The purpose of this blog is to provide a succinct, reader friendly overview of sleep and its importance for rehabilitation, especially as it relates to pain, performance, and injury. You can read it from start to finish or choose the sections that are relevant to your interests.

What is sleep?

“Sleep is a reversible behavioral state of perceptual disengagement from and unresponsiveness to the environment” and is “typically accompanied by postural recumbence, behavioral quiescence, closed eyes, and all the other indicators one commonly associates with sleeping”, as described by Carskadon and Dement [1]. Sleep is further categorized into two distinct states that characteristically alternate throughout the night - rapid eye movement (REM) and non-REM (NREM) [1]. It’s a natural, recurring physiological phenomenon that accounts for roughly ⅓ of our entire lives. We sleep, we wake, and we repeat this process indefinitely. Why is sleep important?

With sleep occupying such a large portion of our time, we can assume that it plays an integral role in human function and should be prioritized on a daily basis. However, data actually suggests a decline in sleep duration over the past 30 years with problems likely to worsen in the future due to our more recent lifestyle and behavioral changes [2]. Insomnia problems present in up to 30% of adults worldwide and 10% may have chronic insomnia [2]. Although sleep has been an area of interest for thousands of years, it wasn’t until more recently that its functions have been better elucidated [2]. When we look to animal models, we see that total sleep deprivation for two to three weeks in rats is actually fatal [3]. Luckily, this experiment has not been reproduced in humans. However, the average sleep duration in the United States is 7.18 hours and roughly ⅓ of the population sleeps less than 6 hours per night [4]. While not fatal, short sleep is associated with a 12% increase in mortality [4,5]. Additionally, individuals who sleep less than six hours per night demonstrate a significantly higher likelihood to have diabetes, cardiovascular disease, and obesity [4]. Research has also indicated a correlation between long sleep duration (greater than 8 or 9 hours) and mortality, stroke, diabetes, cardiovascular disease, obesity, and coronary heart disease [5,6]. Hold on, though! Don’t start cutting hours off your sleep just yet. These results might be confounded by demographics, social correlates, and health correlates such as low level of physical activity, depressive symptoms, and other conditions [5,7]. Therefore, this association might be the result of an illness or some other health-related factor, rather than the cause of it. These less healthy individuals may also report greater sleep than what is actually measured by actigraphy or polysomnography (gold standard for measuring sleep) to provide the expected response [7]. Sleep on! So what is it about sleep that makes it so important? Well, it’s believed to aide in the facilitation of learning and memory, energy conservation, the synthesis of important macromolecules such as proteins and cholesterol, and the recovery of countless bodily processes [3]. An even better understanding of sleep has come by way of how a lack of it can negatively impact immune function, tissue healing, cardiovascular health, depression and anxiety, and a host of other conditions [8]. Sleep restriction also negatively impacts neurocognitive functioning such as sustained attention, executive functioning, including working memory, and long-term memory [9]. Purposely foregoing the opportunity to sleep in favor of work is often seen as a badge of honor. “I’ll sleep when I’m dead” or “No rest for the weary” come to mind. And if you sleep in without the use of an alarm? You’re lazy. Look no further than the scientist who created the light bulb to rescue us from the perils of darkness. “The man who sleeps eight or ten hours a night is never fully awake—he has only different degrees of doze throughout the twenty-four hours. We are always hearing people talk about ‘loss of sleep’ as a calamity. They better call it loss of time, vitality and opportunity.” - Thomas Edison. But Edison didn’t know what we know now. And he surely couldn’t have predicted accidents like Chernobyl or the loss of the space shuttle Challenger may have been avoided if there was just a little more sleep to go around [10]. Let’s dive a little deeper. What is the relationship between sleep and pain?

