Every week, you're giving yourself jet lag.
When you switch from an 11 pm to 6 am sleep schedule on the weekdays to a 2 am to 9 am sleep schedule on the weekends, you are effectively moving over three time zones. Mondays are awful because they are Mondays. But they are also bad because we're forcing ourselves into a circadian misalignment.
This amounts to what researchers call "social jet lag," and scientists are just beginning to learn whether it's as bad as the real thing.
Constant jet lag is terrible for your health
As study designs go, this one was intense.
For three weeks, Orfeu Buxton and his colleagues at Harvard had 24 participants sleep in a lab at Brigham and Women’s Hospital in Boston. There were no windows in the lab. No clocks. No cues to signal what was going on in the outside world. Exercise was not allowed. The human guinea pigs were only allowed 5.6 hours of sleep a night, and they lived on a 28-hour day.
"That’s like going four time zones west every day," Buxton, a sleep researcher, told me in September.
The study was designed to put them in a constant state of jet lag. Three weeks in this sleeping arrangement was equivalent to taking three trips around the globe — but with none of the adventure and all of the bleariness. Most of us know that too little sleep (doctors say ideally we should get at least six hours a night, if not more) and jet lag make us feel bad. But with these tight controls in place, Buxton and his colleagues could study, in precise detail, what effect changes in sleep have on the body.
The results were dramatic.
All participants were healthy at the start of the study. At the end, three participants were pre-diabetic, and "everybody else was headed in that direction," Buxton said. Overall, the participants' resting metabolic rates dropped 8 percent. "Assuming no changes in activity or food intake," the study concludes, that "would translate into ~12.5 pounds increase in weight over a single year."
In our conversation, Buxton was certain: "All things being equal, if you disrupt the circadian clock — whether they’re younger or older, men or women — you can in many of them induce a pre-diabetic state, or they'll be at least well on the way to the pre-diabetic state, in three weeks."
Why disrupting sleep disrupts our health
Buxton's participants grew unhealthy because their circadian rhythms were constantly out of step with their schedules.
The circadian system is the body's internal clock. It keeps time so that our bodies can anticipate our daily actions. For instance: Our bodies expect us to eat a meal after waking up, so we produce the most insulin in the morning. We're primed to metabolize breakfast before even taking a bite. It's more efficient that way. And the circadian system doesn't just prime us for food or sleep. Circadian rhythms regulate many of our organ systems and even the expression of our genes. In animal studies, it's been found that throwing off sleep patterns can cause toxins to accumulate in individual cells.
A circadian misalignment — jet lag — means you're eating when you should be fasting, You're trying to sleep when you should be awake. That burdens the body, and could, in theory, be making us sick.
Buxton's test was one of many studies seeking evidence to confirm that sleep behaviors can have negative health impacts. Over the past few decades, evidence has mounted that poor sleep — either getting too few hours, sleeping on an inconsistent schedule, or waking up too often during the night — is a risk factor for heart disease, diabetes, and obesity. (Sleep may play a role in maintaining the wide health gaps between white and black Americans.)
Should we not sleep in on the weekends?
Buxton's study examined extreme situations that most of us are unlikely ever to face. But there's some evidence that the results apply to our everyday lives.
"If you’ve read the studies, you know they are pretty extreme, taking people and isolating them," Patricia Wong, a PhD candidate and sleep researcher at the University of Pittsburgh, tells me on a recent phone call. "What we were interested in was getting at that in a non-invasive way."
In November, Wong published results of a study that asked: Was the social jet lag caused by sleeping in on the weekends also associated with poor health?
Wong and her team tracked the sleep patterns of 490 people by sending them home with wrist sleep monitors that estimate time spent sleeping via movement. The participants also had their blood drawn and their health assessed.
The first thing Wong found was obvious: Most participants shifted their sleep schedules a least a little bit on the weekends — staying up later and sleeping in.
And here's the concerning part: The more dramatic the shift in weekend sleep, the more Wong found red flags for health. The people who demonstrated the biggest shifts in weekend sleep had showed more warning signs for obesity, heart disease, and diabetes — even after adjusting for behaviors like exercise, smoking, and alcohol use.
These weekend jet-laggers were more likely to already have greater waist circumference and greater BMI. Higher social jet lag was also associated with insulin resistance — a precursor to diabetes — lower HDL cholesterol (that's the good type), and higher levels of triglycerides. And it wasn't because they were sleeping less. The shift in sleep time alone appeared to explain the difference.
Wong cautions that her work isn't conclusive. It was a snapshot study that can make no causation claims. But her overall conclusion remains in congruence with Buxton's study. "What I think we’re showing here is that there’s some sort of importance about us ideally being able to work, wake, and match up our schedule as best as we can to what we are biologically suited for," Wong says.
That means: We're not biologically suited for shift work. We're also not suited to sleep less than six hours a night. (A very small fraction of the population can get away with less and remain functional, but odds are you are not one of them.)
Different people also have different sleep needs. Scientists are finding evidence that being a morning person or an evening person is a real biological distinction. These are called chronotypes: For people who score high on "eveningness," their bodies work best sleeping later in the day and staying up late; those who score high on "morningness" are the opposite. An evening person forced to wake up at dawn every day for work may live with constant social jet lag.
Wong says she'd like to see an experimental test before doling out recommendations. But there is some other research that suggests we should stick to a regular schedule and resist the urge to sleep in. In 2012, researchers in Germany analyzed a self-report data set of 65,000 Europeans. "One-third of the population represented in our database suffers from 2 hr or more of social jet lag," the study found. "Over and above the impact of sleep duration, social jet lag significantly increased the probability of belonging to the group of overweight participants."
So what should I take away from these studies?
Because this work is still early, scientists are wary of making direct take-home conclusions from their studies. But a bigger picture is forming: Sleep is a cornerstone of your health. And the timing of your sleep may be just as important as the quality of it. "It's better to live on a relatively consistent schedule," Buxton says.
And that may mean if you have the choice between staying out late and sleeping in on the weekend and just going to bed at your usual, boring time — maybe boring is for the best.