Low Visibility at Night

Cartoon: Circadian Rhythms, from You Want Proof?.

Eddie Sez:

I think I've actually fallen asleep twice in the pilot's seat in my almost 35 years flying. The first time was in a T-37 trying to build time, the need for sleep had nothing to do with jet lag. The second time was in an Air Force Gulfstream III. I woke after flying halfway around the world up to a Russian air defense controller yelling at me, see Flying Where the Bullets Aren't. I consider myself a pretty conscientious pilot and don't tolerate anything not flight related in my cockpits. And yet there I was. Jet lag?

What the medical community calls desynchronosis has always plagued us who cheat gravity for a living. I think it first became a serious topic for flight safety studies following the China Airlines 006 crew who almost lost a perfectly good Boeing 747 over the Pacific. It can also be used as an excuse. The NASA study quoted below cites Korean Air 801 as a case where fatigue was causal, and yet it really wasn't. Whatever you call it, you need to understand it, recognize it, and know how to combat it.

Most of this stuff is pretty dry reading, from the references listed below. I've tried to pull out the important stuff up front with a medical-to-pilot translation following in blue. What you really want to know, what I wanted to find out, is how to best deal with it. If you like, you can skip down to Coping Strategies.

Desynchronosis, by the way, is simply the desynchronization of one's body clock with the rest of the world. It has two forms: jet lag and shift lag. Jet lag you know. Shift lag is what shift workers going from day to night shift work, or night to day shift work, experience.


[NASA, pg. 10]

  • It is widely believed that sleep is a time when the brain and the body shut off and then re-engage upon awakening. Actually, sleep is a highly complex physiological process during which the brain and body alternate between periods of extreme activity and quiet, but are never “shut off.” Sleep is composed of two distinct states: NREM, or non-rapid eye movement, and REM, or rapid eye movement, sleep. These two sleep states are as different from each other as they are from wakefulness.

  • During NREM sleep, physiological and mental activities slow (e.g., heart rate and breathing rate slow and become regular). NREM sleep is divided into four stages, with the deepest sleep occurring during stages 3 and 4. There is usually very little mental activity during NREM stages 3 and 4. If awakened during this deep sleep, an individual may take sometime to wake up and then continue to feel groggy, sleepy, and perhaps disoriented for 10–15 minutes. This phenomenon is called sleep inertia.

  • REM sleep is associated with an extremely active brain that is dreaming, and with bursts of rapid eye movements (probably following the activity of the dream); during REM sleep, the major motor muscles of the body are paralyzed. If awakened during REM sleep, individuals can often provide detailed reports of their dreams.

  • Over the course of a typical night, NREM and REM sleep occur in a cycle, with about 60 minutes of NREM sleep followed by about 30 minutes of REM sleep. This 90-minute cycle repeats itself throughout a typical sleep period. However, most deep sleep (i.e., NREM stages 3 and 4) occurs in the first third of the night, and REM periods are shorter early in the night and then become longer and occur more regularly later in the sleep period. Overall, about 25% of sleep time is spent in REM sleep and about 50% is spent in NREM stage 2.

You spend about a quarter of your sleep in the Rapid Eye Movement (REM) phase where your major motor muscles are paralyzed and you have those vivid dreams. But this isn't your deep sleep, that happens during another quarter of your sleep time known as Non-REM stages 3 and 4. You normally have 60 or so minutes of Non-REM stages 1 and 2, stages 3 and 4, back to stages 1 and 2, and then 30 minutes of REM sleep.

Circadian Rhythms

[USAARL, pg. 323] The mechanism that mediates the interdependence of human physiology with environmental cycles of light and dark has been referred to as the circadian timing system (CTS). It is generally accepted that the CTS synchronizes physiological and behavioral rhythms to a circadian period (circa, about; dies, a day) and that the phase of these rhythms can be reset by the influence of the environmental light/dark cycle.

