes in sleep occur. Research has demonstrated that older adults have shallower nocturnal sleep with more arousals, meaning that they are more easily awakened during the night. This is primarily due to the fact that older adults have less slow wave sleep. Also, older individuals generally have earlier habitual sleep and wake times. Although it is not clear at this point, it may be that the older adult’s circadian rhythm becomes disturbed, which affects sleep patterns. On the other hand, children, teens and young adults experience more and longer periods of slow wave sleep, which explains why it is often very difficult to wake a younger person. Also, the circadian rhythm may have a stronger influence on younger people, causing them to feel more tired or sleepy during certain parts of the day.

Sleep Problems

Nearly a hundred sleep disorders are classified into four categories. This section focuses on the most common problems such as:

• Insomnia
• Sleep apnoea
• Narcolepsy
• Periodic limb movement (Restless Leg Syndrome)
• Parasomnia.

Diagnosis and treatment are beyond the scope of this section. For more information visit: National Electronic Library for Mental Health or Sleep Disorders on Helpguide.

Insomnia

Insomnia is not generally considered a disorder; rather it is more of a symptom or complaint. Insomnia is characterized by both a lack of sleep and excessive worry about sleep disturbances. Common complaints given by individuals experiencing insomnia are:

• Difficulty falling asleep
• Difficulty staying asleep (many awakenings)
• Waking up too early
• Feeling anxious, worried, depressed or irritable, especially at bedtime
• Feeling physically or mentally tired during the day
• Daytime sleepiness
• Impaired concentration
• Impaired memory

Sleep apnoea

Sleep apnoea describes a condition in which breathing stops for brief periods throughout the night. Each pause in breathing typically lasts 10 to 20 seconds or longer in some cases. These pauses can occur 20 to 30 times an hour.

Sleep apnoea occurs when the muscles at the base of the throat relax, resulting in loud snoring and laboured breathing. When complete blockage of the airway occurs, breathing stops or is impaired so drastically that the individual is prevented from reaching the deeper stages of sleep, which results in extreme daytime drowsiness.

The most common symptoms are:

• Loud snoring
• Morning headaches
• Trouble staying awake during the day
• Memory or learning problems
• Feeling irritable/depressed
• Not being able to concentrate on work
• Dry throat when waking up
• Frequent urination at night
• Waking up sweating

Narcolepsy

People with narcolepsy can fall asleep virtually any time (e.g., while working, talking, driving a car). These sleep attacks can last from 30 seconds to more than 30 minutes. Narcolepsy is a chronic disorder affecting the part of the brain where regulation of sleep and wakefulness take place. Narcolepsy can be thought of as an intrusion of REM sleep into the waking state. It causes a person to:

• Suddenly lose muscle tone and control when awake (cataplexy)
• Not be able to move or speak while falling asleep or waking up (sleep paralysis)
• Have vivid dreams while falling asleep or waking up (hallucinations).

Research suggests that the cause of narcolepsy is a lack of a chemical called hypocretin in the brain. This chemical stimulates brain cells and helps promote wakefulness. It is not known why hypocretin is missing in people who have narcolepsy. Some factors that may work together to cause a lack of hypocretin include:

• Infection
• Loss of certain brain cells due to brain injury, toxins and/or the body’s destruction of its own tissues (autoimmune reaction)
• Changes in hormones
• Stress.

The prevalence of narcolepsy has been calculated at about 0.03% of the general population. Onset can occur at any time throughout life, but the peak onset is typically during the teen years. Narcolepsy has been found to be hereditary as well as being associated with some environmental factors.

Restless leg syndrome

Restless leg syndrome (RLS) is characterized by sensations in the lower legs that make a person uncomfortable and causes the uncontrollable urge to move the legs in order to relieve the discomfort. RLS can make it difficult to fall asleep and stay asleep. People with RLS often do not get enough sleep and may feel tired during the day. This condition can be worsened by stress and can result in decreased quality of sleep.

One variation of RLS is periodic limb movements in sleep (PLMS), which is characterized by leg movements or jerks that typically occur every 20 to 40 seconds during sleep and can greatly disrupt it. These movements fragment sleep, leaving the person with excessive daytime sleepiness. Usually, it is the bed partner that makes the person suffering from PLMS aware that there is a problem.

Key Points

• Sleep is an active physiological process that includes five stages, N1-N4 and REM
• Stages N3 and N4 are most important in recovering from fatigue
• Dreams are more vivid during REM sleep
• More slow-wave sleep occurs in the first part of the night and more REM sleep in the second part
• Two components control the time of sleep:
  • The homeostatic component is based on wakefulness. The longer a person stays awake, the greater the tendency to fall asleep
  • The circadian rhythm, or biological clock, limits the ability to sleep at unusual times. How long a person stays asleep depends on when in the circadian cycle the individual falls asleep
• Temporary sleep disorders (1 to 6 nights) often result from stress, environmental factors or circadian factors
• Long-term sleep problems (3 or more weeks) result from sleep disorders and bad sleep habits
• Regardless of the source of sleep problems, sleepiness on the flight deck is a significant safety risk.

References

1. ^ Human homeostasis refers to the body's ability to physiologically regulate its inner environment to ensure its stability in response to fluctuations in the outside environment and the weather.

Additional OGHFA Material

Briefing Notes:

• Fatigue Manifestations
• Key Training Topics About Managing Jet Lag

Situational Examples:

• Engine Failure at Takeoff
• Takeoff Weight Entry Error and Fatigue
• Unidentified Fire On Board
• Fuel Starvation, Stress, Fatigue and Nonstandard Phraseology

Other References

• Sleep Disorders on Helpguide

Related SKYbrary Articles

• Fatigue

Further Reading

• Human Performance and Fatigue Research for Controllers, Gawron et al., 2011.
• FAA Fact Sheet – Sleep Apnea in Aviation, Feb 2015.
• Diabetes mellitus and its effects on pilot performance and flight safety
• Coping with long range flying. Recommendations for crew rest and alertness., Airbus, Cabon, P., et al., Nov 1995