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Day To Day Rhythms For Sleep
Day-to-day rhythms are the natural daily rhythms that govern our daily lives. The circadian clock, located in the suprachiasmatic nucleus of the hypothalamus, regulates daily variations in numerous physiological processes such as sleep/wakefulness, temperature, and hormone release, as well as cognitive performance.
A daily trough in body temperature occurs about 1.5-2 hr before the usual wake time. This is close to the time of a daily trough in alertness and cognitive performance. Thus, the circadian clock is programming the body for maximal sleepiness in the early morning hours (about 0300-0500 for a person on a typical sleep/wake schedule).
There is a second period of increased sleepiness that occurs in the mid-afternoon, regardless of whether lunch was eaten.
A human allowed to live in an environment free of time cues (i.e., no external light/dark cycle or clocks) will extend the usual 24-hr day to a longer period, closer to about 25 hr. (16) This extension is probably due, at least in part, to the natural internal tendency of the circadian clock to run with a cycle length that is slightly longer than 24 hr.
This basic physiological property of the clock explains the relative ease of staying up later (i.e., lengthening the day) and the relative difficulty of trying to initiate sleep earlier than usual (i.e., shortening the day). Lengthening the day is called a phase delay while shortening it is called a phase advance.
Ordinarily, the clock is synchronized to the 24-hr day by the daily alteration in light and darkness. Problems can arise because the circadian clock cannot adjust immediately to an abrupt change in the timing of the
light/dark or sleep/wake schedule, as in shift work. When humans move to a new time zone or change their shift schedule (e.g., work at night and sleep during the day), the internal circadian clock can take from days to weeks to adapt physiologically to the new schedule.
Manipulation of bright light can facilitate this adaptation. Anytime there is a lack of light cues to keep the clock synchronized, it will tend to move toward its natural, longer than 24-hr rhythm.
Personal vigilance is related to the body’s circadian rhythms rising from a low level early in the morning, peaking during the day, and then falling again. Unfortunately, ATS officers must provide service throughout the 24-
hour cycle, and often perform their critical tasks when their ability to be vigilant is lowest.
1-What causes sleepiness?
People who lose sleep not only become sleepy but also have to make a substantial effort to avoid unwanted sleep onsets. It is easy to underestimate the magnitude of the drive for sleep after an extended period without any.
Sensitive laboratory measures show that, after two days without sleep, subjects will fall asleep immediately (i.e., in
less than 2 min) in a sleep-conducive environment.
Such an environment is more likely to be found in an information center than on the flight deck, and consequently, many of the people doing the planning and other “desk work” are at greater risk to fall asleep on the job than those who are more physically active.
Factors that promote the onset of sleep are:
- Warm temperature
- Reduced social stimulation & interaction
- Dim lighting
- Minimal physical activity
- Low noise levels
- Passive, monitoring-type work
However, if there is a lull in physical activity, even persons with active job demands can fall asleep quickly after 24 hr of wakefulness.
2- What are the physical and mental effects of sleep deprivation?
The problems resulting from sleep loss are pervasive and insidious, affecting virtually all aspects of performance.
Microsleeps are more likely to occur. These are brief (several seconds or less) episodes of total perceptual disengagement from the environment.
Microsleeps can occur during periods of otherwise acceptable performance. Consequently, performance tends to become more variable and uneven — people perform well for periods of time with brief lapses, errors, and other performance failures interspersed.
Lapses are failures to respond to information, or failures to respond in a timely manner. There is also a general tendency toward slower performance, resulting in longer reaction time. The tendency to experience microsleeps, lapses, and slower reaction times combine to result in reduced vigilance.
Short-term memory can become impaired. That is, newly learned information becomes difficult to store and retrieve from memory. Fixation on a particular task or component of a task can also occur, resulting in failures to perceive and process other possibly critical information.
Communication often becomes reduced in amount and effectiveness.
Decision-making can be impaired, with people showing a tendency to choose options that involve less effort, even though they have a known lowered probability of success.
There can be a general loss of situational awareness in all environments. In general, the performance problems described above are more likely to occur on novel or higher-level cognitive tasks, while well-learned tasks are more resistant to the effects of sleep loss.
3- How does sleep loss affect behavior?
