By: Robert Melick
Scientists may still be perplexed about the function of sleep, but they do know that it affects disease risk, especially diabetes, hypertension and heart disease. So, how much sleep do we need? What is the “optimal” amount? Well, that is hard to say. Everyone is different and requires different amounts of sleep, right? Indeed. But there appears to be a range of sleep where below that range or above that range, the chance of developing one of the previously mentioned diseases increases.
Let me preface this report by mentioning the major problems associated with assessing amounts of sleep and relating it to disease risk. Scientists like to prove cause and effect, A causes B. That is why they love the simplicity of using animals in a controlled lab with the ability to control for multiple variables.Unable to prove cause and effect, we are unable to say with certainty if A causes B or if B causes A. Scientists don’t like that.
How does that relates to sleep studies. If someone sleeps 14 hours a night and you find out they are also depressed, is it the depression causing the person to sleep so long or is the person sleeping so long causing depression or is it something else unaccounted for? Scientists are able to adjust for variables and use other methods to find a relationship but this is still not perfect and leaves scientists unable to say with certainty what the cause and effect relationship is.
That is not to say that these types of observational studies (we are observing and not changing or controlling anything like in a lab) are without benefits. We can still connect amount of sleep to disease risk but must interpret these relationships with caution. Having said all that, we can talk about the science behind why too little or too much sleep would cause increased rates of disease. There may be specific chemical or physiological changes induced from sleeping alterations. These in turn may affect heart rate, blood pressure, sugar metabolism, etc and lead to disease. Onto the science!
What the studies say
One of the major reasons scientists believe sleep alterations can negatively affect health is because when we sleep our brains release a variety of chemicals, some associated with appetite suppression, decreasing blood pressure and releasing anabolic growth hormones (1). Over the past few decades the duration of sleep in adults and adolescents has decreased by 1.5-2 hours a night (1).
There are various sleep stages we go through nightly. Scientists believe that the most restorative sleep for us is slow wave sleep. A majority of the slow wave sleep occurs within the first 3 hours of sleep (1). There are a few important physiological processes that only occur in slow wave sleep. There is a reduction in heart rate, blood pressure and sympathetic nervous system, major modulating effects on endocrine release and release of growth hormones (1).
Interesting fact: observed for each 10 year increase in age was a 28 minute decrease in sleep duration. The older we get the less sleep we need. As overall amount of sleep is decreased so is slow wave sleep resulting in reduced release of growth hormone each night (1). Sleep seems to be a major player in controlling blood sugar (glucose) levels, and continuous partial sleep deprivation has resulted in detrimental effects in carbohydrate metabolism (1).
A sleep debt study consisting of 11 young men compared their glucose metabolism after a period of sleep deprivation (4 hrs a night) to a period of normal sleep (8 hrs per night) (1-10). During the period of sleep restriction, the subjects had impaired glucose tolerance, significant reductions in their insulin response to sugar (a bad thing) and in glucose effectiveness, compared to the period of normal sleep (1-10). A study among seemingly healthy young adults undergoing suppression of slow wave sleep, but not overall sleep duration, showed that this led to a decrease in insulin sensitivity, reduced glucose tolerance and an increased risk of developing type 2 diabetes (1-13).
The role of sleep duration in appetite regulation involves production or inhibition of a few chemicals. Insulin and leptin decrease our appetite while increasing the amount of calories our body’s burn (2). Conversely, ghrelin is an appetite stimulating hormone (2). Studies have shown that chronic short duration sleep is associated with a decrease in leptin levels and an increase in ghrelin levels, which combined caused an increased appetite (2). A study comparing two nights of short sleep (4 hours) compared to two nights of long sleep (10 hours) showed that after sleep restriction there was a significant decrease in leptin levels along with a significant increase in average ghrelin levels, compared with long sleep (2-21).
Sleep and Obesity
Does obesity cause health problems reducing nightly sleep duration? Sure. But the previous discussion showing an increase in appetite from increased ghrelin levels observed in short sleep lends some credence to the link between short sleep duration and increased body mass index that has been observed in many studies. In actuality, it probably works both ways and it is hard to separate the two.
