The Science of Sleep – Part 2

Posted on December 20, 2017 at 7:11 am

More on the benefits of sleep

In Part 1 we looked at the relationship between sleep and sport performance.

Considering the scientific support, sleep is paramount for long-term athletic success.

Now we will investigate the importance of sleep for general health and longevity. We will also discuss the testing procedures commonly adopted by coaches and sports scientists to identify sleep issues in athletes.

Snoozing your way to good health  

Sleep is a major determinant of health and wellbeing.

Sleep deprivation is linked to many metabolic and mental illnesses due to the “destabilisation” of the autonomic nervous system. Imbalances in the nervous system leads to altered cardiac, inflammatory and immune responses, negative changes to hunger hormones and decreased insulin sensitivity (1).

Cognitive function also decreases with accumulated sleep loss, with learning ability disrupted by decreases in response time, decision making and concentration (2).

The health issues that arise from sleep deprivation contribute to morbidity and mortality risk, and have massive impacts on an individual’s socioeconomic status and overall quality of life (2,3).

Heart health

Epidemiological research shows a clear connection between sleep and heart health…

Coronary artery disease, congestive heart failure and hypertension are all associated with sleep deprivation. Research shows 50% of individuals suffering from congestive heart failure, 80% of resistant hypertension patients and over 50% individuals admitted to hospital with a heart attack suffer from a severe sleep disorder (1,3). In fact, studies have shown resting arterial blood pressure and heart rate increases after just one night of sleep restriction in otherwise healthy individuals (1,4,5).

Why do these problems occur?

Lack of sleep increases the sympathetic nervous system activity responsible for our natural “flight or fight” response. Sleep loss is therefore an additional stress on the body, and If untreated, can lead to endothelial cell dysfunction (1,5). Dysfunction of these cells means that blood vessels have a reduced capacity to vasodilate, leading to excessive vasoconstriction. Over time, constriction of blood vessels causes atherosclerosis (the narrowing of blood vessels) and arterial plaque accumulation, greatly increasing the risk of cardiovascular disease (6).

Research shows that getting adequate sleep increases parasympathetic activity, reduces stress responses and allows our blood vessels to stay nice and flexible, greatly reducing the risk of heart disease!

Metabolic disease

Obesity and type 2 diabetes are risk factors for those not getting enough shut-eye…

Sleep loss increases diabetes risk through elevated sympathetic activity, activation of inflammatory pathways and changes in adipocyte profiles (7). In a nutshell, chronic sleep deprivation reduces insulin sensitivity and glucose tolerance, leading to long-term insulin resistance. When combined with a poor diet and low exercise, this causes increased fat storage in the liver, contributing to the build-up of dangerous visceral fat located around the belly (8). Visceral fat accumulation and high insulin levels are major risk factors for developing type-2 diabetes and cardiovascular disease (7,8).

Fat gain from sleep loss is also caused by deregulation of the “leptin-ghrelin’ system (1).  The hormone ghrelin is responsible for inducing feelings of hunger and is opposed by leptin which promotes satiety. Sleep deprived individuals experience elevations in ghrelin combined with suppression of leptin secretion, leading to poor nutrition compliance and a hyper-caloric food intake.

Put simply, when you don’t get enough sleep, you’re more likely to eat high calorie food and lots of it!

Reductions in blood testosterone to cortisol ratios, decreased amino acids and slowed metabolism, all contribute to fat gain and muscle loss in sleep deprived individuals (1,7,9). Fat gain from sleep debt can cause obstructive sleep disorders, such as sleep apnoea, due to fat storage around the neck closing airways during sleep cycles. This can initiate a vicious cycle of sleep loss causing weight gain, causing further sleep loss (8).

Fortunately, good sleep health reduces the risk of diabetes and obesity through increasing insulin sensitivity and stabilising hunger hormones. Proper completion of the sleep cycles and ensuring 8-9 hours per night maximises growth hormone, inhibits of cortisol secretion and reduces sympathetic activity, promoting metabolic health and longevity (1,7).


There is a big link between sleep loss and immune system suppression…

Chronic sleep loss increases catecholamine (adrenaline) and cortisol production, which supresses immune function, reduces white blood cell numbers and increases susceptibility to infection. Sleep deprivation decreases natural “killer” secretion and lowers B and T-cell production, leading to attenuated antibody response to infection and immunisations (1,10).

Wilder-Smith’s 2013 study of sleep loss and immunity found that one night of sleep deprivation significantly increases susceptibility to upper respiratory tract infections (10). Some studies even show next-day increases in IL-6 and tumour necrosis factor (pro-inflammatory cytokines) after one-night of sleep restriction. Elevation of these markers suggest a potential link between sleep loss and more serious inflammatory diseases, as well as cancer (1,9).

Athletes are especially susceptible to immune suppression caused by poor sleep (9). Hard training sessions cause tissue damage which elevates white blood cells – not a problem if cortisol secretion is controlled through proper sleep recovery. However, sleep loss increases cortisol concentrations, supressing the immune system (10, 11). This means that in poorly slept athletes, the immune system is both highly stressed (due to muscle damage) and supressed (due to excess cortisol secretion), resulting in an inability to recover to baseline levels of fitness (overtraining syndrome) and a high risk of infection.