It is well documented in the literature that pain and sleep issues have a bidirectional relationship. Over ⅔ of people living with chronic pain present with sleep complaints and at least ½ of individuals with insomnia are living with persistent pain [11]. This presents us with the classic “chicken or egg?” scenario. In our case, which one came first: persistent pain or sleep disturbances? Well, more recent longitudinal studies have led us to believe that sleep disturbances are actually a stronger predictor of pain than pain is of sleep disturbances [11,12]. Insomnia is defined as “a frequent and persistent difficulty initiating sleep, waking up during the night with difficulty returning to sleep, or waking up too early, despite having adequate time and circumstances for sleep” [12]. Insomnia, or insomnia symptoms, are inextricably associated with migraines, tension-type headaches, fibromyalgia, and other forms of chronic musculoskeletal pain [11]. Sleep disturbances increase the likelihood of developing chronic pain in pain free individuals (including back pain), worsen pain outcomes and physical functioning over time, influence daily changes in pain, and may lead to a worse prognosis in individuals with existing pain [11-14]. On the other hand, persistently good sleep can be a predictor of being symptom free and lead to improvements in self reported health outcomes such as as the physical component summary (PCS) of the short form health survey-36 (SF-36) [12,13]. Even individuals with knee osteoarthritis who slept 6 hours or more per night demonstrated a better clinical pain response at 6 month follow-up compared to their counterparts who slept less than 6 hours per night [12]. A survey study published by Dunietz and colleagues in 2018 examined the relationship between insomnia symptoms, pain, and anxiety symptoms over a 3 year period in American adults aged 65 and older. Not surprisingly, participants who had difficulty with sleep initiation and maintenance at baseline had 24% and 28% higher odds of incident pain 2 years later [15]. What was surprising however, is that anxiety symptoms mediated the pathway between sleep disturbances and the future experiences of pain by up to 17%. Therefore, insomnia symptoms may contribute to new onset pain directly and indirectly, secondary to anxiety [15]. Another longitudinal study conducted by Generaal and colleagues in 2017 surveyed 1860 Dutch subjects over the course of 6 years to determine the association between insomnia, sleep duration, chronic multisite musculoskeletal pain, and depressive symptoms. Short sleep duration and insomnia were associated with baseline depressive symptoms and predictive of chronic pain onset [16]. Similar to anxiety symptoms, depressive symptoms accounted for 40% and 26% of the relationship between short sleep duration and insomnia and chronic pain [16]. Consequently, sleep disturbances may contribute to the development of chronic widespread pain directly and indirectly, through the pathway of depression [16]. The exact mechanisms as to how sleep disturbances may increase an individual’s sensitivity to pain is unclear. Since dopamine plays a role in pleasure and arousal, dysfunction in dopaminergic signaling is one hypothesis put forward by Finan et al [11]. Alterations in opioidergic signaling may also play a role as it relates to our ability to modulate pain through descending inhibitory pathways [11]. Other theories include increased inflammatory profiles and changes in arousal or mood, that could potentially contribute to things like pain catastrophization [11,12,15,16]. What we clearly know is that insomnia really hurts [17]. If you get a bad night of sleep, you are more likely to experience pain the next day. Likewise, if you have a bad day of pain, you are more likely to suffer during your sleep. However, the relationship is stronger for sleep affecting pain than pain affecting sleep [17]. Luckily, if you have a good day of sleep, you are more likely to have less pain the next day and vice versa. Unfortunately, the more severe and habitual your insomnia symptoms, the less likely you’ll reap the positive benefits and the more likely you’ll sow the negative consequences [17]. Suffice it to say, sleep should be an important consideration during the rehabilitation of acute and chronic painful conditions. What is the relationship between sleep and performance?

