Fly from New York to London overnight and you land at breakfast time feeling as if someone has stolen a night of your life. Fly back and you arrive tired, but somehow it is easier to shake off. That is not your imagination, and it is not just the airline seat. Jet lag really is worse when you fly east.

The reason lies in a tiny cluster of cells in the brain that keeps time, and in the awkward fact that, left to itself, that clock does not run on a 24-hour day.

Your body clock runs a little slow

Your internal clock sits in the suprachiasmatic nucleus of the hypothalamus. Every day it is nudged back into line with the outside world, mainly by light. Without those cues, studies of people living in isolation show that the clock drifts, because its natural period is slightly longer than 24 hours. Estimates range from about 20 minutes to around half an hour more.

That small offset makes the two directions very different. Flying west, you need to stay up later and lengthen your day, which goes with the clock’s natural tendency. Flying east, you need to go to bed earlier and shorten your day, which works against it.

The physics of a bad night

In 2016, a team led by physicist Michelle Girvan at the University of Maryland modelled the brain’s pacemaker cells as a network of oscillators, published in the journal Chaos. Their model, as reported by ScienceAlert, predicted that recovering from a three-time-zone trip takes a little under four days going west but more than four days going east, and that six time zones takes about six days west but eight days east.

It is not only the brain. Organs such as the liver have their own clocks, which resynchronise at different speeds from the master clock. For a few days after a long flight, different parts of the body are effectively living in different time zones, which helps explain the fog, the poor appetite and the upset stomach.

A long-haul airline captain explains how he deals with jet lag (Captain Joe).

What actually helps

Light is the most powerful tool. Morning light tends to advance the clock, which is what you want after flying east; evening light tends to delay it, which helps after flying west. The timing matters: the wrong light at the wrong time can push the clock the wrong way, which is why many travellers now use apps that calculate a personal light schedule.

Shifting your sleep by an hour or two in the days before departure can shorten the adjustment. Melatonin, taken in the late afternoon or evening, can help advance the clock, though its effect depends on dose and timing, and it is worth checking with a doctor first. Short-acting sleeping pills help some people sleep but do nothing to move the clock itself. Alcohol and heavy caffeine generally make things worse.

How airline crews live with it

For long-haul pilots and cabin crew, jet lag is not a holiday nuisance but a safety issue, and airlines operate under flight and duty time rules designed to manage fatigue. Many crews on short layovers deliberately stay on home time rather than adapting, because by the time their body clock has adjusted they would be flying back.

Pilot Minute: the Federal Aviation Administration on what jet lag is and how to prevent it.

For the rest of us, the lesson is simple. Going west, stay up and enjoy the long evening. Going east, get outside into the morning light, resist the afternoon nap and accept that your body is a few hours behind the rest of the plane for a while.

Why jet lag is worse when you travel east rather than west (WFAA Verify).

Sources: Wikipedia (Jet lag), STAT News, ScienceAlert, Chaos (2016), CDC Yellow Book, Federal Aviation Administration