Circadian Rhythm and Jet Lag
To understand the process of how jet lag occurs and how it can be managed, it is useful to look at how the body signals that it is ready to go to sleep and when it is ready to wake. The circadian rhythm – the ‘body clock’ – is signaling mechanism that tells the body when to trigger various physiological processes, e.g. when to eat, when to adjust temperature, when to sleep, and when to wake up. The rhythm runs every 24 hours (circa-dian translating as ‘around a day’) and is adjusted by light, chemical levels within the body, and cues from the environment. Daily adjustment of the circadian rhythm generally occurs automatically and keeps the rhythm running smoothly, similarly to ensuring that a wristwatch is set to the correct time each day.
Problems can develop when the circadian rhythm is affected by altered patterns of sleeping and wakefulness as it takes time to adjust. During the adjustment period, sleep is impaired, hunger signals are disrupted, and there may be a reduction in alertness and subsequent loss of mental performance. Shift workers who switch from daytime working to night-time working are familiar with sensations of altered temperature and feeling hungry at 3-4 AM which persist until the body clock adapts. Similarly, travellers who cross time zones often experience problems with the body clock adapting and experience disruption to sleep patterns and levels of alertness. This disruption is commonly termed jet lag and refers to the lag between the time frame of the biological clock and the time frame of the destination time zone. In this article, techniques for adjusting the biological clock and minimizing the effect of jet lag are presented.
The Body-Clock Mechanism
The body clock relies on an area of the brain called the supra-chiasmatic nuclei of the hypothalamus. This part of the brain is responsible for generating and sustaining the body clock rhythm. It will continue to produce a clock rhythm regardless of whether a person is continuously awake or asleep, exercising, resting, or in a light or dark environment. These factors only influence the timing of the clock rhythm and not whether it is generated.
Given that the body clock is an important timing signal for bodily processes, it is unsurprising that is relatively resistant to change, adapting slowly over several days to changes in environmental cues. The most important environmental cue is light exposure.
Adjusting the circadian rhythm generally involves one of two strategies; resetting or overriding the clock. Resetting the clock requires that the clock is shifted to match the desired times to signal when a person should feel tired and sleep, and when that person should feel more alert and wake up. Overriding the clock requires the use of medication to suppress the system by taking medication to encourage sleep and/or medication to induce alertness. Common examples of these medications include ‘sleeping pills’ or melatonin to promote sleep and caffeine or medications such as modafinil to encourage alertness.
Effects of Eastbound versus Westbound Travel
When looking at the experience of jet lag, typically in westbound flight, e.g. from London to New York, the body clock is able to shift 90 minutes closer to the destination time zone each day. For example, New York time is five hours behind ‘London time’ so during the flight, five time zones are crossed. To calculate the number of days required for adaptation is relatively straightforward: Multiply the number of time zones crossed by 60 (to work out the number of minutes difference between time zones), then divide by 90 to get the number of days. In this example it would approximately three days to fully adapt (5 x 60 = 300, 300/90 = 3.3). Westbound travel is associated with phase delay of the biological clock.
Eastbound travel requires the body clock to shift forward in order to adapt – phase advancement – and requires 60 minutes of adaptation per time zone crossed. In the example of travelling between New York and London, the return flight still crosses five time zones but now it takes longer for the clock to adapt. After arriving in London, it will then take five days to fully adapt as the body can only adapt by an hour a day rather than an hour and a half as in the westbound direction. In comparing the time taken for the body to adapt, is it clear that westbound travel will produce fewer symptoms than eastbound travel.
In simple, short flights like this it is straightforward to estimate the likely effect of jet lag. During long haul flights, e.g. from London to New Zealand, it is more difficult as the combination of the duration of flight and the time discrepancy can let to you seeming to arrive only a few hours later that you left but on a different day.
How do you minimise jet lag?
Several methods of minimising jet lag are can be used and relate to a combination of resetting and overriding the biological clock. These countermeasures can be behavioural, environmental, or medical.
Behavioural Adjustment Options
Daylight is the most powerful timing signal for the biological clock. Getting adequate daylight exposure in the destination time zone is beneficial. To maximise the effect, try to increase daylight exposure at the start of the day and reduce it towards the end of the day. Increased daylight exposure at the end of the day can cause phase delay of the circadian rhythm and delay the signal to sleep.
The body uses melatonin to adjust the timing of the circadian rhythm and encourage sleep. Normally, the levels of melatonin fluctuate during the day with the highest levels at 8 – 11 PM and the lowest levels at 4 – 5 AM.
Melatonin pills can be taken to override the body’s natural level and shift the timing of the body clock. It has the greatest effect when combined with light exposure. Melatonin is most effective if taken at the desired bedtime in the destination time zone. Usually only a few days of medication are required in order to adjust the timing of the clock.
Several regimes of melatonin dosing have been described, a simplified dosing regimen which will work in the majority of cases would be to take 5mg melatonin en route at the destination bedtime, then 5mg nightly for 3 to 5 nights after arrival, again at the destination bedtime.
Hypnotic medications are sometimes helpful in managing jet lag, particularly when they are short acting and encourage sleep without causing sedation. Drugs such as zolpidem or zopiclone have a typical duration of action of 3 – 5 hours so that they have minimal effect on waking and hence do not interfere with the morning clock signal.