Most people know sleep is important. But few know a lack of it can put us at greater risk of heart disease and obesity.
Many of us don’t get enough sleep on a regular basis. It might be due to a sleep disorder, busy social life, new baby, long working hours, shift work or just staying up too late binge-watching Netflix. But not getting enough quality sleep can have significant implications for health.
A growing body of research is beginning to show how habitual inadequate sleep might alter our physiology and lead to the development of chronic disease.
The three main areas of response to sleep deprivation that have been examined are metabolic (processing and using energy from food), immune (protection against disease) and heart function.
To examine how these systems react to sleep deprivation in healthy people, volunteers are recruited to studies that require them to live in a laboratory environment from several days to weeks. Their sleep time is manipulated and access to food and drink, light, temperature, physical activity and social interaction are all controlled.
In these studies, participants may go without sleep for one or several nights (total sleep deprivation) or reduce sleep time for several weeks (partial sleep deprivation) to examine the impacts of changes to sleep duration on metabolic, immune and heart function.
A good deal of research suggests sleep loss impairs glucose metabolism, the process in which sugars from food intake is processed and stored or used to produce energy. Laboratory studies have consistently found short-term sleep loss decreases glucose tolerance and insulin sensitivity in healthy, young, lean adults.
If long-term, these changes to glucose metabolism could increase the risk of obesity and type 2 diabetes. Combine this with the tendency when sleep-deprived to eat comforting foods, which are higher in fat and sugar, and it’s no wonder people who are sleep-deprived find it harder to lose weight than those who are well rested.
What’s more, both total and partial sleep deprivation have also been found to modify the normal daily rhythms of appetite-regulating hormones. Leptin, a hormone that suppresses appetite, and ghrelin, a stomach-derived peptide that stimulates appetite, both change in response to sleep deprivation. When you do not get enough sleep, changes in these appetite-regulating hormones and an increase in food consumption could lead to weight gain and obesity.
These laboratory results have also been found in a large population-based longitudinal study of sleep patterns known as the Wisconsin Sleep Cohort. In this study, participants reported their sleep habits through questionnaires and sleep diaries and provided a blood sample on one morning, prior to eating, to evaluate leptin and ghrelin levels.
In this study, the people sleeping less than eight hours a night (74.4% of the sample) had an increased body mass index (BMI). Habitual short sleep was also associated with low leptin and high ghrelin. Since reduced leptin and elevated ghrelin are likely to increase appetite, this may explain the increased BMI observed and how insufficient sleep could contribute to developing obesity.
Healthy sleep helps to maintain appropriate immune function. Sleep loss may lead to alterations in immune function, resulting in inflammatory disease, an increased risk of cancer and infectious disease.
One night of total sleep deprivation has been found to cause a reduction of natural immune responses. Total sleep deprivation has also been shown to elevate certain inflammatory markers that may lead to insulin resistance, heart disease and osteoporosis.
In one interesting laboratory study, partial sleep deprivation (six nights of only four hours' sleep per night) at the time of a vaccination was found to reduce the number of antibodies by more than 50% ten days after sleep-deprived participants received a flu shot. This shows that adequate sleep is needed for optimal response to infectious disease.
The prevalence of high blood pressure has increased in the last few decades. Over this period, habitual sleep duration has decreased. Recent studies have shown there is a relationship between sleep deprivation and high blood pressure and heart disease.
The Nurses’ Health Study, one of the largest and longest-running studies assessing influences on women’s health, found the risk of developing heart disease was increased in women who slept less than five hours (short sleepers) and more than nine hours (long sleepers).
Some potential reasons for the relationship between decreased sleep duration and heart disease may be sympathetic overactivity (bodily systems involved in the stress response commonly known as the fight-or-flight response), increases in blood pressure, or decreased glucose tolerance.
Another potential mechanism that might link sleep loss and heart disease is through the activation of C-reactive protein, a protein raised in response to inflammation. C-reactive protein is a marker shown to be predictive of poor heart health. It is elevated in healthy adults following both total sleep deprivation and a week of partial sleep deprivation.
There is some evidence that by improving sleep we can reduce the impact of sleep loss and reverse its negative effects. Habitually extending sleep, taking naps and using weekends and days off to “catch up” on sleep may lessen the long-term adverse health impacts of sleep loss.
PhD Student, Centre for Sleep Research, University of South Australia. research focus is the impact of sleep loss and circadian misalignment (particularly night shift work) on metabolism, mood and performance.