"Social jet lag is driving obesity" is the misleading headline in The Daily Telegraph. A new study only found a link between "social jet leg", obesity, and metabolic markers that may indicate a person has an increased risk of obesity-related diseases, such as type 2 diabetes. A cause and effect relationship was not found.
Social jet lag is the term used to describe the difference in someone's sleep patterns between work days and free days – also known as having a lie-in at the weekend.
The researchers' hypothesis was that regularly disrupting our sleep patterns could upset the body clock (circadian rhythms), which could then have a harmful effect on the metabolism.
The study of more than 800 non-shift workers found people with a greater difference in sleep patterns between free days and work days were more likely to be obese and "metabolically unhealthy" (have markers for obesity-related diseases) than those with little or no difference between these timings.
But the study does not prove regular lie-ins cause obesity or obesity-related diseases, as it assessed sleep patterns and health at the same time. It is possible with this type of study that the reverse is true – that obesity and any associated health conditions may cause people to lie in more.
Overall, this study provides no proof having a lie-in will affect your health, though the occasional early-morning Saturday stroll may improve both your fitness and wellbeing.
Where did the story come from?
The study was carried out by researchers from the Medical Research Council (MRC) and the University of London in the UK, Duke University and the University of North Carolina in the US, and the University of Otago, New Zealand.
It was funded by the US National Institute of Aging and the MRC.
The study was published in the peer-reviewed International Journal of Obesity.
The quality of the UK's media coverage of the study was mixed. The Independent correctly mentioned there was no proof social jet lag causes obesity, but none of the papers mentioned the possibility of reverse causation: that obesity makes people more likely to lie in, rather than lie-ins causing obesity.
The Daily Telegraph's choice of headline was particularly unhelpful, as it implied social jet lag was now a proven partial cause of the obesity epidemic and the related complications. This is not the case.
What kind of research was this?
This was a cross-sectional analysis of a cohort study that aimed to look at the association between obesity and metabolic markers that may indicate obesity-related disease, and social jet lag. Social jet lag is a measure of the discrepancy in sleep timing between our work and free days.
The researchers say travel-induced jet lag results in problems with circadian rhythms (the body's internal clock), which causes temporary problems with metabolic rate (the rate at which the body uses up energy).
However, they suggest social jet lag can become chronic throughout someone's life and therefore have longer term consequences for metabolism, possibly increasing the risk of metabolic syndrome. Metabolic syndrome is the medical term for a combination of diabetes, high blood pressure and obesity.
The researchers also say recent research found people with higher social jet lag and a greater discrepancy between internal and social clocks were found to have a higher self-reported body mass index (BMI).
They consider it possible that if our internal clocks are at odds with external schedules, this may partly underlie the increase in obesity seen in the last few decades.
Cross-sectional studies look at all data at the same time, so they cannot be used to see if one factor (in this case, social jet lag) has caused the others (in this case, obesity or metabolic markers).
What did the research involve?
This study included 815 non-shift workers who were participants of an ongoing health study in New Zealand (Dunedin Longitudinal Study), which is following more than 1,000 people born between 1972 and 1973 to investigate links between health and behaviour.
At the age of 38, each participant was asked to fill in a standard questionnaire to assess social jet lag, as well as sleep duration and chronotype (their "natural" preference in sleep timing).
Social jet lag was measured by subtracting each person's midpoint of sleep on work days from their midpoint of sleep on free days (assuming five work days and two free days a week as standard).
So, for example, if someone slept from 12am to 8am on workdays, the midpoint was 4am. If they then slept from 1am to 11am on free days, the midpoint was 6am, giving a social jet lag of two hours.
Researchers also measured participants' height and weight to calculate BMI, with obesity defined as a BMI of 30 or more. Waist circumference and fat mass were also measured.
The researchers also assessed whether participants had markers of metabolic syndrome, a disorder associated with diabetes and obesity.
