January 2021

Sleep Series Part I: Basics of Sleep and Its Role with Injury

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Sleep – that magical state where our bodies recuperate, our minds rejuvenate, and we are free of the everyday stresses of the world – is a necessary physiological process that all humans require. This article is the first of a series that will explore the need for sleep and its many restorative benefits.

By Jeremy R. Hawkins, PhD, LAT, ATC; Michael Reeder, DO; and Alli Powell, DAT, LAT, ATC

When discussing performance, performance enhancement, recovery from injury and general wellness, sleep has become as prominent of a topic as proper hydration and nutrition. Professional and college sports teams now have “sleep pods” and professional athletes like LeBron James and Roger Federer reportedly get upwards of 12 hours of sleep each night.1 Is this what we all should be getting?  And what of the more recent discussions of the impact on sleep of daylight saving time (DST)? Each year on the second Sunday in March, clocks are moved forward one hour and then thirty-four weeks later, on the first Sunday in November, we move our clocks back one hour.2 The American Academy of Sleep Medicine supports abolishing this annual adjustment so that our internal circadian clocks will not be misaligned for 65% of the year.3 The impact of DST has been evaluated in multiple industries, including healthcare. One example is the recent study of a large health care system which indicates the week after the spring time change is a high-risk period for patient safety related issues.4

Regardless of the type of professional, athlete or health care professional, it is apparent that sleep is an important part of our ability to function optimally. We will begin this sleep series by discussing the basics of sleep, including why we sleep, what it does for the body, and the role of sleep in injury.

Why Do We Sleep?

Until recently, if you were to ask a sleep expert why we sleep, they may reply, “We really don’t know,” as the function of sleep is still being explored.5 Generally it is agreed that sleep has many important functions involving our memory, cognition, and multiple physiological functions. Many experts think that the purpose of sleep is to help the body recover from previous wakefulness and/or prepare for functioning in the subsequent awake period. New developments have built upon this idea,6 but much remains to be understood.

Insight as to why we sleep is also gained by observing what happens when we do not sleep. Three areas of research that illustrate this include loss of sleep and emotional regulation, the effect on cognitive and motor function, and the impact on injury occurrence. With sleep loss, people tend to experience more negative emotions, have less frequent positive emotional experiences, and change the way they understand, express, and modify their emotions. Further, failure to obtain adequate sleep may also place individuals at risk for a variety of psychiatric disorders.7 Similarly, too little sleep hurts cognitive and motor function. In the cognitive realm, multiple studies have looked at how sleep deprivation relates to changes in alertness and performance which results in accidents, increase in healthcare costs, and decreased efficiency and productivity.8 In the military, marksmen demonstrated that the ability to differentiate between friend and foe slowed with less sleep; accuracy in hitting the target decreased as well. Interestingly, the soldiers did not think their performance changed over time.9  In athletics, there are multiple examples of the impact of the lack of sleep including mood changes, altered pain perception, and a decrease in reaction time. The third example, the role sleep deprivation plays with injury, will be discussed later in this paper. Needless to say, there is much more that we do not know about sleep than what we do know.

Sleep Cycles

According to Carskadon and Dement,10 “sleep is a reversible behavioral state of perceptual disengagement from and unresponsiveness to the environment.” Within sleep, two separate states have been identified, rapid eye movement (REM) and non-REM (NREM). NREM is further divided into five stages that progress in depth of sleep. These five stages are:

  • Stage 0 is awake. Characteristics of this stage include open eyes, responsiveness to external stimuli, and the ability to hold an intelligible conversation. Most individuals are in this stage 16 – 18 hours each day.
  • Stages 1 and 2 are considered light sleep. During these stages, an individual transitions between waking and sleep. If awoken, the individual may claim they were never asleep. In stage 1, an individual may experience sudden muscle contractions preceded by the sensation of falling. In stage 2, heart rate slows, and temperature drops in preparation for deep sleep. We spend the majority of the night in these stages of sleep. Because of this, these stages are important for mental and physical restoration.
  • Stages 3 and 4 are considered deep sleep. This is also known as slow-wave sleep or delta sleep. Deep sleep is important for physical recovery and aspects of memory and learning. By spending time in deep sleep, an individual’s sleep drive is decreased. Human growth hormone is released during deep sleep11 and the immune system restores itself.12–15 For those of us who have ever moved a sleeping child, it is because of the deep sleep that they are so easy to move and nearly impossible to wake. Sleepwalking, bedwetting, and night terrors also occur during these stages.

During REM sleep, brain waves are similar to when we are awake, but the body experiences muscle atonia. Some would say we have an activated brain in a paralyzed body. Our most vivid dreams happen in this stage and the muscle atonia is thought to help keep us from acting them out. If you are awakened during REM sleep, you likely will remember your dreams. Other characteristics of REM sleep include increased heart rate, blood pressure, and respiration rate. Finally, REM sleep is important for memory16 and mood.17

An entire sleep cycle (REM plus NREM sleep) takes approximately 90 – 110 minutes to complete. Sleepers usually cycle through all 5 sleep stages, going cyclically from light sleep to deep sleep, back to light, then into REM, though sleep cycles vary naturally. Each stage is thought to last 5 – 15 minutes. Deep sleep tends to predominate early in the night with limited REM. The opposite happens later in the night where we see predominately REM with limited deep sleep. Overall, as stated previously, we spend most of our night in light sleep.

