Minnesota Flyer - Serving Midwest Aviation Since 1960

By Dan McDowell
MnDOT Aeronautics 

In the Rhythm of Flight


Courtesy of David Gonzales Photography

Flight operations for military, airline, cargo flight crews, and even corporate/business flight crews are often disruptive to the normal cycle of human physiology. With frequent changes in flight schedules, (flights departing varying hours of the day and night), sleep for pilots, at least quality sleep, is often hard to come by. Quality sleep sometimes seems like a unique facet of life that busy flight crew members may experience only on occasion. That raises the question, do irregular schedules impact pilots and other flight crew members? The answer is yes. It can impact anyone whose regular sleep cycle is disrupted.

When people deviate from their regular work/sleep schedule, their biological rhythms are thrown out of sync. While many human biological functions naturally vary during a 24-hour day, some of them function and vary systematically within a 24 hour cycle. This is called the "circadian rhythm." The variations are controlled by the "circadian clock" located in the brain. Researchers have discovered the circadian clock is located in the suprachiasmatic nucleus of the hypothalamus gland near the center of the brain.

In an article from the Operator's Guide to Human Factors in Aviation, it states that," Laboratory studies have shown that, in the absence of any time cues (i.e., no sunlight or social time cues), the biological clock for most humans operates on a cycle of about 25 hours. Under ordinary circumstances, however, the biological clock is reset by about one hour each day such that the biological clock is synchronized with the 24-hour solar day. The cues that serve to reset the biological clock are called "zeitgebers," a German word that means "time givers." Evidence supports morning sunlight as the most important zeitgeber. Other cues in the social environment that serve as zeitgebers have not been identified with any amount of certainty. However, cues that may serve as zeitgebers include work/sleep schedule, eating schedule, social activities and, in the absence of other cues, subtle environmental factors such as building vibration and traffic noise."

The article goes on to say, "both laboratory studies and field studies have demonstrated variations related to circadian rhythms in behavioral functions such as alertness, reaction time, short-term memory, long-term memory, search tasks, vigilance, and sleep. The circadian variation throughout a normal solar day is not the same for all biological and behavioral functions. Performance efficiency tends to decline to a low point in the early morning hours, (2AM - 6 AM). The important implication of this research is that circadian rhythms influence performance efficiency even when the circadian variations are in synchrony with the solar day and the normal work/sleep schedule."

Research also shows that when an individual experiences circadian rhythm disruption (CRD), a series of problems can result. One problem is that the person with CRD may be working during the phase of their circadian cycle when their performance efficiency is near or at its lowest point. Another problem is, if the work/sleep cycle has been disrupted, the individual may have a decreased amount of quality sleep. That in turn leads to fatigue. Some of the causes of fatigue include:

The shortage of quality sleep

Sleep disturbances

Poor diet


Exertion from heavy exercise

Mental and/or emotional stresses

Interruption of the circadian rhythm

Pilots or passengers who may be experiencing CRD may show one or more of the following symptoms:

Increased day-time sleepiness

A general lack of energy in the morning

Difficulty concentrating or accomplishing mental tasks

Maintaining alertness

Increasing negative moods

Impaired sensory perceptions and decision making

Diminished decision-making skills, including making rash decisions, or no decision at all.

So one might ask, does CRD really affect your flying and/or driving skills? The answer is, yes, absolutely! The FAA's publication, Circadian Rhythm Disruption and Flying, states that, "CRD- induced fatigue that goes untreated or ignored will have both physiological and psychological ramifications that not only can jeopardize your personal health but can also become a safety- of-flight issue." This author would add the exact same wording for driving. Think about road rage, or missing your turn and then doing a U-turn instead of going around the block, or getting off at the next exit to turn around safely.

A few of the known and undesired effects of CRD are:

Increased reaction time

Decreased attention

Impaired memory

Personal conduct of isolation

Impaired decision making

Mood changes


All of the above listed effects lead to the possibility of increased risk in (your) aviation operations including; increased frequency and severity of pilot errors and increased operational incidents. Remember, the FAA publication Circadian Rhythm Disruption and Flying, says, "circadian rhythm disruption can lead to acute or even chronic fatigue. Fatigue in the cockpit has shown to be just as debilitating as drugs and alcohol. Do not let CRD-induced fatigue become a hindrance to aviation safety."

Today, many people and organizations use electronic devices to help track and manage individuals' fatigue and fitness levels. These fitness tracking devices are often small enough to be worn on the user's wrist. Other types of devices include mounted cameras that monitor the individual's facial area and expressions. When the individual shows signs of fatigue for instance, the unit sends a warning in the form or various tones, bells, or other attention getting methods to wake up that person. These devices and systems also help the individual to keep track of their sleep hours and various other personal health/fitness data.

If you want to mitigate the issue of CRD, you can use guidelines that many professional pilots use. Read them along with other detailed information in the Flight Safety Foundation/NBAA document: Duty/Rest Guidelines for Business Aviation. You can find it as a free download at: https://flightsafety.org/files/DutyRest2014_final1.pdf .

Bear in mind that the period of circadian low is between 0200 and 0600 for people who have already adapted to a common day-wake/night-sleep schedule. A single (professional) pilot may have a maximum of a 12 hour duty day. That includes a maximum of 8 hours of flying during that day. Then he/she must have a minimum of 12 hours of off duty rest before starting another duty day.

These are the guidelines some organizations use when they have operations within the circadian low window. That includes a duty day that begins at 0400 or earlier, or flight through both sides of the circadian low, or a landing during the circadian low window time period.

Courtesy of Gary Chambers

Safety of course, should always be priority one. It is your responsibility to be sure you, as the pilot-in-command, are fully prepared and completely ready to take your planned flight. Proper rest prior to your planned long duty day is vitally important, as is the proper rest at the end of your long duty day, or even your regular duty day. Getting proper rest and always using your checklists are two key elements of your safe flying practices.

For additional information on CRD and its potential impacts on your flying (and driving, too) check out the following publication:


Publication No. AM-400-09/3 Written by: J.R. Brown Melchor J. Antuñano, M.D. Federal Aviation Administration Civil Aerospace Medical Institute.

To order copies of this brochure, contact:

FAA Civil Aerospace Medical Institute Shipping Clerk, AAM-400

P.O. Box 25082, Oklahoma City, OK 73125 Telephone: (405) 954-4831

For more pilot safety information, see: w w w.faa.gov/pilots/safety/pilotsafetybrochures


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