Circadian Rhythm and the Impact of Time Zone Changes: How to Travel Smarter and Minimize Jet Lag

In today's world, air travel between multiple time zones has become remarkably easy and more accessible than ever. However, while we can now change our geographic location within a matter of hours our internal biological clocks remain poorly adaptable to these abrupt changes. Time zone shifts are unnatural and go against the grain of our evolutionary biology, but with the globalization of business, sport and leisure travel, these shifts have become inevitable.  This disconnect between biology and reality often gives rise to jet lag, a condition deeply rooted in the disruption of our circadian rhythm. Jet lag doesn't just affect sleep, it actually affects widespread physiology including neurological function, hormone regulation, metabolism, and our immune response. In today’s newsletter I want to discuss the difference between jet lag and travel fatigue, the basis of circadian biology, our physiology during time zone shifts, and how we can change our behaviour during travel to minimize the negative health consequences.

Jet Lag vs. Travel Fatigue: A Physiological Distinction

You may have heard the terms jet lag and travel fatigue used interchangeably, however they represent two very distinct entities. Think of travel fatigue as being “run down” after travel. It is a transient condition resulting from the general stressors of travel: extended sitting, poor hydration, disrupted routines, sub-optimal nutrition, uncomfortable seating positions and cabin pressure changes. Travel fatigue can happen acutely with one trip, or accumulate chronically with repeated exposures, as in the course of an athletic season or for individuals who travel frequently for business. It occurs regardless of time zone changes and typically resolves with rest, movement, and proper recovery strategies.

Jet lag, on the other hand, is a genuine chronobiological disturbance. In other words, it disrupts the normal "clocks" that regulate our body's normal biology. This occurs when the body’s internal circadian rhythms—regulated by our “master clock”, the suprachiasmatic nucleus (SCN) in the hypothalamus—are out of sync with the local time at the destination. This temporal misalignment leads to disrupted sleep-wake patterns, impaired cognitive function, mood changes, hormonal dysregulation, gastrointestinal discomfort, and compromised physical performance. While travel fatigue is typically short-lived, jet lag can persist for several days, especially when crossing multiple time zones.

Circadian Biology and Zeitgebers

Our circadian rhythm is a near-24-hour cycle (24.2 hours, if we want to get specific) that governs numerous physiological functions, from hormone secretion and sleep architecture to metabolism and immune responses. Although this rhythm is naturally generated, it is further synchronized—or entrained—by external cues known as zeitgebers (“time givers” in German).

Think of a zeitgeber as any cue that reminds you of what time of day it is. This could be something like breakfast time, a regular morning workout, or a typical wind-down routine in the evening. However, the most potent zeitgeber of all is light. Light directly influences the SCN through the retinohypothalamic tract, a neurological connection between the retina of the eye to the SCN in the hypothalamus. Ultimately these cues help align our internal clock and physiology with the reality of our external environment, enabling a coherent sleep-wake cycle and physiological stability. This is why strategic light stimulus can be an effective way of helping us “phase shift” more efficiently during travel. Phase shifting is essentially us trying to trick our physiology into a different time zone, but the outcomes can be very real. More on this below.

Eastward vs. Westward Travel: Why Direction Matters

One of the key determinants of jet lag severity is the direction of travel. Eastward travel (e.g., Toronto to Amsterdam) generally induces more pronounced jet lag symptoms than westward travel (e.g., Amsterdam to Toronto). The reason lies in the natural tendency of the human circadian rhythm to run slightly longer than 24 hours as I mentioned above—shown to be about 24.2 hours in experimental settings absent of zeitgebers . This makes it inherently easier for the body to delay its internal clock (as required during westward travel) than to advance it (as required during eastward travel).

When you travel west, your day is effectively extended. For example, arriving in Vancouver from Toronto at 9:00 p.m. local time may feel like midnight, but staying awake for a few extra hours is generally more achievable. Light exposure or gentle exercise in the evening can help facilitate staying awake and delay the natural circadian rhythm associated with Toronto time.

In contrast, travelling east shortens your day. A 9:00 p.m. bedtime in Amsterdam after flying from Toronto might feel like 3:00 p.m. to your body as it’s still on Toronto time. From a biological perspective, forcing an earlier sleep time is more challenging because the circadian system is more resistant to phase advancement. Due to this added physiological demand it may be beneficial to prepare for eastward travel in the days before departure. More on that below...

Another factor that can increase the severity of jet lag is the number of time zones you cross during travel. Jet lag is more pronounced when crossing more than three time zones. Adaptation typically takes one day per time zone crossed when travelling east, and slightly less when heading west. This is one more reason why we sometimes hear in the professional sporting world that travelling “east is beast”, and travelling “west is best” when it comes to physical performance. This holds true for everyone when it comes to perceived energy levels, mental clarity and cognitive performance.

Something I've considered while writing this newsletter is if the locations of certain sports teams and the locations of various international company headquarters, are in fact, strategic? Arguably, this may be a subtle way to improve sports performance or employee performance when subject to frequent travel. I'm not sure if this has actually been done, but @NHL, @NBA, @NFL, if you're looking for some input, I got you!