They assessed five biomarkers, and people with "high-risk values on three or more" were defined as having metabolic syndrome. These were:
- waist circumference (88cm or more for women, 102cm or more for men)
- high blood pressure (130/85mm Hg or higher)
- low levels of high-density lipoprotein (HDL, or "good") cholesterol
- high triglycerides (another blood fat)
- high blood levels of a glycated haemoglobin (an indicator of blood glucose control – a marker for diabetes)
They also assessed blood levels of an inflammatory marker called C-reactive protein.
The authors say recent research has shown a subset of obese individuals who are "metabolically healthy". They therefore created a measure for obesity status with three levels:
- non-obese (a BMI of below 30)
- healthy obese (a BMI of 30 or above, but no metabolic syndrome)
- unhealthy obese (a BMI of 30 or above and metabolic syndrome)
Researchers also asked people about their current smoking status (since smoking is positively associated with jet lag and may also keep weight low) and socioeconomic status, assessed by their current or most recent occupation.
They were then allocated to one of six categories (from 1 – unskilled labourer to 6 – professional). Those not working were rated according to their educational status.
Researchers analysed their results to determine if social jet lag was associated with "unhealthy" obesity. They created three models, with one adjusting the figures for potential confounders, including smoking, socioeconomic status, sleep duration, and sleep preferences.
What were the basic results?
The researchers report social jet lag was associated with numerous measures of metabolic dysfunction and obesity, with higher social jet lag levels in "metabolically unhealthy" obese individuals.
Among metabolically unhealthy obese individuals, social jet lag was additionally associated with high blood levels of glycated haemoglobin and CRP (an indicator of inflammation).
Individuals with higher social jet lag scores were more likely to be obese (odds ratio [OR] 1.2, 95% confidence interval [CI] 1.0 to 1.5) and to meet the researchers' criteria for metabolic syndrome (OR 1.3, 95% CI 1.0 to 1.6) – though both of these risk increases are only of borderline statistical significance.
How did the researchers interpret the results?
The researchers say the findings are consistent with the possibility that, "living against our internal clock may contribute to metabolic dysfunction and its consequences".
They suggest a two-hour difference in sleep patterns at the weekend is the "threshold" for a higher BMI and other biomarkers, although they also point out this association was weakened or non-significant once smoking and socioeconomic status were taken into account.
Further research is needed, they say, to determine the physiological mechanisms underlying these associations.
The study involved 815 non-shift workers. It found people with a greater difference in sleep patterns between free days and work days (so-called "social jet lag") were more likely to be obese and "metabolically unhealthy" (have markers for obesity-related diseases) than those with little or no difference between these timings.
This study adds to previous research in both animals and humans that has explored the possible effects altering the body clock may have upon our metabolism, being overweight or obese. A recent UK survey found a link between shift work and chronic diseases, which we discussed at the end of 2014.
However, this new study cannot prove regular lie-ins cause obesity or obesity-related diseases.
The study is cross-sectional, assessing sleep patterns and health at the same time. It is possible with this type of study that the reverse is true – that obesity and any associated health conditions may cause people to lie in more whenever possible.
There may be many underlying factors this study has not taken into account that are influencing the apparent relationship between obesity, metabolic markers, and higher levels of social jet lag.
For example, the study did not take account of people's diets or their exercise levels, which are two key factors that influence BMI and may also influence our sleep patterns.
The increased risks of obesity and metabolic syndrome with social jet lag were only of borderline statistical significance in any case, which further indicates the overall lack of strength in these associations.
Experts tend to agree it is best to keep to a regular sleep schedule on week days and weekends to prevent sleep problems. Whether following this advice can also keep the weight off is uncertain. Overall, this study provides no proof having a lie-in will affect your health.
Still, we can't help but agree with the recommendations of one of the authors of the study, as quoted on the Mail Online website: "I don't want to tell people not to have a lie-in because I enjoy one myself," lead study author Michael Parsons said. He then went on to recommend that employers could offer flexible hours, so staff could synchronise their week days with their weekends.