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.
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. 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, 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. 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. 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. There is much more that we do not know about sleep than what we do know.
According to Carskadon and Dement, “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 much 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.
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.
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. 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. 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 studies 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. 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.
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. Similarly, in a 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. 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.