Is jet lag a medical condition?

Yes. Jet lag disorder is classified as a circadian rhythm sleep wake disorder. It is defined by insomnia or excessive sleepiness that follows travel across at least two time zones, accompanied by impaired daytime function, and it resolves as the body clock realigns.

Jet lag is caused by a mismatch between the internal body clock and local time. Aircraft move a traveller across time zones in hours. The body clock moves at about one to 1.5 hours per day. The gap between the two is what produces the symptoms.

Does jet lag get worse with age?

Older travellers report slower realignment on average. Circadian amplitude falls with age and the clock becomes less responsive to light, so the same time zone shift takes more days to close. Individual variation is wide.

Can you get jet lag without flying?

Yes. Night shift work, rotating rosters and large weekend sleep swings produce the same mismatch between the body clock and the clock a person lives by. The label differs but the mechanism is the same.

Does jet lag affect anything besides sleep?

Yes. The body clock times digestion, core temperature, cortisol release, alertness and physical output. A misaligned clock therefore shows up as appetite loss, stomach upset, poor concentration, low mood and reduced physical performance alongside broken sleep.

The condition

What jet lag is

Jet lag is a circadian rhythm sleep disorder. It appears when air travel carries the body across time zones faster than the body clock can follow, leaving internal time and local time out of step until the clock catches up.

Jet lag defined

Jet lag is the set of symptoms produced by a mismatch between the internal body clock and the external clock at a destination. Clinically it is named jet lag disorder and sits within the circadian rhythm sleep wake disorders. Its older name, desynchronosis, states the mechanism directly: the internal rhythm and the external day are no longer synchronised.

Jet lag at a glance
Category	Circadian rhythm sleep wake disorder
Cause	Crossing time zones faster than the body clock can shift
Onset threshold	About two zones, reliable at three or more
Primary symptoms	Insomnia, daytime sleepiness, impaired concentration
Secondary symptoms	Appetite loss, digestive upset, low mood, reduced physical output
Natural duration eastward	About one day per time zone crossed
Natural duration westward	About two thirds of a day per time zone crossed
Treatment	Timed light exposure, timed sleep, timed melatonin

Why the body clock cannot keep up

A master clock sits in the suprachiasmatic nucleus, a small region of the hypothalamus. It runs on a cycle slightly longer than 24 hours and is reset daily by light reaching specialised cells in the retina. That daily reset is what normally holds the clock to the local day.

Air travel breaks the arrangement. A flight from London to Tokyo moves the local clock eight hours in twelve. The body clock can move about one hour per day. The traveller lands with an internal clock still set to London, telling the body to sleep while Tokyo is having lunch. Every symptom of jet lag follows from that single disagreement.

The clock does not only time sleep. It times core temperature, cortisol release, digestion, alertness and physical output. When it is misaligned, all of those arrive at the wrong hour, which is why jet lag reaches well beyond tiredness.

Direction changes the cost

Crossing the same number of zones costs more going east than going west. Eastward travel asks the clock to run earlier, against its natural drift. Westward travel asks it to run later, with the drift. A traveller flying New York to London loses more days than the same traveller flying London to New York.

North to south travel crosses no time zones and produces no jet lag, however long the flight. Fatigue after a 14 hour flight from London to Johannesburg is travel fatigue, which clears with one night of sleep. Jet lag does not, because the clock itself has moved out of position.

What shortens it

Three interventions move the body clock, and all three depend on timing rather than quantity. Bright light at the right hour moves the clock fastest. A fixed sleep window holds the new position. A low melatonin dose at the right hour adds a smaller push in the same direction.

Timing is the whole task, and timing is what a plan supplies. The Jet Lag Calculator converts your route into the exact hours those three interventions belong in, and how the calculator works sets out the rules it applies.

Questions about jet lag

Is jet lag a medical condition?
Yes. Jet lag disorder is classified as a circadian rhythm sleep wake disorder. It is defined by insomnia or excessive sleepiness that follows travel across at least two time zones, accompanied by impaired daytime function, and it resolves as the body clock realigns.
What causes jet lag?
Jet lag is caused by a mismatch between the internal body clock and local time. Aircraft move a traveller across time zones in hours. The body clock moves at about one to 1.5 hours per day. The gap between the two is what produces the symptoms.
Does jet lag get worse with age?
Older travellers report slower realignment on average. Circadian amplitude falls with age and the clock becomes less responsive to light, so the same time zone shift takes more days to close. Individual variation is wide.
Can you get jet lag without flying?
Yes. Night shift work, rotating rosters and large weekend sleep swings produce the same mismatch between the body clock and the clock a person lives by. The label differs but the mechanism is the same.
Does jet lag affect anything besides sleep?
Yes. The body clock times digestion, core temperature, cortisol release, alertness and physical output. A misaligned clock therefore shows up as appetite loss, stomach upset, poor concentration, low mood and reduced physical performance alongside broken sleep.