Social jetlag is defined as being a sort of mismatch between the body's internal clock and the realities of our daily schedules.
Our natural circadian rhythms are no longer in synch with the demands of modern life, where we stay awake long after nightfall and are artificially woken by the dreaded alarm clock. According to a large-scale epidemiological study carried out in Europe, it is making us more than just sleepy.
"We have identified a syndrome in modern society that has not been recognised until recently," says Till Roenneberg of the University of Munich, which collected the data from thousands of participants.
"It concerns an increasing discrepancy between the daily timing of the physiological clock and the social clock. As a result of this social jetlag, people are chronically sleep-deprived. They are also more likely to smoke and drink more alcohol and caffeine. Now, we show that social jetlag also contributes to obesity. The plot that social jetlag is really bad for our health is thickening."
Roenneberg's team is compiling a vast database on human sleeping and waking behaviour, which it will eventually use to produce a world sleep map.
Its analysis shows that people with more severe social jetlag are also more likely to be overweight. Living "against the clock", they say, may be a factor contributing to the epidemic of obesity. The circadian clock is known to regulate energy homeostasis or stability and its disruption may contribute to weight-related problems.
"Waking up with an alarm clock is a relatively new facet of our lives," Roenneberg says. "It simply means that we haven't slept enough and this is the reason why we are chronically tired."
Dr Alex Bartle is a sleep specialist with Auckland's Sleep Well Clinic. He says the term "social jetlag" may be a new one, but the link between sleep deprivation and obesity is undoubted.
"Absolutely. If you are not sleeping at the right times you get fatigued, and that results in two major problems. You lose the motivation to do exercise and hormones such as leptin and ghrelin drop and rise. This tells us we are hungry. So, you tend to eat more - not lettuce and celery, but chocolate and other high-calorie stuff."
Dr Bartle points out that we are still influenced by thousands of years of evolution, of watching the sun coming up and going down, even though our environments are no longer natural.
"What our bodies like, what entrains us, is the light/dark cycle. What keeps us to 24 hours is the rotation of the Earth.
In the last 100 years that's been increasingly eroded. Electricity has allowed us to spend more time awake and in light environments.
TV has now started to go for 24 hours. Then there are laptops and more recently iPads and texting - everything happens throughout the night. It's a 24-hour society which has done nothing to change our brains."
Our circadian cycle hasn't changed over the past 100 years, but society has altered beyond recognition. "We've still got prehistoric brains, but we are making huge demands on our bodies to conform to things like shift work."
He says a wide range of health issues is related to poor sleep, besides obesity. These problems include lethargy and lack of motivation, medical problems such as hypertension and cancer and "emotional stuff" such as poor memory, irritability, depression and anxiety.
Top tips
Dr Alex Batle's tips on how to avoid social jetlag:
* Spend time outside during the day to absorb natural light
* Keep your bedroom dark when it's time to sleep
* Avoid using computers for at least an hour before bedtime
* Avoid taking any communication devices to bed. No texting
* Try to stick to a regular waking time, even on weekends and days off
* Avoid caffeinated drinks after 2pm
* Go to bed when you're sleepy - not too early or too late
