Jet Lag And Why Direction Matters

Travellers consistently report that eastward flights feel worse than westward ones of the same length. The reason is a property of the internal clock rather than of the flight.

The internal clock runs slightly long

The human circadian clock, measured without external time cues, has a period a little longer than twenty-four hours for most people.

Daily light exposure corrects this, nudging the clock slightly earlier each day so that it stays synchronised with the solar day rather than drifting later.

Because the system is already built to be pulled earlier against a natural drift later, it delays easily and advances with difficulty. That asymmetry is the whole explanation.

East demands advance, west demands delay

Flying east shortens the day and requires the clock to move earlier. That is the direction the system resists, and adaptation is correspondingly slower.

Flying west lengthens the day and requires a delay, which is the direction the clock drifts naturally. Staying up later is something most bodies do willingly.

A rough working figure is that adaptation takes around a day per time zone eastward and rather less westward, though individuals vary considerably.

Light is the input that moves the clock

Light in the hours after the body's temperature minimum, which is shortly before habitual waking, advances the clock. Light before that point delays it.

This is why the timing of light exposure matters more than the amount, and why getting it wrong can move the clock in the opposite direction to the one intended.

Because the temperature minimum itself shifts as adaptation proceeds, the correct timing changes over the days following arrival, which is what makes practical advice complicated.

Jet lag is not the same as travel fatigue

Much of what people call jet lag on short trips is simply exhaustion from disrupted sleep, cabin conditions and long transit, and it resolves after one good night.

True jet lag is misalignment between the internal clock and local time, producing sleepiness at the wrong hours, poor concentration and digestive disturbance for days.

Crossing only one or two zones rarely produces genuine misalignment, which is why short-hop travel feels tiring without feeling disorienting.

Why shift workers face a harder version

A traveller eventually receives consistent local light cues, so the clock has something to align to. A shift worker returns to daylight and social cues that contradict their schedule.

That means the misalignment is repeatedly reimposed rather than resolved, which is why rotating shifts are associated with sustained circadian disruption.

Where sleepiness is affecting safety at work or driving, that is a medical matter rather than a scheduling inconvenience, and occupational health services deal with it specifically.