Are the declining sleep duration and the greatly increased obesity levels seen in the past few decades related? Some believe so, and there are many studies backing up that idea. I believe if it is a factor, it is a small factor. Although small, it should still be taken seriously as it is an easily modifiable risk factor for obesity. Obviously, sleeping less means more time awake and hence more food eaten throughout the day.
Studies from cultures around the world have found associations between sleep duration and mortality risk, although there were differences in the optimal amount of sleep duration. In Gifu for instance, researchers found that only short sleep (< 7 hours) increased mortality risk compared to those sleeping 7-8.9 hours per night (3). A study of an elderly population in Ohgimi found that only short sleep duration (< 6 hours) was associated with a higher mortality risk compared to those sleeping more than 7 hours (3). I can name countless other similar studies but I do not want to bore you.
Researchers measure a marker of cardiovascular disease called C-reactive protein (CPR) to determine a person’s risk of cardiovascular disease (3). Those exposed to sleep deprivation have elevated CPR concentrations and markers of inflammation (3).
Why does poor sleep increase mortality? Scientists showed that sleep deprivation in rats led to death within two weeks (3). Short duration sleep may not directly affect mortality risk, but indirectly affect something in the body that leads to increased mortality. Those exposed to short sleep duration, (< 6 hours) usually perform worse on neurobehavioral tests such as the Psychomotor Vigilance Task not to mention they have shorter attention spans (4). Anyone that has stayed up all night or got insufficient sleep for a few days knows how this can affect mood, patience, attention and makes one extremely clumsy and more prone to accidents, especially car accidents.
I often speak with various coworkers, of all different ages, who get by sleeping only 5 or less hours a night. This may not be the case for some people, but I believe often times our brain/body merely “adjusts” to short sleep duration and we believe we are functioning just fine but in reality we are not. I can personally attest to this effect. I have had weeks where I slept for 5 or 6 hours a night (I usually get a bit under 9 hours). After getting 5 or 6 hours a night my body got used to it and I thought, “hey maybe I can get by on 6 hours, that would be great,” but come weekend I was sleeping 10 or more hours.
Oddly enough, oversleeping also leaves me tired and grumpy. I guess one must find that perfect amount of sleep, not too little and not too much, since either one of those can leave one feeling less than optimal. Well, it may not be so odd after all. It has been found that when adults sleep longer than normal, they have certain feelings such as lethargy, irritability, fatigue, and sleepiness lasting for hours after awakening. They also feel sleepy which leads to them sleeping even more the next night or napping during the day.
Well, that is it for the effect of sleep on disease risk. Studies have shown that short and long duration sleep increase various disease risk. Even so I still remain skeptical because there are so many confounding variables that cannot be properly accounted for. Mental disorders, sleep apnea, snoring, inaccurate sleep recall, stress, and quality of sleep all play a major role in increasing or decreasing ones need for sleep and I believe that long or short sleep is usually a manifestation of some underlying disorder. Each person is slightly different, so find the right amount of sleep for yourself where you are not tired all day and you are able to fall asleep at night and stick with that. I would not get too hung up on the exact hours of sleep you get each night.
- Vancauter, E, K Spiegel, E Tasali, and R Leproult. “Metabolic Consequences Of Sleep And Sleep Loss.” Sleep Medicine 9 (2008): S23-S28. Print
- Gangwisch, J. E.. “Short Sleep Duration as a Risk Factor for Hypertension: Analyses of the First National Health and Nutrition Examination Survey.” Hypertension 47.5 (2006): 833-839. Print
- Grandner, Michael A., Lauren Hale, Melisa Moore, and Nirav P. Patel. “Mortality associated with short sleep duration: The evidence, the possible mechanisms, and the future.” Sleep Medicine Reviews 14.3 (2010): 191-203. Print.
- Grandner, Michael A., Nirav P. Patel, Philip R. Gehrman, Michael L. Perlis, and Allan I. Pack. “Problems associated with short sleep: Bridging the gap between laboratory and epidemiological studies.” Sleep Medicine Reviews 14.4 (2010): 239-247. Print.