Is Jet Lag Leaving You Tired and Unwell? Here’s What a Study Reveals.

After a long-haul flight or a night of work, you usually don't feel very good about yourself. This is mainly because of what scientists call “circadian desynchronization”. Our internal clock is no longer in sync with daily changes in the external environment. British researchers recently examined the effects of being out of sync for a few hours. They found that a time difference of just five hours causes many changes in OUR metabolism.

Circadian rhythms refer to cyclical, approximately 24-hour biological processes that respond to the environment. They are regulated by a circadian clock, which coordinates these processes so that they occur at the appropriate time. They adjust to the local environment via external temporal signals called zeitgebers (from German Zeitgeberwhich means “giver of time”). Circadian rhythms are important for multiple aspects of human physiology, including cardiometabolic health.

In mammals, the master clock is located in the suprachiasmatic nuclei (SCN) of the hypothalamus. It synchronizes with the light-dark cycle. Peripheral clocks, connected to the main clock, are scattered throughout the rest of the body (liver, muscles, fat, etc.). They regulate metabolism and other key tissue functions. Certain life situations, such as working night shifts or long plane trips, cause our internal clocks to become out of sync with environmental rhythms. However, this is not without consequences.

Exploring the effects of a behavioral lag of a few hours

Several studies have already highlighted the importance of time in the short and long term response to caloric intake. This is also the basis of chrononutrition, which determines which foods to eat and at what time of the day.

It is known, for example, that postprandial metabolism, particularly glucose and insulin responses, follows a 24-hour rhythm. Thus, a study showed that delaying meals by 5 hours induces a delay of 5 to 6 hours in carbohydrate rhythms. In addition, eating late in the evening increases hunger upon waking while reducing energy expenditure. This is why eating late can lead to an increased risk of obesity.

To date, most studies of circadian desynchronization rely on a 12-hour shift in environmental/behavioral cycles. A 2015 study showed that such a shift reduces glucose tolerance. This effect could explain the increased risk of diabetes observed in shift workers.

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In contrast, the effects of more moderate desynchronization – which occurs more commonly, for example, in transmeridian flights – remain unclear. To fill this gap, a team of British researchers subjected volunteers to a 5-hour shift in the light-dark, sleep-wake and food-fasting cycles.

They hypothesized that circadian desynchronization the day after the shift would alter daytime energy metabolism. They thought, in particular, that the thermic effect of food (i.e. the body energy expended for its transformation) would be weaker and would only partially return to its baseline value over the following days. The results of their study were published in iScience.

A significantly modified central clock

The 14 participants were 45.6 years old on average and had no health problems. The average BMI was 31.7 kg/m² (corresponding to overweight). They arrived at the laboratory on day 0, then after an acclimation period, they delayed their bedtime by 5 hours, starting in the evening of day 2. That evening, to maintain a consistent fasting period between the last food intake and the following breakfast, they had a snack 5 hours after dinner (around 11 p.m.).

Participants kept this delayed cycle for 5 days, so that researchers could examine how biological rhythms adapted to the new schedule. Meals, shifted by five hours from the third day, were isocaloric and taken at a fixed time. Nighttime sleep duration was 8 hours throughout the study.

Study protocol. Credits: Flanagan et al., iScience (2024)

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The researchers carried out various measurements, before the shift, then over the following 5 days. They measured blood markers of metabolism, the central clock biomarker (melatonin), energy expenditure and appetite. They also collected participants' subjective feelings of hunger, sleepiness, and alertness throughout the day.

THE Dim light melatonin onset (DLMO) corresponds to the start of melatonin production in the evening, in low light conditions. This is the reference marker of the central clock. However, the time difference significantly affected him. By day 3, the time interval between waking up and DLMO had decreased by around 3.5 hours! Then, as the days went by, it tended to return to normal.

Effects of a gap of 5 hours on the timing of the production of melatonin. Credits: Flanagan et al., iScience (2024)

Immediate but short-lived metabolic effects

Alongside the change in melatonin production, researchers noted significant changes in sleepiness and alertness. Between 14 and 17 hours after waking up on day 3, participants experienced significantly higher sleepiness and significantly lower levels of alertness », they note. Then, between day 3 and day 7, the temporal profiles of sleepiness and alertness also gradually readjusted, but without completely returning to their initial value.

Effects of a 5-hour time lag on daily sleepiness and alertness profiles. Credits: Flanagan et al., iScience (2024)

The team reports that the 5-hour shift had no impact on energy expenditure during the 16 hours of waking. In contrast, the thermal effect of the power supply first dropped on day 3, before realigning with the central clock. At 7^e day, it had regained its initial value.

Energy expenditure over all a day and thermal effect food. Credits: Flanagan et al., iScience (2024)

>>Also read: Children and teenagers who skip breakfast could be less happy, study suggests

The researchers also observed a significant effect of jet lag on gastric emptying following breakfast. This was delayed by around 1h30! However, she quickly adapted to the new behavioral cycle.

Finally, differences also emerged in blood sugar and fat regulation. Fasting blood glucose was lower, while postprandial plasma blood glucose and triglycerides were higher than before jet lag.

The team took subjective measurements of hunger, appetite and fullness every hour from waking up. However, she found no significant differences between the eight days of the study.

Potentially greater effects in some people

This study showed that even a small time difference can impact many aspects of metabolism. The researchers nevertheless emphasize the temporary aspect of these metabolic changes. Indeed, most of them recover within 2 to 3 days following the 5-hour shift. “ However, it is important to eat a good quality diet and, where possible, minimize late evening and nighttime meals. », add the researchers in The Conversation.

Note that this study only involved healthy participants. The team believes that individuals with elevated cardiometabolic risk factors could potentially exhibit more altered metabolic responses to a 5-hour phase delay.

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If the alteration of metabolism was here of short duration, this was not the case for the changes in the production of melatonin, and the sensations of drowsiness and alertness. At the end of the 8 days of experience, they had not yet regained their initial rhythms. Thus, people who are frequently subject to circadian desynchronization are advised to apply certain measures to reset their biological rhythms. Researchers suggest exposing yourself to light (or avoiding it) and taking a melatonin supplement at appropriate times.

“ Our research highlights the importance of maintaining a consistent sleep schedule, especially in our fast-paced world where long travel and shift work are increasingly common said Jonathan Johnston, professor of chronobiology and integrative physiology at the University of Surrey, who led the study.

“ Understanding the impact of circadian rhythms on our health can help us make informed choices about our lifestyle. By optimizing our sleeping and eating habits, we can improve our overall well-being », concludes the specialist.