Travel across time zones causes jet lag because the body’s internal clock remains synchronized with the departure time while the destination follows a different light-dark cycle. This disrupts sleep and can delay physical and psychological recovery.
The body’s approximately 24-hour circadian rhythm is regulated mainly by the suprachiasmatic nucleus (SCN) in the hypothalamus. Light is the principal zeitgeber, or environmental cue, that synchronizes the SCN with local time.
After rapid travel across several time zones, light exposure, mealtimes and activity occur at times that conflict with the traveller’s internal rhythm. The release of melatonin, which normally increases in darkness and promotes sleepiness, may therefore occur at an inappropriate local time. This can cause difficulty falling asleep, early waking, fragmented sleep and daytime fatigue.
| Direction of travel | Circadian adjustment and likely effect |
|---|---|
| Eastward | Requires a phase advance, meaning sleep and waking must occur earlier. This is usually more difficult because the natural circadian period is slightly longer than 24 hours. |
| Westward | Requires a phase delay, meaning sleep and waking occur later. This is generally easier for the body to achieve. |
Reduced sleep duration and quality impair recovery by decreasing alertness, reaction time, motivation and decision-making. They may also interfere with glycogen restoration, tissue repair, immune function and the regulation of anabolic and catabolic hormones. Consequently, an athlete may experience greater perceived fatigue and reduced performance even when muscular work during travel was limited.
Recovery strategies include gradually adjusting sleep times before departure, obtaining appropriately timed light exposure, maintaining hydration, scheduling meals according to destination time and allowing several days for adaptation. Light exposure must be timed carefully because exposure at the wrong biological time can shift the circadian rhythm in the opposite direction.
For A.3.3 Fatigue and recovery (HL), explain the causal chain: time-zone change disrupts circadian synchronization, altered melatonin timing reduces sleep quality, and inadequate sleep impairs recovery and performance. Avoid the misconception that jet lag is simply tiredness caused by a long flight.
