What actually happens to your body when you experience jet lag?
The exact effects of a relatively slight time shift—the
equivalent of flying from New York to Iceland—on our body's metabolic processes
have been clarified by a study published in the journal
iScience. According to the research, we may still feel "left out"
even after shorter travels rather than just lengthy journeys across time zones.
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The human body operates on a complex 24-hour cycle called
our circadian rhythm - essentially our internal clock that helps regulate
everything from sleep patterns to hormone production to metabolism. This
biological timekeeper is controlled primarily by a small region of our brain
called the suprachiasmatic nucleus (SCN), which responds to environmental cues
such as light and dark cycles to keep us us in sync with the outside world.
For a rigorous eight-day laboratory investigation,
researchers gathered 14 participants—eight men and six women—to precisely
determine how this response impacts our metabolism. The volunteers, who were
generally healthy but overweight or slightly obese, agreed to change their
entire daily schedule by five hours - similar to what happens when you fly from
the East Coast to Iceland .
The research team carefully controlled everything from meal
timing to light exposure to sleep schedules. In-depth information about the
individuals' metabolism was gathered, including how fast their bodies
metabolised meals, how much energy they expended, and how their blood sugar and
fat levels fluctuated during the day.
The results revealed that even this relatively small five hour shift had significant effects on the body's metabolic processes. Immediately after the time shift, the participants showed slower digestion of their breakfast, changes in blood sugar patterns, and altered fat metabolism. The thermic effect of eating is the term used to describe how their bodies grew less effective at producing heat from the food they consumed.
The most intriguing aspect was perhaps how fast the body adjusted to the new routine. Most of these metabolic disturbances began to normalize within 48 to 72 hours, showing that our internal systems have remarkable flexibility. This adaptability helps explain why most people can adjust to new time zones within a few days rather than suffering an indefinite delay.
The study also revealed that some metabolic processes are
more resilient to changes over time than others. For example, the participants'
resting energy expenditure - how many calories they burned while resting -
remained surprisingly stable despite the scheduled shift. This suggests that
some aspects of our metabolism are "anchored" more firmly than
others.
One fascinating finding involved melatonin – often called
the “sleep hormone” – which acts as a reliable marker of our internal
biological clock. After the five-hour schedule shift, the participants'
melatonin patterns gradually adjusted at a rate of about 1-1.5 hours per day.
This orderly adjustment shows how our bodies not only flip a switch to adapt to
new time zones but instead go through a careful recalibration process.
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Beyond only aiding in our understanding of jet lag, the
discovery has significant ramifications. In our modern world, many people
regularly experience "social jet lag" - the mismatch between their
natural body clock and social obligations that require early wake-up times.
This study suggests that even these smaller time shifts may have metabolic
consequences that we should be aware of.
The findings also show why sticking to daily
schedules—especially when it comes to meal times and sleep patterns—may be
especially important for metabolic health. Although our bodies can adapt to
changing schedules, forcing them over and over again may not be ideal.
"Our research highlights the importance of maintaining
a consistent sleep schedule, particularly in a fast-paced world where long
commutes and shift work are common," says Jonathan Johnston, professor of
chronobiology and integrative physiology at the University of Surrey. , in a
statement.
"Even a small shift can affect many aspects of
metabolism, but currently reactive metabolic effects seem to reverse faster
than sleep and wakefulness disturbances. Understanding the effects of circadian
rhythms on our health can help us make informed lifestyle choices. By
optimizing our sleep and eating patterns, we can improve overall health."
we can improve our well-being.”
So, while we may not be able to cross time zones on long
flights around the world, we can understand why our bodies experience jet lag
or other significant schedule changes like angry chronological critics. Indeed,
the most difficult time frame is actually more than a quick twist of arms to
adapt.
Summery
Methodology
The researchers conducted the study in a highly controlled
laboratory environment where they could observe every aspect of the
participants' daily activities. 14 participants spent 8 days in the laboratory,
with the first day serving as an adjustment period. The next day they went on a
normal schedule, but on the 3rd day everything was moved 5 hours later - food,
sleep, light, everything. For the rest of the day they maintained this delayed
schedule.
During the study, researchers collected blood samples to
measure various metabolic markers, used special equipment to measure energy
expenditure, and monitored how quickly the participants digested their food
using breath tests. They also monitored the participants' levels of melatonin
(a sleep hormone) to see how their internal body clocks adjusted to the time
shift.
Main results
Following the 5-hour shift, the study discovered a number of
significant changes:
- Breakfast slowed the stomach down considerably
- Blood sugar fluctuates, with fasting levels lower but higher postprandial
- Fat metabolism is altered, triglycerides are higher after eating
- The body became less efficient at producing heat from food
- The duration of melatonin gradually changed, 1-1.5 hours per day
- Most breakouts begin to return to normal within 2-3 days
- Despite the time shift, energy costs remained stable
Study limitations
The researchers acknowledge several limitations of the
study. The participant group was relatively small (14 people) and consisted of
metabolically healthy but overweight individuals. Findings may not apply
equally to people with different body compositions or metabolic disorders.
Additionally, when research attempts to simulate real-world shifts, highly
controlled laboratory conditions may not reflect real-life conditions well,
where other factors may affect adaptation to time-shifts.
Discussion
Research shows that relatively modest shifts in our daily
schedule can temporarily disrupt various metabolic processes. But it also shows
that our body clocks are incredibly adaptable, with most disruptions being
resolved within a few days. This suggests that while occasional schedule
changes may be manageable for our bodies, frequent interruptions may not be
ideal for metabolic health. The research has practical implications for jet
lag, shift work, and even social jet lag from weekend schedule changes.
Financing and opening
The study was funded by an MRC (Medical Research Council)
grant. Some researchers have disclosed relationships with food companies: Dr.
Johnston collaborates with Nestle and consults for Kellogg's and International
Flavors and Fragrances (IFF). Dr Johnstone holds a voluntary position as an
advisory committee member at the British Nutrition Foundation. However, these
relationships did not affect the study design or outcomes.
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