Wednesday, January 06, 2010
Image by studentofrhythm.
In the past, I have mentioned that physical activity and sleep time are positively related - the more physical activity you perform, the more sleep you are likely to get. Now most of these past studies have been observational, so we have a bit of a chicken and egg problem. By that I mean that we don't know whether:
A) Sleep deprivation causes reductions in physical activity,
B) High levels of physical activity make people sleepier, or
C) Some combination of A & B
Fortunately, an interesting experiment has just been published in the American Journal of Clinical Nutrition, which may help to untangle the cause and effect in this relationship.
This new study was performed by Sebastian Schmid and colleagues, and I will warn you up front that the methods were a bit complicated. I will do my best to explain them, but if you'd rather just take my word for it, skip the next paragraph.
In this study, 15 healthy young men experienced 2 conditions: 2 consecutive nights with 8 hours of sleep, or 2 consecutive nights of 4 hours of sleep. Otherwise, both conditions were identical. Half the men did the 4 hour condition first while half the men did the 8 hour condition first, and the two conditions were separated by at least 6 weeks for all subjects. After the first night in both conditions, the subjects were allowed to live a normal day, with the sole instructions being that they follow their usual eating habits, and avoid intense exercise. After the second night, the subjects spent a day in a controlled laboratory setting, so that researchers could see how much food each participant ate from a series of buffets, as well as changes in appetite-related hormones. Physical activity was directly measured using accelerometers, which is the gold-standard for measuring physical activity in this type of study.
So, what did the authors find? Not surprisingly, physical activity was significantly lower following 4 hours of sleep as compared to 8 hours of sleep. In addition to an overall reduction in physical activity following sleep deprivation, there was also a significant reduction in the proportion of the day spent in high intensity physical activity, and a greater proportion of the day spent in low intensity physical activity (unfortunately the amount of time engaging in sedentary behaviours is not reported, although one would assume that it increased following sleep deprivation). Surprisingly, the authors did not find any effect of sleep deprivation on appetite-related hormones, which is in contrast with past research that has shown sleep deprivation to result in increases in both appetite-hormones and feelings of hunger. There are of course some limitations (two that jump to mind: all subjects were healthy young men so we don't know if the findings will translate to overweight and obese individuals, and we have no idea how sleep-deprived these individuals were prior to the study) but the results are certainly interesting nonetheless.
So, what's the take-home message?
This is the first experiment to suggest that a reduction in sleep time can cause a reduction in physical activity. This means that if you or your kids are hoping to live an active lifestyle, it's in your best interest to get a good night's sleep. I don't think these results will shock anyone - when I'm tired, all I really want to do is sit down and relax. It's important to remember that the amount of time watching TV, having a TV in the bedroom, and watching TV before bedtime, are all associated with sleep problems in children. So if you want your kids to get a good night's sleep followed by plenty of physical activity, removing the TV from their bedroom is a great way to start (since screen time also predicts both caloric intake and obesity, this is really a no-brainer). Spending less time around TV's and computers is probably a good first step for most of us adults as well. As Peter mentioned earlier this week, the road to a healthier lifestyle usually involves many small steps, rather than drastic changes. Getting a good night's sleep is a great way to start.
Schmid, S., Hallschmid, M., Jauch-Chara, K., Wilms, B., Benedict, C., Lehnert, H., Born, J., & Schultes, B. (2009). Short-term sleep loss decreases physical activity under free-living conditions but does not increase food intake under time-deprived laboratory conditions in healthy men American Journal of Clinical Nutrition, 90 (6), 1476-1482 DOI: 10.3945/ajcn.2009.27984
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