Circadian desynchrony
Woman undergoing phototherapy in front of a light screen. Phototherapy is used to treat jet lag.
Jet lag is the temporary disruption to sleep and activity patterns. It is due to rapid travel across more than two time zones, with the body’s “internal clock” (or circadian rhythm) being synced to the original time zone rather than the sleep and wake cycles of the final destination.
The primary cause of jet lag is the rapid change in local time cues (for example, sunrise, sunset, light and dark) after travelling across several time zones. The body’s “internal clock” (or circadian rhythm) is unable to adjust rapidly to such a quick change. It takes approximately a day for each time zone crossed for the internal clock to readjust with local time. This means that for a number of days, depending on the number of time zones crossed, the clock is in ‘night mode’ during the day and ‘day mode’ during the night. Our ability to sleep, stay alert, perform different tasks and digest our food is best when the internal clock is in sync with local time. For example, sleep is best taken when the internal clock is in ‘night mode’. It is thought that our inability to rapidly adapt to new time zones may be linked to melatonin (a hormone made naturally by the body to promote sleep), which is released in high levels during the night when we sleep and lower levels during the day.
Jet lag is usually experienced as sleep problems for a few days after a long flight. Travelling westwards (i.e., when we “gain” several hours during travel), we may wake up early after a short sleep and/or feel tired during daylight hours, as our body would normally be asleep back home. Travelling eastwards (i.e., when we “lose” several hours during travel), we may have problems getting to sleep at the ‘normal’ time in our destination time zone, this is because our body would normally be alert and awake at home. If we have not slept on the flight, we will be sleep-deprived, and the first night, we may sleep reasonably well as our body catches up on its sleep. However, the rhythms of sleepiness, alertness, body temperature, the hormone melatonin and many other rhythms controlled by the body’s internal clock will not yet have readjusted with local time. This means we may feel sleepy, cold and generally ‘unwell’ (such as feeling sick and changes in appetite and bowel habits) during the following daytime and have problems sleeping the following night in the destination time zone.
Anyone can experience jet lag. It is possible that 60–70% of long-haul travellers will suffer from some form of jet lag. People can experience jet lag in different ways and to varying degrees. Some may be badly affected and others not at all (especially children). The same person may experience jet lag on one occasion and have little problem on another. Much depends on the direction and timing of the flight, together with the timing of exposure to bright light during and after the flight in the destination time zone – bright light is the main influence on the internal clock. In general, older people suffer more than younger travellers, and travelling eastwards is worse than travelling westwards (because our body can readjust more quickly to “gaining” time than “losing” time).
There is some evidence that genes may influence the ability to adapt better eastwards than westwards and vice versa.
Jet lag is not a disease; it is a temporary and fully reversible condition. However, it is a (temporary) form of a “sleep-wake cycle” disorder (or circadian rhythm disorder) and, as such, any persistent sleep problems may be diagnosed in the same way as this disorder (see the article on circadian rhythm sleep disorders).
‘Shift work sleep disorder’ is similar to jet lag. Changing work time from day shift to night shift puts the body’s work–sleep clock out of sync, with essentially the same effects as travelling across time zones. However, whilst everyone eventually adapts to a new time zone providing that they stay in the destination long enough, some shift workers experience sleep problems each time they change to night work.
Most people do not require treatment for jet lag and their bodies naturally adjust to the destination after a short period in the new time zone.
It is possible to prevent jet lag or reduce its effects by using appropriately timed exposure to bright light and/or specifically timed treatment with the hormone melatonin to help our body adjust to new sleep patterns. At the same time, it is important to change behaviour, such as eating and sleeping, to match with the new time zone. Once the internal clock has adapted, jet lag problems resolve.
Timed exposure to bright light before travelling and for the first three to four days after travelling can help adjust the body’s internal time clock to the new location. For westward travel, it is best to ensure exposure to light in the late evening and avoid the early morning of the departure time zone to help adjust to a later than usual time zone. For eastward travel, it is best to ensure exposure to light in the early morning and avoid the evening of the departure time zone to help adjust to an earlier than usual time zone. However, there are no agreed guidelines on dose, type, strength or length of light exposure.
Timed treatment with melatonin before and after travelling in the early morning of the departure time zone (when travelling westward) or the very early evening of the departure time zone (when travelling eastward) may also help body clock adjustment.
It is not worth attempting to adjust the body clock for a short stay in a new time zone. Multiple changes of time zone with short stopovers (often experienced by long-haul pilots, flight attendants and business travellers) are also difficult. For people who know they suffer badly from jet lag, it is worth attempting to change internal clock timing before departure. This can be done by sleeping increasingly earlier (going eastward) and increasingly later (going westward) for a few days before travelling, as well as using melatonin and bright light treatment according to specific instructions.
Melatonin can induce sleepiness for several hours after the dose. There are few reported side effects of melatonin; the commonest are headache and nausea. If melatonin treatment or light exposure is wrongly timed, they can worsen jet lag.
People who do not travel often are unlikely to develop long-term problems as a result of jet lag. It is possible that long-term exposure to frequent time zone change may result in an increased risk of some diseases, but there is little evidence and no real agreement as to the risk.
Last reviewed: Aug 2023