Light/Dark Cycle

[USAARL, pg. 323]

Physiological and Behavioral Rhythms Exhibiting 24-Hour Periodicity

[USAARL, pg. 325]

Rhythm Acrophase
Sleep Night
REM Sleep Night
Exercise capacity Day
Activity Day
Core temperature Day
Hydroxicorticosteroid excretion in urine Day
Adrenalin and noradrenalin production Day
Cognitive performance Day

Internal and External Synchronization

[USAARL, pg. 324] Under a consistent environmental LD cycle, physiological and behavioral rhythms maintain a relative synchronization with each other (internal synchronization) and with the daily rhythm of sunrise and sunset (external synchronization). That is, in steady state conditions, they exhibit their peaks and troughs at approximately the same time of day or night, depending on their natural expression.

Your body does have a normal day/night cycle to it and this forms a rhythm where your normal active period is better suited to the day and your normal inactive period is better suited to night.

The Body's Natural Clock and Sleepiness

[USAARL, pg. 326]

[NASA, pg. 22] When people live alone in environments from which all possible time cues have been carefully excluded (deep caves, underground bunkers, or specially designed apartments), they begin to live “days” that are generally longer than 24 hours. Regardless of how long someone’s subjective “day” becomes in a time-free environment, however, the circadian clock still enforces an approximately 25-hour cycle in many functions. Some people even develop “days” as long as 50 hours with, for example, 36 hours of wakefulness followed by 14 hours of sleep.

[NASA, pg. 20] We are physiologically programmed for two periods of maximal sleepiness in a usual 24-hour period. The period 3–5 A.M. is a circadian low point for temperature, performance, and alertness. During this time, the brain triggers sleep and sleepiness. The other period of increased sleepiness is roughly 3–5 P.M. Most individuals have experienced an afternoon wave of sleepiness. These windows can be used to schedule sleep periods or naps because the brain provides a period of maximal sleepiness and an increased opportunity for sleep.

[FASMB, pg. 1] Maximal sleepiness occurs between 0600 and 0800. Although not as imposing, another episode of sleepiness occurs between 1400 and 1600.

Your natural rhythm is not based on a local clock but an internal one. For most of us it ends up being a 25 hour day, but that varies with individuals. Most of us, however, are forced to live in a 24-hour day. If you are programmed to a normal 2400 midnight and 1200 noon day/night schedule, then you will be sleepiest between 0300 - 0500 and 1500 - 1700 according to most studies.

Desynchronosis: Jet Lag

[USAARL, pg. 324] Individuals who engage in rapid transmeridian travel (e.g., airline crews) undergo alterations in the timing of the CTS. These changes manifest in the desynchronization of physiological and behavioral rhythms. Upon arrival, travelers encounter a new rhythm of sunrise and sunset and social schedule. The impact of these changes on body physiology results in several days of fatigue, sleepiness, lethargy, insomnia, gastrointestinal track disorders, and poorer mental agility and performance.

"Morning" versus "Night" People

[USAARL, pg. 333] Morning people ("larks"), with strong preferences for early morning wake up times, exhibit early morning peaks and large amplitudes in their temperature rhythms. These characteristics make the circadian system rigid and result in a greater resistance to resynchronization. In contrast, evening people ("owls") appear to have a more labile circadian system exhibiting smaller amplitudes in temperature rhythm than "larks," their peaks occur later in the day.


[NASA, pg. 29] A NASA study of the effects of aging found that as flight crews get older, they become more morning-type and the amplitude of their circadian temperature rhythm declines. Daily percentage sleep loss during trips was 3.5 times greater among long-haul crews aged 50–60 than among long-haul crews aged 20–30.

Not everyone suffers equally from jet lag. Generally speaking, younger people and "night owls" do better crossing time zones.

Subjective vs. Physiological Sleep and Alertness

[NASA, pg. 18]

Odds are that you are falling asleep faster than you think and sleeping longer than you give yourself credit. But you are also probably more sleepy thank you think too.

The Effects of Jet Lag on Performance

[USAARL, pg. 327] In general, psychomotor performance after a 6-hour transmeridian flight suffers a reduction of 8-10% of preflight levels during the first day after arrival.

