Jet lag happens when the body's internal timing is out of step with the local schedule after travel across time zones. It can affect sleep, daytime alertness, concentration, and how someone feels during the first part of a trip.[1]
The clock takes time to adjust
Light and darkness help coordinate circadian rhythms, which influence sleep, hormone release, and other daily functions. Moving to a new time zone changes those cues faster than the body necessarily adapts.[1][2]
Short sleep during travel can add to the problem. Travel fatigue and a shifted body clock can occur together, so simply spending enough hours in bed may not immediately restore the usual pattern.
The direction of travel, number of time zones crossed, and schedule at the destination all affect the experience.
Describe the sequence
Useful details include departure and arrival times, opportunities to sleep, caffeine or alcohol use, and when sleepiness or wakefulness becomes difficult. Include the return trip if symptoms continue after coming home.
A diary can distinguish a pattern tied to travel from fatigue that was present beforehand. It also makes practical sleep advice more specific to the itinerary.
Cortisol is not a jet-lag score
Cortisol follows a daily pattern. Clinical cortisol tests investigate specific disorders and depend on collection timing and other conditions.[3]
When the timeline does not explain enough
Fatigue that persists, predates travel, or continues despite adequate rest deserves a broader assessment. Sleep disorders, medicines, anemia, thyroid conditions, and other causes may be relevant.[4]
Keep the travel record, but describe the symptoms on ordinary days too.
