Research Review: Should Your Doctor Prescribe Naps?
A few months ago I was channel surfing and ran across a TV program on the Discovery Channel. Initially, it seemed to be a documentary on some sort of torture. It started with a scientist in a white lab coat talking to a reporter while looking at various monitors. Then the scientist points to the monitor and says, “He just entered into slow-wave sleep.” She presses a button. The scene switches to a sleeping young man with electrodes attached to his forehead and temples. Suddenly a short loud horn blares into the room. The young man doesn’t wake up but his sleep is clearly disturbed –- his brows furrow and he shifts his body. Back to the scientist: she points out to the journalist that the young man is out of slow-wave sleep now.
So every time this poor guy gets into slow-wave sleep the scientist presses the button to trigger the “suppression of slow-wave sleep” device (aka loud horn). (Hmm… maybe my neighbour is running a sleep deprivation study with me as the subject. Anyway, back to the study.) The next morning the young man is interviewed. He’s asked if he slept through the night and if he felt rested.
“I slept through the night, but for some reason I’m exhausted,” he replies.
This suppression of slow-wave sleep continues for two more days. Each day this poor man is more exhausted and more perplexed. As far as he knows he has been sleeping eight hours a night without any disruptions, but for some reason he’s exhausted.
What were the scientists trying to figure out? Certainly, frequent burst of a loud horn while you’re trying to sleep would disturb your sleep regardless of whether you consciously noticed it. So I didn’t think that “Loud noises disturb sleep!” was going to be the big finding.
Nope. The big finding was that lack of this specific type of sleep (slow wave sleep) messes up glucose regulation. Thus, not only can the usual culprits of candy, cake and cola disrupt proper glucose regulation, but all those nights of restless sleep have an impact too.
Sleep
Before we any further, let’s go through the different parts or stages of sleep. Sleep is grouped into either rapid-eye-movement (REM) sleep or non-REM (NREM) sleep. NREM sleep is sub-divided into four stages conveniently named stages 1, 2, 3 and 4. Stages 3 and 4 are deeper stages of sleep, also called slow-wave sleep, and considered the most restorative.1 But for the most part, even though we know sleep is important, the specifics are still pretty fuzzy.
Eventually the young man got to go home and sleep undisturbed. I wondered if he ever found out why he felt so exhausted. I also wanted to know more about the study, so I went on PubMed and found the original article that the TV program depicted. And this is the article I’ll be reviewing this week:
Tasali E, Leproult R, Ehrmann DA, Van Cauter E. Slow-wave sleep and the risk of type 2 diabetes in humans. Proc Natl Acad Sci U S A. 2008 Jan 22;105(3):1044-9. Epub 2008 Jan 2.
Methods
I’ve already given you an idea of how the study was run, but here are some more details.
In total there were nine volunteers for this study: five males and four females, ages 20-31, with lean BMIs (19-24 kg/m2.)
All the volunteers were screened for normal sleep patterns and were excluded if they smoked or took any medications. Shift workers and anybody who had travelled across time zones within the previous month were excluded.
Glucose tolerance
All the volunteers had normal glucose tolerance as tested by an oral glucose tolerance test.
What is an oral glucose tolerance test? You start by fasting for eight to twelve hours (no food or drink). A fasted blood sample is taken. Then you drink a ridiculously sweet syrupy-type drink and your blood is sampled again one, two, and sometimes three hours after your syrup drink. You think I joke about the syrup… I’ve done it once and all I can say if you think soda pop is sweet, then you’ll probably have trouble getting this stuff down. How much glucose is in this drink? 75 grams! Glurk!
Once collected, the blood samples are tested for glucose and checked to see if they are in the normal ranges.
Normal glucose levels are 60-100 mg/dL for the fasting sample, less than 200 mg/dL one hour after the glucose challenge and less than 140 mg/dL after two hours.
If your glucose levels are greater than 200 mg/dL, there is a good chance you have diabetes. If your levels are between 140-200 mg/dL then you would be considered pre-diabetic.
Suppression of slow-wave sleep
Each participant had two consecutive baseline nights of sleep where they were not disturbed and three nights of consecutive suppression of slow-wave nights of sleep. Which occurred first was randomized and each round of testing was four weeks apart.
On the mornings of the last night the volunteers underwent intravenous tests for glucose tolerance and insulin levels.
Results
When slow-wave-deprived the volunteers had a 25% decrease in insulin sensitivity! This is big. After only three days of slow-wave deprivation the volunteers had decreases in insulin sensitivity that rivalled levels reported for people at risk for diabetes.2
Not only was insulin sensitivity decreased, but insulin didn’t increase, which usually occurs to compensate for decreases in sensitivity. The disposition index, a measure that factors in both insulin sensitivity and insulin levels, also decreased by 20%. In case you’re wondering, disposition index is calculated as:
Disposition index = insulin sensitivity x insulin levels
Since the volunteers weren’t as sensitive to insulin and didn’t make more insulin, it’s not a surprise that glucose tolerance was impaired by 23% when they were sleep deprived.
Conclusion
Everybody knows that when they don’t sleep they feel fuzzy, have difficulty remembering and thinking clearly. Now take this one step further: even if you sleep eight hours a night, but wake feeling exhausted, it’s just as bad as if you’re sleep deprived. So it’s not only about quantity but quality.
Still, the suggestion that lack of sleep could lead to diabetes might pretty far-fetched. What’s clear with this study is that sleep quality can affect your glucose metabolism. As you age you get less and less slow-wave sleep, which could compound the problem and increase your diabetes risk.3
One other thing to ponder is that most obese individuals have sleep-disordered breathing that leads to reduced sleep quality and yes, that means less slow-wave sleep.4 Thus, less slow-wave sleep is just another factor that increases the risk of type 2 diabetes in obese individuals.
I guess this is good news! You can sleep and decrease your chance of diabetes. How easy is that? Usually, some new finding comes out and you need to exercise, avoid or eat a certain food or avoid some environmental toxin, but with this all you need to do is take a nap.
References
1. Marshall L, Helgadóttir H, Mölle M, Born J. Boosting slow oscillations during sleep potentiates memory.
Nature. 2006 Nov 30;444(7119):610-3. Epub 2006 Nov 5.
2. Bergman RN. Lilly lecture 1989. Toward physiological understanding of glucose tolerance. Minimal-model approach. Diabetes. 1989 Dec;38(12):1512-27. Review
3. Van Cauter E, Leproult R, Plat L. Age-related changes in slow wave sleep and REM sleep and relationship with growth hormone and cortisol levels in healthy men. JAMA. 2000 Aug 16;284(7):861-8.
4. Young T, Peppard PE, Taheri S. Excess weight and sleep-disordered breathing. J Appl Physiol. 2005 Oct;99(4):1592-9.