While the ideal amount of sleep each individual requires varies, age-based recommendations from the Sleep Foundation are considered a good rule of thumb (Table, page 41).

*In each group, the guidelines present a recommended range of nightly sleep duration for healthy individuals. In some cases, sleeping an hour more or less than the general range may be acceptable based on a person’s circumstances.

Sleep and Injury

Adequate sleep, as measured by sleep quantity, is related to multiple biological functions, including motor function, mood and cognition, and moderate sleep loss is associated with deteriorating psychomotor performance.18 In the athlete, the effect of sleep on performance has been examined previously, with studies demonstrating improved sports-specific skills with increased sleep1 and decreased cardiovascular performance in cyclists with sleep deprivation as examples.19 These same factors influencing biological functions and performance likely contribute to an increasing injury risk across a number of populations.

In the last decade, three survey based studies20–22 were published that looked specifically at injury rates and amount of sleep in athletes. The studies compared the number of musculoskeletal injuries with average amounts of sleep for adolescent athletes within a school system, a sports institute, and a group of related medical clinics. Recognizing the multifactorial nature of athletic injuries, these studies suggest a link between sleep and injury risk in adolescent athletes (Figure, page 43). Educating athletes, coaches, and parents on the possible relationship between quantity of sleep and injury prevention may reduce the number of musculoskeletal injuries in adolescent athletes. These findings are also supported by a recent systematic review and meta-analysis.23

Injury rates and lack of sleep have also been studied in other populations.  For example, in a study of a construction worker population, risk of accidents increased by 9% when the worker slept less than 8 hours in a night.24 Similarly, in a very large study involving more than 60,000 Korean adolescents, individuals who slept less than 7.5 hours per night had a higher correlation with accidents and falls than those who slept more than 7.5 hours per night.25 Finally, it was observed that on-call healthcare workers had twice as many attentional failures when working overnight than if they were just working a 16-hour shift. These attentional failures resulted in 36% more serious medical errors. Further, they reported making a greater number of fatigue-related medical errors that led to a patient’s death.26


This is the first article in a series on the benefits of sleep. Why we sleep is poorly understood, but there is mounting evidence as to the impact on individuals and society because of inadequate sleep. Each of the five distinct stages of sleep serves a different function and without an individual stage, the entirety of the benefits of sleep are not experienced. Subsequent articles will focus on sleep and performance as well as sleep’s role in healing, and sleep hygiene.

Jeremy R. Hkins, PhD, LAT, ATC, is an associate professor and the Department Head for the Department of Kinesiology, Colorado Mesa University, Grand Junction, Colorado. He also serves as the Program Director for the Master of Science in Athletic Training program. Hawkins routinely conducts therapeutic modality research, focusing on whether common treatment approaches help or hinder the healing process.

Michael Reeder, DO, board-certified in emergency medicine, is Director of the Monfort Family Human Performance Lab, Colorado Mesa University, Grand Junction, Colorado. He is Director of Medical Education for Cycling CME, a provider of continuing medical education for physicians, Physician Assistants, and other healthcare providers. Reeder holds a Certificate of Added Qualifications Fellowship in Primary Care Sports Medicine.

Alli Powell, DAT, LAT, ATC, is an assistant professor in the Department of Kinesiology, Colorado Mesa University, Grand Junction, Colorado. She also serves as the Coordinator of Clinical Education for the Master of Science in Athletic Training program. Powell’s scholarly interests focus on quality patient care through a variety of novel treatment paradigms.

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  2. The Old Farmer’s Almanac. Daylight saving time 2021: when does the time change? Available at https://www.almanac.com/content/when-daylight-saving-time  Accessed Jan. 8, 2021.
  3. Rishi MA, Ahmed O, Barrantes Perez JH, et al. Daylight saving time: an American Academy of Sleep Medicine position statement. J Clin Sleep Med. 2020;16(10):1781-1784.
  4. Kolla BP, Coombes BJ, Morgenthaler TI, Mansukhani MP. Increased patient safety-related incidents following the transition into daylight savings time. J Gen Intern Med. Published online August 12, 2020. doi:10.1007/s11606-020-06090-9
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  9. Smith CD, Cooper AD, Merullo DJ, et al. Sleep restriction and cognitive load affect performance on a simulated marksmanship task. J Sleep Res. 2019;28(3):e12637.
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  15. Dimitrov S, Lange T. Gαs -coupled receptor signaling and sleep regulate integrin activation of human antigen-speci fi c T cells. J Exp Med. 2019;216(3):517-526.
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  17. Get Sleep. Sleep and Mood. Available at  http://healthysleep.med.harvard.edu/need-sleep/whats-in-it-for-you/mood. Accessed Jan. 8, 2021.
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  22. von Rosen P, Frohm A, Kottorp A, Fridén C, Heijne A. Multiple factors explain injury risk in adolescent elite athletes: Applying a biopsychosocial perspective. Scand J Med Sci Sport. 2017;27(12):2059-2069.
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