Jet lag is the mismatch between your internal body clock and the local time at your destination after a long flight across time zones. It is a normal, temporary response to fast travel, but it can leave you tired during the day, wide awake at night, and generally below your best for several days. A few evidence-based measures can noticeably reduce how long it lasts and how bad it feels, and for some travellers a short course of prescription-only melatonin has a place too. This guide explains why jet lag happens, what helps most, and how melatonin is used in the UK.
What jet lag is and why it happens
Your body runs on an internal 24-hour rhythm — the body clock — that governs sleep, alertness, digestion and the natural release of the sleep hormone melatonin. When you fly across several time zones, this clock stays on home time while the world around you has moved on. Until it catches up, you feel sleepy and hungry at the wrong moments and struggle to sleep or wake at the right ones.
Jet lag is generally worse the more time zones you cross, and it is often harder travelling east than west. Flying east asks your body to go to sleep and wake earlier than it wants to, which most people find more difficult than staying up later when travelling west. Crossing just one or two time zones usually causes little trouble; it is longer hauls that tend to hit hardest. As a rough guide, the body clock shifts naturally by only about an hour a day, so after a long eastward flight it can take several days to feel fully in step with local time.
How jet lag feels
Jet lag is more than just tiredness. As well as disturbed sleep and daytime sleepiness, common effects include difficulty concentrating, low mood or irritability, headaches, a general feeling of being unwell, and digestive upset such as an unsettled stomach, constipation or reduced appetite as your gut adjusts to eating at unfamiliar times. Symptoms are usually at their worst in the first day or two after arrival and then ease steadily. Knowing this pattern is reassuring in itself, because it means feeling below par on arrival is expected and temporary rather than a sign that something is wrong.
Light, sleep timing and practical measures
Light is the most powerful tool for resetting the body clock, because it is the main signal your brain uses to tell the time of day. Getting bright light — ideally daylight — at the right point in the day nudges your clock towards destination time, while avoiding light at the wrong time stops it drifting further out of step. Broadly, after an eastward trip, morning light at your destination helps; after a westward trip, light in the late afternoon and early evening helps. Adjusting your sleep timing in the same direction reinforces the shift. In the days before you fly, gradually moving your bedtime and mealtimes an hour or so towards destination time can give you a head start.
Practical habits make a real difference too. Set your watch and phone to the destination time as you board and try to sleep and eat on that schedule. Once you arrive, get outdoors in daylight, stay active, and try to hold off sleep until a sensible local bedtime. Stay well hydrated, go easy on alcohol and heavy meals late at night, and be cautious with caffeine close to bedtime — although a well-timed caffeinated drink can help you stay alert during the day. If you do nap to get through the first afternoon, keeping it short, ideally 20 to 30 minutes, reduces the risk of finding it harder to sleep that night. If you are only away for a day or two, it can sometimes be easier to stay on home time rather than fully adjust.
Melatonin for jet lag — how it is used
Melatonin is the hormone your body naturally releases in the evening to prepare for sleep, and a supplementary dose can help shift the body clock towards destination time. In the UK, melatonin 3mg is licensed specifically for jet lag. Importantly, this licensed medicine is a prescription-only medicine rather than an over-the-counter supplement, and it is not routinely available on the NHS as a travel medicine, so it is usually obtained privately. It is worth being aware that products sold as melatonin supplements abroad or online are not the same regulated medicine, and their strength and quality can be uncertain.
When used for jet lag, melatonin is generally taken at bedtime after you arrive at your destination — the guidance is to take it at local bedtime but not before 8pm or after 4am — and continued for up to about five nights. Taking it too early in the day can make you drowsy at the wrong time and may work against the adjustment you are trying to achieve. Because it can cause drowsiness, it is sensible not to drive or operate machinery for several hours after a dose. It is used alongside, not instead of, good light and sleep-timing habits, and it tends to be most useful for the eastward journeys that people find hardest.
Who might consider it
Melatonin for jet lag may be considered for some adults crossing several time zones, particularly on eastward journeys or where being functional quickly on arrival matters. It is not the right choice for everyone. A clinician will advise on suitability, which is especially important if you have existing health conditions such as epilepsy, liver problems, or an autoimmune condition, if you are pregnant or breastfeeding, or if you take other medicines that could interact — for example some medicines for blood clotting, diabetes, epilepsy or mental health. It is not licensed for children for this purpose.
Side effects are usually mild when they occur and can include daytime sleepiness, headache or dizziness. For many travellers, careful attention to light exposure and sleep timing is enough on its own, and these measures cost nothing. Melatonin is best thought of as an addition for the right person and trip, not a routine requirement, and it does not remove the need to give your body clock time to adjust. If you regularly struggle with sleep even at home, our guide on insomnia and when to seek clinical help may be worth reading, and you can find more articles in our General Wellbeing section.
When to seek advice
Jet lag is temporary and usually improves on its own within a few days as your body clock catches up. It is worth seeking advice if you want to consider melatonin and are unsure whether it is suitable for you, if you take regular medicines or have a long-term condition, or if sleep problems persist well beyond the trip. Frequent flyers and shift workers who find their sleep is disrupted much of the time may also benefit from a proper review rather than repeatedly managing alone. It is also sensible to plan ahead for travel more generally — for instance, our guide to travellers' diarrhoea covers another common travel concern.
If you take medicines that must be taken at set times — such as insulin, the contraceptive pill, or medicines for the heart or epilepsy — it is worth planning how you will manage timing across time zones, and a clinician or pharmacist can help you work this out before you travel. A Farmeci consultation is a simple way to check whether melatonin is appropriate for your circumstances. A UK-registered clinician will review your health and medicines and discuss whether treatment, alongside light and sleep-timing measures, is right for you.