Jet lag symptoms
Jet lag symptoms appear within 24 hours of landing and reach well beyond tiredness. Each one traces back to a single cause: a body clock still keeping time in the zone you left.
Sleep symptoms
Sleep carries the clearest signal, and the shape of the disruption depends on direction. After an eastward flight the body is not ready for sleep at local bedtime, so sleep onset is late and the alarm arrives before the body has finished. After a westward flight sleep arrives early and then breaks in the small hours, because the body clock has already completed its night.
Daytime sleepiness follows the same pattern in reverse. The body schedules its strongest sleep pressure at the hours that were night in the zone you left, which lands in the middle of a local working day.
Physical symptoms
The body clock times more than sleep. It times core temperature, cortisol release, digestion and physical output. When it sits out of step with local time, each of those arrives at an hour that no longer fits the day around it.
| Insomnia at local bedtime | Sleep pressure and melatonin release still on origin time |
|---|---|
| Waking hours before the alarm | Body clock has completed its night early |
| Afternoon collapse in alertness | Circadian alertness low point landing mid local day |
| Appetite loss and irregular hunger | Digestive rhythm still expecting origin meal times |
| Constipation or stomach upset | Gut motility rhythm out of step with local eating |
| Headache and general malaise | Sleep loss compounding circadian misalignment |
| Reduced strength and reaction time | Peak physical output arriving at the wrong local hour |
| Irritability and low mood | Circadian misalignment affecting emotional regulation |
Cognitive symptoms
Attention, working memory and decision quality all track the body clock. In the first days after a long crossing, travellers make more errors on tasks that need sustained attention, and they make them at predictable hours: the hours that were night at home.
This is the symptom that costs the most in practice. A negotiation, a presentation or a drive scheduled into that window meets a traveller operating well below their normal level, whether or not they feel it.
What makes symptoms worse
Four factors raise severity. Zones crossed sets the size of the mismatch. Direction decides how hard the clock has to work, with east costing more than west. Sleep debt carried onto the flight stacks ordinary tiredness on top of misalignment. Light exposure at the wrong hour after landing can push the clock further from local time rather than towards it.
The last factor is the one a traveller controls most directly, and it is the one the Jet Lag Calculator schedules.
Questions about jet lag symptoms
How soon do jet lag symptoms start?
Symptoms usually appear within 24 hours of landing. The first night is often the clearest signal: sleep arrives late and ends early on an eastward trip, and arrives early and breaks in the middle on a westward trip.
Why does jet lag upset digestion?
The digestive system runs on its own circadian rhythm, timed by the master clock and reinforced by meal timing. After a long crossing that rhythm still expects food at the hours of the zone you left, so meals at local times arrive when the gut is not prepared for them.
Is jet lag fatigue the same as travel fatigue?
No. Travel fatigue comes from the journey itself: dehydration, cabin pressure, cramped seating and lost sleep. It clears after one good night. Jet lag comes from a misaligned body clock and persists until the clock realigns, which takes days.
Can jet lag affect mood?
Yes. Circadian misalignment is linked to irritability, low mood and reduced emotional regulation. The effect is strongest in the first days after landing and eases as the clock and the local day come back into step.