A circadian rhythm is any biological process involving a built-in, self-sustained, oscillation during periods of about 24 hours, which are created by Earth's rotation. These 24-hour rhythms have been widely observed in plants, animals, fungi and cyanobacteria. The rhythms are adjusted (entrained) to the local environment by external cues, which include light intensity, temperature, humidity and redox cycles.
Disruption of the Biological Clock
An individual's circadian rhythm can be described essentially as the internal biological clock that regulates our body functions, based on our wake/sleep cycle. Circadian rhythms are not only important in determining sleep cycles but also in feeding patterns. There are clearly recognizable patterns of brain-wave activity, hormone production, cell regeneration, and other biological activities linked to each individual's daily cycle. Possibly because days on Earth were longer in the distant past, the circadian cycle is a 25-hour cycle. The risk in aviation is that any time that our normal circadian rhythm is altered or interrupted, physiological and behavioural effects occur. This risk is known as circadian rhythm disruption, or CRD.
Effects of Circadian Rhythm Disruption (CRD)
Shift work almost always causes CRD because the internal biological clock is at odds with the shift pattern, impacting on performance and increasing the risk of accidents and health problems. Shift workers experiencing CRD may experience difficulty falling and staying asleep, increased daytime sleepiness, a general lack of energy in the morning, an increase in energy in the evening or late at night, difficulty concentrating, oversleeping and trouble waking, and increased negative moods.
The most debilitating symptom of CRD is Fatigue but people experiencing CRD may also experience insomnia, headaches and digestive system problems. CRD-induced fatigue, as noted, can have physiological and psychological ramifications including increased reaction time, decreased attention, impaired memory, distraction, irritability and indifference.
Rapid time zone changes — common amongst flight crews and cabin crews operating long haul routes — cause CRD, which is more commonly referred to as jet lag.
Strategies for Minimising the Impact of CRD
Guidance for Flight Crew
- Sleep well at home before any flight;
- Try to get at least as much sleep per 24 hours as you would normally at home;
- If you are sleepy, try to sleep. Employ strategic (combat) napping techniques;
- Whenever possible, take a 30-minute nap prior to a long flight. Avoid naps of more than 30 minutes, as they involve deep sleep;
- Taking a nap is better than not sleeping at all;
- Avoid adaptation to a local circadian rhythm following transmeridian flights with short layovers. Try to maintain the circadian rhythm from your place of origin, and at the same, time try to sleep longer;
- Use caffeine strategically during the flight to counteract circadian rhythm-induced sleepiness; and,
- While in the cockpit seat, converse with others, stretch your legs, and take regular breaks.
Guidance for Schedulers
- Try to avoid scheduling night flights following a transmeridian flight.
- Where possible, transmeridian flights should be alternated with intrameridian flights, enabling flight crews to return to their normal circadian rhythm.
Resetting the Biological Clock
- Exposure to daylight: Bright light helps to reset circadian rhythms and increases serotonin levels in the brain, improving well-being and promoting a positive mood.
- Physical Activity: To adapt quickly to a new time zone, adopt the sleep and eating patterns of that timezone: Get up at dawn, eat breakfast and get out and exercise in the daylight. If you are tired when you arrive in the new time zone, and go to sleep, then your body will set the circadian rhythms back to the original timezone. (There are occasions when this might be desirable — for example if you are returning to the original timezone after a short layover.)