Physiological Impact of Jet Lag

As I mentioned, jet lag is not just a matter of feeling tired or groggy—it reflects a systemic disruption of physiology. The central nervous system is particularly sensitive, with reductions in brain connectivity resulting in attention deficits, decision-making errors, and increased reaction times all of which are critical for athletes, high performers and those looking to make the most out of their hard-earned vacation time.

Sleep quality may be significantly compromised, with fragmentation, decreased REM and slow-wave sleep, and difficulty with sleep onset. The endocrine system is affected through alterations in cortisol rhythms, blunted melatonin secretion, and disrupted patterns of growth hormone and thyroid hormone release. Metabolic function suffers too—circadian misalignment can increase insulin resistance, impair glucose tolerance, and elevate appetite hormones like ghrelin. This suggests why repeated exposure to time zone changes (e.g., flight attendants, pilots) may result in the onset of various non-communicable diseases. More research on this population needs to be done in order to determine the exact physiological disruptions and how severe they may be.

Immune function is also modulated by the circadian system. Jet lag has been shown to suppress the immune system and lead to dysregulation of the inflammatory process, potentially increasing vulnerability to illness and impairing recovery.

Overall, it's quite clear that jet lag is much more than just sleep dysregulation. It affects so much of our physiology.

Evidence-Based Strategies to Reduce Jet Lag

Mitigating jet lag requires a multifaceted approach that targets circadian realignment through strategic manipulation of light, behaviour, and biology. One of the most effective tools is light exposure timing. Because light is the strongest zeitgeber, controlling exposure to it can help shift your body clock more efficiently. For eastward travel, morning light at the destination can help advance your circadian phase. In contrast, for westward travel, exposure to light in the evening helps delay the clock. Using bright light therapy—either natural sunlight or 10,000 lux light boxes—can amplify these effects when timed properly, and when used strategically in the days prior to departure.

Melatonin supplementation is another well-researched intervention. There still remains some uncertainty with respect to the optimal dosing for melatonin, however starting melatonin one or two days before departure and continuing for a few days upon arrival appears to enhance adaptation.

Adapting sleep and wake schedules prior to departure can help pre-shift your circadian rhythm. For eastward flights, gradually going to bed and waking up earlier in the days leading up to travel can reduce the phase shift required upon arrival. Conversely, staying up and waking later may help with westward travel. Upon arrival, it's essential to try to align sleep to local nighttime hours as quickly as possible, even if that means staying awake a few extra hours. I find that a jet lag calculator can really help clarify the timing of your phase shift based on realistic goals and the amount of time you have to work towards your circadian shift. Calculators take into account your typical sleep times, your departure and arrival time zones, and how many days you’re willing to commit to phase shifting prior to travel. Give this a try before your next trip!

Caffeine can be a useful tool, but like melatonin, its effectiveness depends on timing and your individualized response to caffeine. When consumed in the early part of the day, it can promote alertness and help shift your clock forward. However, habitual coffee drinkers or those who do not reap the stimulant effects from caffeine may not find any benefit.

Nutrition and meal timing also function as zeitgebers. Eating meals according to your arrival time zone can help shift and reinforce the circadian signal that is driven by feeding. Hydration is equally important, especially given the dehydrating nature of air travel and dry cabin conditions.

Physical activity—even light movement such as walking, stretching, or mobility work—can also support circadian alignment, particularly when performed in natural daylight. Exercise has been shown to influence the peripheral circadian clocks in skeletal muscle, and may do so independently and synergistically with light. However the effectiveness may be partially determined by your underlying chronotype (e.g., if you’re and early riser or a night owl). Either way, it's just another excuse for us all to get outside and move during our travels!

My final suggestion is one that I've used personally since I was younger, and long before I knew anything about circadian biology, because I just felt it made sense. As soon as I arrive at departures I immediately change my watch and phone clock to the time at my arrival destination. I find this helps as every time I look down at my wrist during the flight, I'm referencing the new time zone in which I will be arriving at. This may just be a subtle psychological trick, and I don't have anything other than anecdote to support it, but it helps me shift my mindset during the flight and be prepared for the time of day I will encounter upon arrival. Give this a try next time you travel.

Final Thoughts

It's clear that jet lag is more than just an inconvenience—it's a complex, multisystem response to a biological mismatch between internal time and external cues. By understanding the nuances of circadian biology, the role of zeitgebers and how we can use them to our advantage, we can hopefully minimize the negative physiological toll that we experience during transmeridian travel.

Whether you're an athlete travelling to compete, a busy entrepreneur who travels for work, or someone who doesn’t want to miss out on valuable hours during a vacation, considering the above strategies to help you adapt to travel across time zones may be a game-changer.

As I get ready to travel eastward from Toronto to Amsterdam next week I’m trying to be realistic with my goals based on work and prior obligations before I depart. The plan is to give myself two days to begin phase shifting by going to bed earlier, and waking up earlier emphasizing early light exposure. We’ll see how it goes this time around, because it’s going to be a busy trip.

If you have an upcoming vacation, conference or other reason for travel, give some of these evidence-based strategies a try, and let me know in the comments what works for you!

Yours in good health,

Andrew

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