Long-haul travel, body clocks and the international football calendar

The body runs on a schedule

Core temperature, hormone release, alertness and several measures of muscular power all follow a rhythm that repeats across roughly a day. That rhythm is generated internally and then corrected by external cues, of which light exposure is by a considerable margin the strongest.

Peak physical capacity for most people sits in the later part of the day, which is one reason evening kick-offs suit human physiology. When an athlete crosses time zones, the internal schedule remains on the old pattern while the demands of the new location have already moved.

Why the two directions are not equivalent

Travelling in a direction that lengthens the day is generally tolerated better, because the internal clock finds it easier to delay than advance. Travelling in the direction that shortens the day requires the clock to move forward, which it resists and does more slowly.

The realignment proceeds at a limited number of hours per day, so a long eastward journey can take most of a week to settle. Squads that arrive shortly before a fixture may therefore play while several internal systems are still on the wrong schedule.

Not everything adjusts together

Different tissues carry their own clocks, and they do not all resynchronise at the same speed after a change in time zone. The disorientation of travel comes partly from this internal disagreement, where sleep, appetite and digestion are running on different times. Digestive discomfort after long journeys is common for exactly this reason and is not simply a consequence of unfamiliar food.

The practical effect is a period in which an athlete may feel adjusted while parts of their physiology plainly are not. Recovery processes that are themselves scheduled by the clock may therefore run at unhelpful times during the adjustment period.

Sleep debt arrives on top

Long flights rarely deliver quality sleep, so athletes typically land already carrying a deficit before the clock problem is addressed. That deficit interacts with the misalignment, because sleep taken at the wrong internal time is shorter and more fragmented.

The first nights in a new location are often the worst, which is exactly when squads are trying to train at intensity. Accumulated across an international window, the effect is a group of players returning to their clubs less recovered than the fixture list implies.

What teams do about it

Managing light exposure is the primary tool, since light at the right internal time shifts the clock and light at the wrong time delays it. Scheduling training, meals and sleep to the destination time from arrival gives the clock consistent cues to work with.

Some squads adjust schedules in the days before departure so that part of the shift has already occurred by the time they land. Any use of medication to manage sleep or alertness is a medical decision that sits with the team doctor and with anti-doping rules in view.

Why this is a calendar problem

The measures available to teams reduce the disruption rather than remove it, because the underlying constraint is biological. Fixtures scheduled shortly after long journeys therefore ask players to perform during a period when several systems are misaligned.

Expanding competitions across more distant hosts increases the total amount of travel absorbed by a relatively small group of players. This is a design question for the bodies that set calendars rather than something a sports science department can resolve internally.