[NASA, pg. 28] Sleep loss leads to increased waking sleepiness. Many people equate sleepiness with being lazy or acknowledge it only humorously. Sleepiness can have severe consequences for us as individuals and as a society. Sleepiness can degrade essentially every aspect of human performance. Sleep loss and sleepiness can decrease physical, psychomotor, and mental performance, and can increase negative mood and decrease positive mood. Therefore, a principal consequence of sleepiness is an increased vulnerability to performance decrements. It is important to consider this as a performance vulnerability because, like the effects of alcohol on performance and memory, sleepiness can lead to a reduced safety margin and an increased potential for operational incidents and accidents. Sleep loss and sleepiness resulting from extended duty or altered work/rest schedules have been suggested as contributory factors in many accidents and catastrophes. Many people put themselves at personal risk by driving when too sleepy, sometimes experiencing a near incident or an actual accident.

[FASMB, pg 2]

It is said that the effects of fatigue are similar to alcohol. You don't react as quickly or accurately, you don't remember things as well, and your motivation to do things correctly suffers as well. On top of all that, you become less critical of yourself and therefore become a poor judge of your own performance.

Coping Strategies

Preventive Strategies (Before the trip)

[NASA, pg. 45] In general:

[NASA, pgs. 47-48] Good sleep habits:

[FASMB, pg. 5]

Make a ritual about sleeping at home. Get rid of the bedroom television to train your brain that the bed is for sleep. Get into a regular schedule so that prior to bedtime you do the same things, avoid heavy exercise, eating, drinking, and go to bed. Get up at regular times too. If you can't fall sleep in 30 minutes, don't lie in bed awake. Get out of bed for a while and do something relaxing.

Strategic Napping

[NASA, pg. 46]

If you can take a nap, time it appropriately. If you have to be on duty within an hour or so, limit the nap to 45 minutes to prevent deep sleep. If you have enough time to fully awaken following the nap, then a nap longer than 2 hours is permitted.

Operational Strategies

Cartoon: Special coffee, from Chris Manno.

[NASA, pg. 49]

  • Operational countermeasures are challenged by FARs that require crewmembers to remain seated at their assigned duty stations with their seat belts fastened. This poses a challenge because one of the most successful technique for combating sleepiness, according to the earliest sleep-deprivation experiments, is physical activity. Whenever possible, engage in physical activity, even if it is only stretching. Take regular stretch breaks and while seated remain as active as possible—even writing helps. Engage in conversations with others and be sure to participate; don’t just nod and listen.

  • Caffeine, a stimulant, can be consumed strategically to acutely increase alertness. It is best not to continually consume caffeine before, during, and after a trip. Instead, determine the potential periods when caffeine could be used to combat a specific period of sleepiness (e.g., 3–5 A.M. or 3–5 P.M.). Avoid using it when already alert, for example, when just beginning a daytime duty period or immediately after a nap. Though affected by several variables (e.g., body size, previous food intake), caffeine will usually take 15–30 minutes to take effect and then last for up to 3–4 hours. Therefore, continually consuming caffeine throughout a flight could interfere with subsequent sleep on layover. Stop caffeine consumption far enough in advance of a planned bedtime so that it will no longer be active.

  • Be sensible about nutrition. Whenever possible, maintain a balanced diet. Obviously, flight operations can interfere with regularly scheduled, balanced meals. Try to carry appropriate snacks as needed. Drink plenty of fluids and stay hydrated. Between reduced cockpit humidity and caffeine (a diuretic), it is easy to become dehydrated.


Adjusting to the New Time Zone

[USAARL, pg. 328] Transmeridian travelers who expect to remain in the new time zone for weeks should quickly change the timing of behavior patterns to match local events, that is, sleep and wake-up times, meal times, and social events should match local schedules as soon as possible. To speed up the rate of readjustment, it is necessary to (1) immerse oneself in local society, maintaining normal social activity patterns, (2) engage in physical exercise, and (3) maintain regular exposure to daylight.

Remaining on Home Base Time

[USAARL, pg. 328] In certain circumstances, travelers can attempt to maintain the time schedule of their departure location by not resetting their watches and by scheduling events, meals, and sleep times to match their home base clocks. This technique avoids most of the CTS disruption of travel except for the change in LD cycle.

Sleep Timing

[USAARL, pg. 328] Because getting uninterrupted restful sleep upon arrival is often difficult, the traveler should be fully rested at departure so there is no "sleep debt." A wristwatch can be set to the destination time zone and activities pursued according to the next clock time. Bedtime should be shifted progressively earlier each night (phase advance) for a few days before an eastward flight and progressively later (phase delay) in anticipation of a westward flight. The time of awakening must also be adjusted.

