Body Clocks, Obesity and Diabetes
Ben - Obesity is a growing problem in the western world, to the extent that it's being dubbed an epidemic. The concern is that it's linked to diabetes, heart disease, stroke, and a wide range of other conditions. But why are obesity rates rising so rapidly? New research suggests that poor sleep may be partly to blame, as Hannah Critchlow has been finding out.
Hannah - Our worlds are becoming increasingly 24/7. About 20% of the working population - so about 1 in every 5 people - work non-standard time, so not your usual 9:00 to 5:00. People are also regularly travelling across multiple time zones for work or pleasure. And on top of this, about 30% of adults report one or more of the symptoms of insomnia, so difficulty getting to sleep or difficulty staying asleep.
At the same time, obesity rates are on the up. Here in the UK, over a quarter of adults are obese and 3 in 10 boys and girls aged 2 to 15 are either overweight or obese.
Diabetes is also on the rise. In the UK, the number of people diagnosed with diabetes has increased from 1.4 million to 2.6 million since 1996 to 2010. So, are these increased problems linked?
A new study by Dr. Orfeu Buxton and his team at Harvard Medical School has uncovered the physiological link between lack of sleep, changing circadian rhythm, diabetes, and weight gain.
Orfeu - What we did was schedule the participants to a 28-hour day. So, each day, their light and dark cycle, their sleep and wake, and their meal timing was shifted to 4 hours later. This would be similar to being on a planet with a 28-hour day. Our internal circadian clock can't synchronise to such a schedule. Imagine the last time you had a large meal at 2:00 in the morning. Your body is just not ready to process that food and our participants experienced that for 3 weeks. That was the circadian disruption.
Also, because night workers and shift workers have difficulty sleeping during the day, we included sleep restriction along with that circadian disruption. So our circadian clock has an alerting signal during the daytime and a sleep signal at night, helping us to be awake and asleep at the usual time. So, most night workers sleep a couple of hours less during the day because of both the endogenously generated rhythm of alertness and sleepiness, and also because during the daytime, there's bright light, more noise, and the phone is always on.
Hannah - So they elongated the circadian rhythm and decreased the amount of sleep that the volunteers were allowed, effectively giving them a harsh version of rotational nightshift work.
Your body receives circadian cues which regulate how you respond to food. Muck about with your circadian clock and you muck about with your metabolism. Your body has evolved to eat during the day and not at night, with dozens of genes and enzymes involved in digestion and metabolism highly expressed during the day. Eat in the middle of the night and these enzymes are not readily available to digest the food and your body is confused. It can't process information to tell you when you're feeling full. Your satiety system is sleepy at night and it can't send the proper signals telling you that you've eaten enough.
Orfeu - We're very interested in what we call ecologically valid assessments of metabolism. So, we used the response to a meal. Everybody has to eat after all. And when you give a controlled and identical meal, we saw that when participants had circadian disruption, their glucose levels go very high and stay high for longer.
Hannah - And is that comparable to having diabetes in some way then?
Orfeu - Well three of our participants out of 21 achieved glucose levels 2 hours after the start of the meal that would meet the clinical criteria for pre-diabetes when they weren't that way at baseline. Fortunately, they recovered after 9 days of sleep at the normal circadian time. When we take in food, our pancreas responds by secreting insulin which promotes the storage of that glucose, and the use of that glucose in peripheral tissues, such as muscle. We saw a 27% decrease in the amount of insulin to the exact same meal when subjects were circadian disrupted.
Hannah - What kind of impact would this have in terms of weight gain?
Orfeu - Our exposure to circadian disruption increased obesity risk is via the change in another parameter called resting metabolic rate. That's our basal metabolism, subtracting out voluntary activity like exercise and so forth. So it's the amount of calories our body uses, just in the normal course of things without the addition of exercise. What we saw was an 8% decrease and while this number sounds small, it's sufficient that if someone had this 8% decrease in basal metabolic rate, and no change in diet or activity levels, they would put on 10 to 12 pounds in a year, taking a normal weight person to obesity within several years.
Hannah - So your results are showing a clear alteration in metabolism and also, glucose levels in those people that have altered circadian rhythms and diminished sleep. What can shift workers do about this?
Orfeu - Night workers have several options. The first is to better adapt their circadian rhythms to the imposed work schedule. So for example, by appropriately timing light and dark, and meals, it's possible to synchronise your circadian rhythms and be eating at a time when your body is prepared for the infusion of that fuel. In addition to adapting your circadian rhythms, the other key strategy is to allow time for recovery. There's chronic sleep restriction associated with night work because the circadian drive for alerting during the day makes it very hard to sleep. So the idea would be to sleep in a silent, dark room with blackout shades, and a cool room.
Ben - Harvard's Orfeu Buxton speaking with Hannah Critchlow. This study he was describing was published very recently in the journal Science Translational Medicine.