Identifying poor sleep patterns  

Now that we understand the importance of getting enough sleep, let’s touch on how to test for sleep debt in athletes. The main testing tools for determining sleep debt include:

  • Psychological assessment tools
  • Blood analysis
  • Salivary tests
  • Heart rate variability

Sports scientists agree that the most effective and non-invasive way to determine fatigue is through psychological assessment (9,12). A simple, verbal assessment of how an athlete is feeling, their mood and motivation gives insight into potential stressors contributing to fatigue, such as sleep loss. Common assessment tools include The Profile of Mood States (POMS) and The Recovery-Stress Questionnaire for Athletes (RESTQ-Sport) (12).

Heart rate variability can also indicate sleep deprivation. Variable gaps between heart beats as well as an elevated resting heart rate indicates under-recovery and potential sleep issues in athletes, and can be assessed through ECG analysis and heart rate monitoring. (9,12).

A more thorough investigation of sleep debt can be achieved through blood and saliva testing. These tests can show anabolic hormone concentration, cortisol and indicators of fatigue such as white blood cell count. In a clinical practice, biochemical profiles, thyroid function tests and C reactive protein analysis can also be ordered to identify potential sleep issues (13).

Note: Sleep deprivation could be caused by a serious sleep disorder. If an athlete complains of tiredness despite several nights of good sleep, they should see a doctor for a check-up and sleep study.

Up next

In Part 3 of this series on sleep, we will look at practical recommendations for sleep schedules in athletes and how to perfect the pre-bed routine.  

Reference –


  1. Tobaldini, E., Costantino, G., Solbiati, M., Cogliati, C., Kara, T., Nobili, L. and Montano, N. (2017). Sleep, sleep deprivation, autonomic nervous system and cardiovascular diseases. Neuroscience & Biobehavioral Reviews, 74, pp.321-329.


  1. Halbach, M., Spann, C. and Egan, G. (2003). Effect of sleep deprivation on medical resident and student cognitive function: A prospective study. American Journal of Obstetrics and Gynecology, 188(5), pp.1198-1201.


  1. Lee, C.H., Khoo, S.M., Chan, M.Y., Wong, H.B., Low, A.F., Phua, Q.H., Richards, A.M., Tan, H.C., Yeo, T.C., 2011. Severe obstructive sleep apnea and outcomes following myocardial infarction. J. Clin. Sleep Med. 7 (6), 616–621.


  1. Zhong, X., Hilton, H.J., Gates, G.J., Jelic, S., Stern, Y., Bartels, M.N., Demeersman, R.E., Basner, R.C., 2005. Increased sympathetic and decreased parasympathetic cardiovascular modulation in normal humans with acute sleep deprivation. J. Appl. Physiol. 98 (6), 2024–2032.


  1. Sunbul, M., Kanar, B.G., Durmus, E., Kivrak, T., Sari, I., 2014. Acute sleep deprivation is associated with increased arterial stiffness in healthy young adults. Sleep Breath. 18 (1), 215–220.


  1. Kohansieh, M. and Makaryus, A. (2015). Sleep Deficiency and Deprivation Leading to Cardiovascular Disease. International Journal of Hypertension, 2015, pp.1-5.


  1. Al-Abri, M., Jaju, D., Al-Sinani, S., Al-Mamari, A., Albarwani, S., Al-Resadi, K., Bayoumi, R., Hassan, M. and Al-Hashmi, K. (2016). Habitual Sleep Deprivation is Associated with Type 2 Diabetes: A Case-Control Study. Oman Medical Journal, 31(6), pp.399-403.


  1. Wood PA. How Fat Works, Chapters 5 and 16. Harvard University Press, Cambridge MA, 2006.
  2. Fullagar, H., Skorski, S., Duffield, R., Hammes, D., Coutts, A. and Meyer, T. (2014). Sleep and Athletic Performance: The Effects of Sleep Loss on Exercise Performance, and Physiological and Cognitive Responses to Exercise. Sports Medicine, 45(2), pp.161-186.


  1. Wilder-Smith, A., Mustafa, F., Earnest, A., Gen, L. and MacAry, P. (2013). Impact of partial sleep deprivation on immune markers. Sleep Medicine, 14(10), pp.1031-1034.



  1. Joo, E., Yoon, C., Koo, D., Kim, D. and Hong, S. (2012). Adverse Effects of 24 Hours of Sleep Deprivation on Cognition and Stress Hormones. Journal of Clinical Neurology, 8(2), p.146.


  1. Kellmann, M. (2010). Preventing overtraining in athletes in high-intensity sports and stress/recovery monitoring. Scandinavian Journal of Medicine & Science in Sports, 20, pp.95-102.



  1. Fallon, K. (2006). Clinical utility of blood tests in elite athletes with short term fatigue ,Commentary, British Journal of Sports Medicine, 40(6), pp.541-544.

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