Do you set an alarm to wake you up on weekdays, then hit the snooze button at weekends because you need more sleep? If so, you could be experiencing social jetlag – a condition associated with weight gain, reduced mental performance and chronic illness.
“Social jetlag promotes practically everything that’s bad in our bodies,” says Till Roenneberg, professor of chronobiology at Ludwig-Maximilian University in Munich, who coined the term. It occurs when we go to bed later and wake up later at the weekend than on weekdays. Like normal jetlag, it is a consequence of being forced to shift our bodies between two time zones: one dictated by work and social obligations, the other by our internal timing system, the circadian clock. It is estimated that two-thirds of us experience at least one hour of social jetlag a week, and a third experience two hours or more – equivalent to flying from London to Tel Aviv and back each week.
How much social jetlag you experience is down to the magnitude of the mismatch between your two time zones. People’s sleep preference, known as their chronotype, is largely dictated by genes. “Owls” – whose natural tendency is to stay up late and not wake until 10 or 11am – experience more social jet lag than “larks”, because they struggle to get enough sleep in the week and sleep in at weekends to catch up. However, an extreme lark pressured into staying up late at weekends by friends will also suffer.
As anyone who has experienced normal jetlag will know, one of the most obvious symptoms is trouble sleeping. The body takes a while to adapt to a new time zone – typically a day for every time zone you cross. Similarly, says Roenneberg: “Social jetlag and sleep deprivation are practically inseparable.”
Chronic sleep deprivation has been linked to many of the same illnesses as social jetlag, including type 2 diabetes, heart disease, obesity and depression; and has been declared a public health epidemic by the World Health Organization and the US Centers for Disease Control and Prevention. A study by the Rand Corporation calculated that inadequate sleep costs the UK £50bn a year – equivalent to 1.9% of GDP – due to decreased productivity and sickness. Sleep deprivation also takes its toll on our daily lives, affecting our vigilance, hand-eye coordination, memory, logical reasoning and emotional stability.
However, social jetlag doesn’t only disrupt the amount of sleep. A study of undergraduates found that those who kept irregular bedtimes had poorer quality sleep than those with more consistent sleep schedules, even though they got roughly the same amount overall. Irregular sleep was also associated with poorer academic performance. Andrew Phillips, now at Monash University in Melbourne, who led the study, says: “This suggests sleep regularity is very important – it’s not just about getting the right amount of sleep, for example by sleeping in at weekends.”
It has other consequences, too. Inside each of our cells is a molecular clock that governs the timing of almost every physiological process in our bodies. The most obvious of these is when we feel sleepy or alert, but circadian clocks also control when we secrete hormones, the activity of our immune cells, our body temperature, even our mood at different times of day and night.
These clocks run on roughly (though not precisely) 24-hour schedules – larks tend to have slightly faster clocks, and owls slightly slower ones – synchronising through signals from a tiny patch of brain tissue known as the suprachiasmatic nucleus (SCN), which acts as a kind of internal Greenwich Meridian. It knows the time through its interactions with light-responsive cells at the back of the eye, which evolved to register the rising and setting of the sun.
If you change the timing of your light exposure – as you do when you go to bed and wake earlier on weekdays, or when you fly across time zones – the timing of the clocks in your organs and tissues also shift, although at different rates. If you constantly shift the timing of your sleep and light exposure, as you might if you regularly sleep in at weekends, your clocks will be perpetually out of synchrony.
“Almost all the hormones in your body are on some sort of circadian rhythm and when you are shifting your sleep time, the entire system is not going to be working as efficiently as it should,” says Sierra Forbush, a research assistant at the University of Arizona College of Medicine. She recently presented data from a study of 984 adults that suggested that, for every hour of social jetlag a person experienced each week, there was an 11% increase in their likelihood of having cardiovascular disease. Social jetlag was also associated with worse mood and greater levels of sleepiness and fatigue.
Another study found that adults with higher levels of social jetlag were more likely to be overweight or obese and have metabolic syndrome (which is associated with the development of type 2 diabetes) compared with those with more regular sleep patterns – even after controlling for how much sleep they got.
“We found that just one hour of social jetlag led to the accumulation of about two additional kilograms of fat mass, on average, by the age of 39,” says Michael Parsons, a circadian biology researcher at the Medical Research Council’s Harwell Institute in Oxfordshire, who led the study. “Although you often see this increase in fat and body mass index in shift workers, we were surprised that a relatively small amount of social jetlag – equivalent to flying across one time zone each week – could be associated with a relatively large increase in such things.”
He cautions that factors such as eating too much and not getting enough exercise play a greater role in weight gain. However, he says: “As a society, it’s something we need to consider when deciding whether or not we should keep such things as daylight savings time [another source of social jetlag] or introduce more flexible working hours to better fit with people’s chronotype.”
Which leads us to the question of what to do about it. It is unlikely that the occasional lie-in will be detrimental to your health – indeed, if it is the only way of catching up on sleep you have missed during the week, it is probably a good idea. “If you have accrued a sleep debt, it needs to be paid back,” says Phillips. “However, a much healthier alternative is to try to maintain a regular sleep pattern throughout the week and get more sleep each day.”
One way of achieving this would be to allow greater flexibility in people’s working hours, so that owls could start work later and therefore get the recommended seven to eight hours of sleep each night. If he were an employer, Roenneberg says, he would ban the use of alarm clocks and instruct employees to start work only once they had had adequate sleep. “The majority of employees would still be in the office by 10am or 11am, but this would increase productivity, sick days would go down, and I would get your best time as an employer,” he says. “It’s a win-win situation.”
However, there may be a simpler solution: light. Although people’s chronotype is genetically determined, environment also influences sleep timing. Studies have shown that when people are sent camping – removing them from the influence of artificial light and exposing them to more daylight – they become more lark-like and fall asleep about two hours earlier. Inspired by these studies, Phillips used a mathematical model of human sleep and circadian physiology to probe why this should be. He found that, regardless of whether they were exposed to natural or modern lighting conditions (where people spend most of the daytime indoors and their evenings exposed to artificial light), people with longer molecular clocks went to sleep later. However, in the modern condition, variation in the timing of people’s sleep more than doubled because exposure to bright (or blue-enriched) light at night pushes the timing of the SCN, the brain’s master clock, later – meaning a night-owl’s natural tendency to stay up later is amplified. “We found that these large inter-individual differences in sleep timing only manifest themselves in the presence of electric light,” says Phillips. “This is what’s causing a lot of individuals to become very delayed in their sleep timing and hence experience social jetlag.”
He suggests turning off overhead lights two hours before bed, and switching to dimmer table lamps. And if you do use a computer or smartphone in the evening, install an app that automatically dims the screen and filters out the blue light.
We evolved on a planet where day was day, and night was night. For the sake of our health, it is time to reacquaint ourselves with those extremes.
• Linda Geddes’s book Chasing the Sun: The new science of sunlight and how it shapes our minds and bodies is published by Wellcome Collection
