As we watch athletes from all over the world competing in the World Cup games, what do we know about how “jet lag” affects their sleep and what do they do to manage the disruptions?

First, to clarify, jet lag is not an official medical term. We instead refer to “Circadian rhythm sleep disorders” as described in the DSM-5. This is a group of sleep disorders that disrupt the body’s natural sleep-wake cycle, leading to sleepiness and alertness occurring at inappropriate times of day. However, when the term jet lag is used in this article, we refer to sleep-wake issues stemming from travel across time zones.

What is the biology behind our rhythms?

The body is equipped with a “central clock” that keeps our circadian rhythm, located in the suprachiasmatic nucleus (SCN) in the hypothalamus of the brain. Cells outside of the brain located in different organs also keep their own “peripheral clocks” that drives their functions – think of how the pancreas and liver coordinate insulin secretion and fat and glucose metabolism, what time the adrenals regularly secrete cortisol, diurnal variations in core body temperature, and when muscles repair themselves in response to exercise. These clocks are normally synchronized with each other, and the most important cue they respond to is the light-dark exposure we subject ourselves to. Melatonin, the body’s “sleep hormone” secreted by the pineal gland in the brain, is secreted during hours of darkness, and is inhibited by light exposure. Other cues include meals, physical activity, social cues, and medication.

De-synchronization of the normal biological clocks by external cues lead to Circadian rhythm sleep disorders. This can refer to changes in light-dark cycles related to traveling across time zones, having a job working non-traditional hours (night shifts), and can even occur without travel and simply from regular exposure to unnatural light sources at night (such as device screens, LED lights, light pollution in urban areas).

Biological effects of circadian rhythm disruption

Sleep disturbance symptoms include daytime sleepiness, insomnia, fatigue, and fluctuating energy levels. The gastrointestinal system can be subject to indigestion and poor food absorption, loss of appetite, diarrhea, and constipation. Changes in mood, cognitive performance, reaction time, hand-eye coordination, loss of drive, and reduced vigilance and alertness can occur. Irregularities in insulin and blood glucose response, and menstrual cycle abnormalities are also observed.

Factors affecting the severity of jet lag

The more time zones crossed, the more it takes to recover from the jet lag. Anecdotally, it takes about one day per time zone crossed to adjust.

The direction of the flight matters. East-to-west flights are easier to deal with (phase delay), because the natural rhythm of the body is actually just a little longer than 24 hours. Basically, left to our own devices without external light-dark cues, our bodies will naturally want to go to sleep later and later. This means that it is easier to deal with having to go to bed at a later time, than making ourselves go to sleep when we are not yet sleepy. The time required to adjust from an east-west flight can be as much as 50% less as compared to west-east flights. Traveling northwards or southwards also has an effect on sleep-wake cycles, but for other reasons. In this situation, the difficulty has to do with when the sun rises and sets, which can vary greatly depending on distance to equator and the season.

Interestingly, because older age is associated with naturally wanting to go to bed earlier and rise earlier (phase advance), older athletes may do better with west-to-east flights.

Exposure to environmental cues and light cues are highly influential and can be the most effective way to manage jet lag. The sooner the athlete is exposed to the day/night cycle in the new place, the faster the resolution of the jet lag. Other factors include meal timing, physical activity, genetics, and use of pharmacologic agents.

Does jet lag affect sports performance?

There is already a hypothesis that athletic performance has its own circadian rhythm, so it is interesting to find out how a disruption in circadian rhythm will affect sports performance. A study on swimmers documented that performance was much worse in the early morning hours (2 to 8 a.m.) compared with later in the day (11 a.m. to 11 p.m.)

With the effects on the body mentioned in the previous section, one can speculate that performance will be accordingly affected by jet lag. Also, consequences of sleep deprivation per se (not just jet lag) on aspects of sport performance are well-documented. Nevertheless, research indicates performance deterioration due to jet lag appears to be the most pronounced in the first 1-2 days after travel in a study on jump performance. Meanwhile, teams who crossed fewer time zones (at least 3 hours) compared to the other team had a higher winning percentage better than even a home-field advantage according to Major League Baseball retrospective data.

The timing of competition may also have a role, for instance athletes travelling west-to-east for an early afternoon event may not perform as well if they are still operating on their home time, which would correspond to morning hours.

More to come on how to help athletes minimize the effects of traveling on sleep rhythms and sports performance.