Jet lag is exhausting, disorientating, and can even make you lose your appetite and libido.
According to Air & Space magazine the term "jet lag" may have been first coined in February 1966. "If you're going to be a member of the Jet Set and fly off to Katmandu for coffee with King Mahendra," wrote Horace Sutton in the Los Angeles Times, "you can count on contracting Jet Lag, a debility not unakin to a hangover. Jet Lag derives from the simple fact that jets travel so fast they leave your body rhythms behind."
It's only in the last few decades that humans have been able to jump time zones, and we've yet to find a way to adapt. To overcome its effects, travellers have turned to everything from taping plant seeds to acupressure points behind their ears to swallowing Viagra.
But what actually happens to our bodies when we leap across time zones, and how is research helping us understand how to ease the effects?
Jet lag is the result of a disruption in our body's internal clocks. Our biological clocks drive our circadian rhythms, which anticipate dawn and dusk, and controls everything from blood pressure to how hungry we are. The system is synchronised to a period of around 24 hours – a "master clock" in our brain that is sensitive to our exposure to light coordinates all the body clocks within our organs and tissues.
All these clocks are controlled by the hormone melatonin, which is produced by the master clock when it gets dark to make us feel drowsy, and controls our body temperature when we sleep. When we fly to a different time zone (or work night shifts), all the clocks in our body go out of sync – or, to use the technical term, "desynchronise". Each clock then takes a slightly different amount of time to re-adjust, which is why we feel so bad.
It takes most people a few days to fully adjust, depending on not only how many time zones have been crossed, but also the direction of travel. Adjusting to eastward travel is harder because of the way our internal body clock functions. As this clock follows a period of just over 24 hours, our bodies compensate every day by contracting this to synchronise with the regular 24-hour sun cycle. When you travel west, you gain several hours, and so your body has extra time to make this adjustment. Travel east, though, and your day is shortened, which makes the adjustment more difficult.
So how possible is it to fool our biological clocks? "There is no silver bullet – so far – to treat jet lag," says Horacio de la Iglesia, professor of biology at the University of Washington. "Typically, the best way involves a combination of strategies that include restricting light exposure to specific times, restricting rest and meals, as well as activities such as walking and running to specific times, and the use of the hormone melatonin in very small amounts." Simon Archer from Surrey University agrees, at least in principle. "A combined approach can be helpful, but in practice it may be difficult to get all the timings right – it may be very difficult not to be exposed to light at an inappropriate time." Also melatonin isn't available in every country, and people with epilepsy or those that use warfarin, a drug used to treat blood clots, are advised against taking it.
All these strategies are aimed at speeding up the adjustment of our multiple clocks to the new time zone. Work over the past 15 years has led to an amazing increase in our understanding of the circadian clock, light input and jet lag, says Stuart Peirson of the Nuffield Laboratory of Ophthalmology at Oxford University. The master clock is in the hypothalamus of the brain – the suprachiasmatic nuclei (SCN). This mechanism relies on so-called "clock genes", expressed in the neurons of the SCN. Altering these genes also alters the 24-hour behaviour of the organism – "be it a fly, a mouse or a human," says Peirson.
Together with his team, he has been studying how light changes the expression of clock genes in the SCN. Increased expression of certain clock genes effectively moves the molecular 'hands' of the clock to the correct time, says Peirson – but there seems to be a limit on how much the clock can be shifted: it's just one hour per day. "Our work in this area has identified a natural brake – a protein called SIK1 – that is activated by light and actively prevents the clock shifting. Normally it would take five or six days to adjust to a six-hour shift in time zone. "When this [SIK1] mechanism is blocked, it enables mice to shift their clocks much more quickly – shifting six hours in two-to-three days," says Peirson.
Work published last year by researchers in Japan suggests a hormone plays a key role in re-setting the body clock. Mice lacking receptors for the hormone arginine vasopressin were much less affected by jet lag and adjusted more quickly to time shifts, says Peirson. However, turning this intriguing finding into any potential jet-lag cure will be far from simple.
Some scientists are looking at how we can avoid jet lag by preparing for it in advance. "We believe it is important for many people to start the process of shifting your internal rhythms before the flight, so that you arrive with little or no jet lag," says Charmane Eastman at Rush University in Chicago, Illinois. "Most recommendations and computer-based programs only make recommendations for what to do after landing in the new time zone."
Together with colleagues, Eastman has been investigating how people could reset their body clocks before flying. The scientists gave volunteers melatonin sold over the counter, and altered their light patterns using a big light box. They found that a combination of as little as 0.5 mg of melatonin (common doses are 3 and 5mg) with morning intermittent bright light advanced people's sleep schedule, resetting the circadian clock earlier in time, and thus making all the circadian rhythms of the body happen earlier.
So to avoid jet lag when flying east, says Eastman, you have to take melatonin in the evening for a number of days before the journey, and use the light box in the afternoon to wake up earlier each day. This can be done for the number of days equal to the number of time zones that will be crossed, helping the body to completely adjust to the new time before the trip. Most people, however, use fewer days, and then jet lag upon arrival is reduced and you get over your remaining jet lag sooner, says Eastman. When flying west, Eastman recommends using the light box at night, and perhaps taking melatonin in the morning.
There might also be ways to adjust the body clock without drugs. Another group of scientists has designed an app called Entrain that uses mathematical modelling to determine how our body can shift from one time zone to another in the quickest way possible. If users type in the timezone they are travelling to the app calculates a schedule that tells the user when they should expose themselves to light.
The researchers say it's probably easiest for travellers to divide their day into two parts, one where they seek as much light as possible and another where they should try to be in darkness – essentially corresponding to dawn and dusk. The software "should allow an individual to get over jet lag in less than half the time than if one used other well-known recommendations," says Daniel Forger of the University of Michigan, a co-author of the study.
The schedules themselves haven't been tested in controlled environments; the data people submit will be used to test the efficacy of the recommendations in the real world – and the team says they have data from over 5,000 people so far. "Hopefully our schedules can be integrated into more devices so that they become easier to follow," says lead developer of Entrain, mathematician Olivia Walch at the University of Michigan. "Maybe hotels could offer 10,000 lux lights so that people can get bright daylight when they need it to push their clocks in the right direction."
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