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Why you feel tired, or wide awake at the wrong time, when you change time zones

Jet lag is neither a lack of willpower nor simple travel fatigue. It is a mismatch between your body clock and the time in the country you are in. Here is how it works, and how to correct it.

A clock at the heart of the brain

In the middle of the brain, in the hypothalamus, a small group of neurons called the suprachiasmatic nucleus acts as the conductor. It imposes a rhythm of about 24 hours on the body: the circadian rhythm.

This clock regulates far more than sleep: body temperature, melatonin release in the evening and cortisol in the morning, alertness, appetite, digestion.

It is reset every day, mainly by the light picked up by the eyes.

Why you are exhausted… then unable to fall asleep

Two mechanisms decide how sleepy you are. The first is a sleep pressure that builds up with every hour spent awake and empties during the night. The second is the clock's wake signal, which rises through the day and peaks in the evening, precisely to offset accumulated fatigue, then collapses at night.

At home, the two are in sync: you last all day and fall asleep easily at night. After an eastbound flight, the clock has stayed on departure time. Result: a slump in the early local afternoon, which is your body's night, then a surge of alertness just when you want to sleep.

Travel fatigue

Short night, waiting, dry and noisy cabin: it disappears after a good night's sleep.

Jet lag

It persists as long as the clock has not been reset to local time, even after a good sleep. This is what Fuseo targets.

The whole body is out of sync, and not at the same pace

Each organ has its own clocks, synchronized by the central clock. After a time-zone change, they do not all reset at the same speed. For a few days, the brain, liver or gut no longer agree on what time it is.

This is why jet lag is about more than sleep:

Sleep

trouble falling asleep, waking at night or very early

Alertness

daytime sleepiness, poorer concentration and memory

Digestion

hunger at odd times, digestive discomfort

Mood

irritability, low mood

Performance

less reliable reflexes and decision-making

Immunity

lack of sleep makes you more vulnerable to infections

17 h

Alertness

After 17 to 19 hours awake, measured performance is comparable to a blood alcohol level of 0.05% (Williamson & Feyer, 2000).

× 4,2

Immune defences

Among 164 volunteers exposed to a cold virus, sleeping less than 6 hours multiplied the risk of falling ill by 4.2 (Prather et al., 2015).

East

Direction of travel

Eastbound trips are generally the hardest, because they require advancing the clock (Sack, 2010).

How long does it take to adapt?

There is no single rule. Without help, the clock only shifts gradually, day after day, and the speed of adaptation depends on:

  • the number of time zones crossed;
  • the direction of travel: eastward, the clock must advance, which is generally harder than delaying it westward;
  • your sleep schedule before and during the trip;
  • the light you get at your destination, and above all when;
  • individual differences, including age and chronotype.

On a round trip, you pay for the time difference twice: on arrival, then on return.

Common myths

“Just stay hydrated.”

Drinking makes the flight more comfortable, but does not reset the clock.

“One big night's sleep on arrival and you're sorted.”

It clears travel fatigue, not the clock's time difference.

“You should always get morning sun.”

It all depends on the direction of travel and your biological time: at the wrong moment, light shifts the clock the wrong way.

“You can take melatonin whenever you like.”

Its effect depends on when it is taken, and it is not suitable for everyone: ask for medical advice.

“A long nap on arrival helps you recover.”

A long nap delays falling asleep at night. Better to have 20 to 30 minutes in the early afternoon.

“Jet lag is all in the mind.”

It is a measurable biological phenomenon: shifted melatonin, temperature and organ rhythms.

What Fuseo does

Fuseo applies these principles to your trip: from your flights and usual schedule, it estimates where your clock is each day and derives three levers.

Light

When to seek it, when to avoid it, including on board.

Sleep

Schedules before departure, in flight and at your destination, naps included.

Caffeine

The time of your last coffee, to protect the following night.

And for your next trip?

Fuseo calculates your sleep, light and caffeine times from your flights.

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