Giving up speed for accuracy
The nature of the performance difficulty can depend on the type of task. On tasks where people can proceed at their own pace, there is a marked tendency to slow down in order to maintain accuracy. This occurs despite explicit instructions and attempts to work as quickly as possible.
Sacrificing routine maintenance
When sleep deprived there is a general tendency to change how one allocates limited mental and physical resources. In the operational environment, this tendency often can result in shedding maintenance and other routine tasks in order to perform one’s primary task. This is a particular problem for those working with complex systems or systems requiring maintenance during the period of sleep deprivation.
There is a general degradation of mood with sleep deprivation. While there is a tendency to dismiss this outcome, the mood is undeniably important to morale and to effective crew communication and resource management.
People can function physically when sleep deprived but will want to quit earlier due to feeling that they do not have the energy to continue. There is a marked reduction in motivation. Sleep loss itself tends to become
the predominant theme. People change their behavior as a result. Among other things, they eat less and may need to be encouraged to obtain meals and look after their own basic needs.
Nevertheless, effort increases at the same time that motivation (i.e., desire to continue) and performance both drop. One keeps trying to do a task if asked or required, even though one does not want to continue. In order to keep going a greater compensatory effort is expended.
This effort is often accompanied by a reduced ability to control what is happening in the environment and so can lead to anger, frustration, emotional outbursts, and cutting corners to conserve energy. These performance decrements will increase despite increasing compensatory effort on the part of motivated individuals.
Shift Work – A Better Understanding
In the literature on air traffic control, some attention has focussed on how shift work and work schedules result in fatigue, and on how they affect performance, sleep, mood, and health.
Authors investigating fatigue among ATCOs find that fatigue related to shift work is twofold:
1) ATCOs working at night are at the lowest point in their circadian rhythms, which results in fatigue, sleepiness, and performance decrements.
2) Shift schedules often create sleep debt, which reduces alertness and performance, particularly during night shifts and at the beginning of early morning shifts.
The sleepiness and fatigue reported by ATCOs can be attributed to the circadian trough occurring at night, but also to sleep deprivation and its associated sleep debt.
For the shift worker, night shifts entail sleeping during the day. Again, because of circadian rhythms, and also because of the diurnal orientation of social life, ATCOs working at night get the shortest amount and poorest quality of sleep.
Also, the quality of sleep ATCOs gets before a night shift is poor compared to sleep before a day or evening shift, according to subjective reports of ATCOs and results obtained with sleep lab measures.
Fatigue, sleepiness, circadian trough, sleep deprivation, low traffic load, and low lighting levels have been linked as factors contributing to decreased performance and vigilance during night shifts in ATMs.
Working the day shift can also entail sleep loss because ATCOs do not go necessarily go to sleep earlier at night before working an early day shift, and get less sleep in the morning (compared to the evening shift and days off) due to early rise.
Shift workers can have difficulties in compensating for an early rise the next morning by going to sleep earlier because there is a period before usual sleep onset when the biological clock seems to prevent sleep.
Compared to performance later in the day, early day shift performance is decreased. Considering the higher frequency of accidents during early morning shifts among other groups of workers, operational safety may be threatened by fatigue and performance decrements experienced at the start of early morning shifts.
Scheduling The Shifts
In addition to the problems inherent in night and day shifts, scheduling the shifts also introduces difficulties. Scheduling is a sensitive topic and a satisfactory solution to the optimal scheduling system for all air traffic control situations remains elusive.
As can be seen in Table 1, various types of shift schedules exist to cover the 24-hour period of operations in air traffic control facilities.
Permanent schedules involve always working the same shift. The slow rotation schedule, a variation of the permanent schedule, involves working five straight days on a specific shift, then rotating to another shift the following week. Other schedules imply rapid rotation of shifts during the week.
Although the exact configuration may vary, two main kinds of rapid rotation schedules exist clockwise rotation and counter-clockwise rotation.
In the clockwise rotation (also called forward or delayed rotation), the work week starts with a day shift, rotating later in the week to an afternoon shift, and finally changing to a night shift.
In the counter-clockwise (backward or advancing rotation), the work week starts with an afternoon shift, then advances to an early day shift, to finally ends with a night shift.
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