Scientists have found that jet lag is more severe when travelling eastwards.
In the largest study of its kind, researchers found that travel-related sleep disruption takes even longer to recover from than previously thought.
Scientists at the Centre of Sleep and Cognition at the NUS Yong Loo Lin School of Medicine, in Singapore, found that while sleep duration recovers quickly, sleep timing can take “significantly longer” to realign when travelling across time zones, particularly when headed east as one is in effect “losing” time.
The body’s circadian rhythm, or body clock, is less confused when travelling westward because the direction “prolongs” the body’s experience of a normal day to night cycle (which is roughly 24.5 hours). Travelling eastward, on the other hand, runs in opposition to the circadian rhythm.
Shorter trips of up to three time zones were severely impacted by jet lag, according to the study published in Sleep, the official journal of the Sleep Research Society.
The researchers analysed sleep patterns during 60,000 trips of more than 600 miles with the help of Oura Health, a Finnish tech firm.
Wearable devices like the Oura ring track sleep, physical activity and overall health behaviours
The team used 1.5 million nights of de-identified data from the Oura ring, a smart device used to track sleep and physical activity, to provide the first large-scale, real-world study of jet lag recovery to date.
The findings also showed that young adults feel the negative sleep effects on long flights more than people in their 60s.
Previous jet lag studies have either been conducted under controlled laboratory conditions or involved specific groups, such as athletes or military personnel, whose characteristics may not reflect those of the general travelling public.
Dr Adrian Willoughby, the study’s lead author and a senior research fellow at NUS Medicine, said: “We’ve known for some time that jet lag is a challenge for travellers, but this study provides data-driven evidence of just how persistent the impacts are, particularly when it comes to adjusting sleep timing to a new time zone.
“Crossing time zones complicates this recovery by disrupting the ability to fall asleep at the appropriate local time.”
Factors contributing to sleep disturbance
While jet lag plays a significant part in sleep disturbance, it is not the only factor. Travellers who wake up early to catch flights shorten their sleep the night before, which results in an early bedtime after the journey and causes the body to crave an extended recovery sleep the next night.
Those who sleep during a long-haul flight are also negatively affected, experiencing impaired performance and increased daytime sleepiness.
The study found that the sleep duration usually returns to normal within around two days. But changes in sleep timing and structure, such as more night time awakenings, can take more than a week, with eastward travel and crossing multiple time zones amplifying the disruption.
The researchers found minimal differences between men and women in travel-related sleep disruption.
Older travellers experienced slightly less impact, with a 20-year-old experiencing a 15-minute greater reduction in sleep compared to a 60-year-old over the initial post-travel days.
Prof Michael Chee, the director of the Centre for Sleep and Cognition at NUS Medicine, said: “Wearable devices like Oura ring that track health behaviours over time, are opening new doors for health data collection on a large scale.
“Travellers looking at this dataset can determine how much better or worse off they are than the population average given their existing sleep habits, trip parameters, and number of days after travel. People like to know where they stand compared to others, and this dataset provides a solid basis for assessing that.”
The team said there was a need for future research to assess lifestyle factors contributing to slower or faster recovery from jet lag and whether efforts to time light exposure and melatonin ingestion help.
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