[NASA, pg. 36] The problem with having to get up earlier than usual is that it is very difficult, if not impossible, to fall asleep sufficiently early the night before to compensate (even when the duty schedule permits). It is not simply a question of discipline or motivation. The circadian clock effectively opposes falling asleep earlier than the habitual bedtime. Just as there are preferred times in the circadian cycle for falling asleep, there are also times when sleep onset is very unlikely. These times have been labeled “wake maintenance zones,” and one of them occurs just before the habitual bedtime. In addition, because the “biological day” dictated by the circadian clock tends to be longer than 24 hours, it is easier to go to sleep later than to go to sleep earlier. Going to sleep later also means staying awake longer, which allows more time for the homeostatic “sleep pressure” to build up.

Meal Timing and the Use of Diets

[USAARL, pg. 329] Diet has not, however, been shown to aid in actual resynchronization of the CTS. . . . Although choice of meals may possibly aid in inducing sleep or alertness, it is the consistent timing of meals and fasting periods that will serve as phase information for the oscillator influencing the GI track and that will ultimately play a significant role in facilitating CTS resynchronization.


[NASA, pg. 14.] Alcohol has a profound effect on the usual sleep cycle. After more than a couple of glasses of wine or a couple of beers (with individual variations), alcohol can essentially eliminate all of the REM sleep in the first half of a sleep period. This can lead to subsequent alcohol withdrawal effects in the second half of the sleep period, including sleep fragmentation. Unfortunately, the most widely used sleep aid in the United States is alcohol. Ironically, although often used to promote relaxation and the ability to fall asleep, it will generally have major disruptive effects on the subsequent sleep. One NASA study found that short-haul pilots increased their alcohol consumption threefold during trips compared with home consumption. The pilots used alcohol within FAR guidelines to unwind after long duty days that included multiple flight segments and to promote sleep onset before an early wake-up for the subsequent duty day. Alcohol also interacts in a synergistic fashion with sleepiness. A sleep-deprived individual who is already sleepy will demonstrate more severe performance and alertness impairment following alcohol consumption.

Bright Light Therapy and Jet Lag

[USAARL, pg. 329] Bright artificial lights may be used to mimic the destination LD cycle before the transmeridian flight. In this manner, travelers may reduce to desynchronosis experienced upon arrival.

[NASA, pg. 52] Bright light has been shown in laboratory studies to facilitate rapid circadian adaptation. Two to three hours of bright light (i.e., 2,500–10,000 lux) administered at the appropriate phase of the temperature cycle for three successive days may facilitate an 8- to 12-hour shift of the circadian clock. Separate from its effects on the circadian clock, bright light also can have an independent alerting effect.


[USAARL, pg. 332] In general, the human CTS exhibits a preference for quicker reestablishment of internal synchronization after a delay in the activity/rest rhythm or LD cycle.

[NASA, pg. 13] Another commonly held belief is that after sleep loss, an individual has to “make up” that sleep by sleeping a number of hours equal to those lost. Scientific laboratory studies have demonstrated that following sleep deprivation, recovery sleep is deeper (more NREM stages 3 and 4), rather than extended. During recovery sleep, an individual might sleep somewhat longer, but the most notable feature is the increase in deep sleep.

As corporate pilots we rarely have the opportunity to fully "sync up" with our new environment before it is time to go home or move further away from our normal biological clock. It has been my experience the following strategy works best for me:


USAARL 1990: Circadian Rhythm Desynchronosis, Jet Lag, Shift Lag, and Coping Strategies, Carlos A. Comperatore and Gerald P. Krueger, United States Army Aeromedical Research Laboratory, Biomedical Applications Research Division, September 1990.

NASA 2002: Crew Factors in Flight Operations XV: Alertness Management in General Aviation Education Module, NASA, February, 2002.

FASMB 2006: Fatigue and Desynchronosis in Air Crews, Federal Air Surgeon's Medical Bulletin, Summer 2002.

"You Want Proof? I'll Give You Proof!" More Cartoons from Sydney Harris, Sydney Harris, 1990