Jet lag is the disruption to sleep or activity patterns and the ability to be alert and function effectively. This is due to rapid travel across time zones, which leads to the body’s internal timing systems being out of alignment with the external time cues of the final destination, particularly the light–dark (day–night) cycle and local clock time.
The primary cause of jet lag is the abrupt change in local time cues (for example sunrise, sunset, light and dark) after flight across several time zones. The body’s internal circadian clock is unable to adapt rapidly to such a swift change. It takes approximately a day for each time zone crossed for the internal clock to realign with local time. Thus, 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 correctly aligned 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 the hormone melatonin, 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, we will tend to wake up early after a short sleep. Travelling eastwards, we may have problems getting to sleep at the ‘normal’ time in our destination time zone. 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 governed by the internal circadian clock will not yet have realigned with local time. This means we may feel sleepy, cold and ‘under par’ during the following daytime and have problems sleeping the following night in the destination time zone.
Anyone can experience jet lag. There are no strong statistics on exactly how many. However, possibly 60–70% of long haul travellers will experience some form of the condition. Individual experience is very variable. Some may be badly affected and others not at all. The same person may suffer 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 personal exposure to bright light during and after the flight in the destination time zone – bright light is the main influence on the internal clock. Generally speaking, older people suffer more than younger travellers.
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 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 related to jet lag. Changing work time from day shift to night shift leads to misalignment of the internal clock with the desired work–sleep schedule, with essentially the same consequences as time zone change. However, whilst everyone eventually adapts to a new time zone providing that they stay there 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 adapt after a short period in the new time zone.
It is possible to increase the rate of adaptation of the body clock to a new time zone using specifically timed exposure to bright light and/or specifically timed treatment with the hormone melatonin. At the same time, it is important to change behaviour, such as eating and sleeping, to coincide with the new time zone. Once the internal clock has adapted, jet lag problems resolve.
Timed exposure to bright light, before flying and for the first three to four days after the flight, should start and speed up the internal time clock's adjustment to the new location. For westward travel, it is best to ensure exposure to light in the late evening and avoidance in the early morning of the departure time zone. For eastward travel, exposure in the early morning or avoidance in the evening of the departure time zone is best. However, there are no agreed guidelines on dose, type, strength or length of light exposure.
Timed treatment with melatonin pre and post-flight in the early morning of the departure time zone (westward) or the very early evening of the departure time zone (eastward) may also help body clock adjustment.
It is not worth attempting to adapt 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) 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) during pre-departure days, at the same time using melatonin and bright light treatment according to specific instructions.
Melatonin can induce sleepiness for several hours after the dose. If melatonin treatment or light exposure are wrongly timed, they can worsen jet lag. The reported side-effects of such doses of melatonin are very few, the commonest being some cases of headache and nausea.
It is possible that long-term exposure to frequent time zone change may lead eventually to an increased risk of some diseases, but there is little evidence and no real agreement as to the risk.
Last reviewed: Oct 2019