What would you be willing to do or sacrifice for a 10% improvement in performance? Wake up at the crack of dawn to “outwork the competition”? That might not be the best idea. In 2009, Oliver et al. studied the effects of 30 hours of total sleep deprivation on a self-paced endurance test within individuals. Compared to their baseline testing, participants had a 2.9% decrement in running performance (measured in meters) when asked to forego sleep [18]. Similarly, Temesi and colleagues in 2013 tested fairly experienced endurance sport athletes on a cycling test to task failure. Compared to the group who was allowed normal sleep, the group undergoing total sleep deprivation showed impaired endurance by roughly 8% (1137s ± 253s to task failure vs 1236s ± 282s) [19]. Lastly, Reilly and Piercy revealed that restricting sleep to 3 hours per night for two nights reduced maximal lifting capabilities on a bench press, deadlift, and leg press. Submaximal lifts were even more affected, and included the biceps curl [20]. Perhaps the most pertinent takeaway from these articles is that despite showing a reduction in performance for running, cycling, and weightlifting, the ratings of perceived exertion between conditions were the same for all participants [18-20]. In order to lift, run, or cycle until failure, motivation and determination must be high. It seems these might be negatively impacted by a reduction in sleep. Furthermore, load monitoring and management strategies that utilize RPE-based protocols for injury risk reduction and performance enhancement are becoming more commonplace. Surely these training considerations would be affected as well. The greatest effects of sleep on performance were arguably demonstrated by Mah et al. in 2011. No sleep deprivation took place, though. Instead, the researchers had athletes on the Stanford University men’s varsity basketball team increase their total sleep time per night by nearly 2 hours on average for 5-7 weeks with a minimum goal of 10 hours in bed per night [21]. What happened? Times significantly decreased from 16.2 seconds to 15.5 seconds during a 282 feet sprint (baseline to half-court and back, then to full-court and back) [21]. Additionally, free throw and 3-point field goal accuracy increased from 79% and 68% to 88% and 77% [21]. If that’s not a slam dunk already, daytime sleepiness, mood, and subjective ratings of mental and physical well-being improved [21]. These athletes were able to improve performance significantly by finding time during their busy athletic and academic schedules to increase their opportunity for sleep. Mah and colleagues also recently demonstrated the effects of sleep restriction on maximal jump performance and joint coordination in elite cyclists with an average age of 28. Prior to restricting their sleep by 3 hours per night for 3 nights (6.7 hours/night to 3.7 hours/night measured via actigraphy), the athletes were tested on a maximal vertical jump from a box drop with 3D motion capture to assess physical performance and biomechanical changes. They were also tested using the Psychomotor Vigilance Task to assess changes in response time. The authors found that maximum vertical jump height decreased, psychomotor response time slowed, and joint coordination variability at the hip and knee increased as a result of the change in cognitive performance [22]. More important than just a worsening in performance, it’s possible that these alterations in movement patterns during dynamic tasks may influence non-contact injuries seen in sports such as ACL tears. If you want to be the best, it might take a lot of hard work and sleep. What is the relationship between sleep and injury?

As important as sleep is for pain and performance, can it actually prevent injuries? Well, nothing can completely prevent injuries, but we can try to systematically reduce the likelihood of them by optimizing various aspects of training and recovery. In this regard, sleep is an extremely low hanging fruit. A study by Milewski et al. in 2014 demonstrated that middle school and high school athletes (ages 12-18) who slept less than 8 hours per night on average had a 1.7 greater risk of being injured than those who slept more than 8 hours per night [23]. Two more studies by von Rosen and colleagues in 2017 confirmed this same relationship between sleep and injury in elite teenage athletes [24,25]. The players who slept more than 8 hours during the week reduced their risk of injury by 61% [24]. If an athlete had an increase in training load and training intensity, as well as a decrease in overall sleep, risk for injury increased 2-fold [25]. This is not an uncommon practice for elite athletes during the competitive season. Surgery and rehabilitation can cost athletes thousands of dollars and months of time. Imagine being able to cut the cost of both for free by simply prioritizing 8 hours of sleep per night? Does sleep affect my diet and overall body composition?

What does diet have to do with rehabilitation? Well, short sleep duration, insomnia, and sleep disordered breathing such as obstructive sleep apnea are associated with Type 2 Diabetes, high blood pressure, and obesity [26,27]. Depending on the severity, medications taken, and various other factors, these could respectably affect healing, injury, and performance. Datillo et al. in 2011 hypothesized that increasing sleep debt would hinder muscle recovery and favor muscle loss secondary to decreases in anabolic hormones such as testosterone, growth hormone, and IGF-1, and simultaneous increases in catabolic hormones like myostatin and cortisol [28]. A study by Nedeltcheva and colleagues in 2010 seemed to somewhat confirm that theory. Participants were placed on a moderate calorie restriction for 14 days with either the ability to sleep 8.5 or 5.5 hours per night. Both groups lost roughly 6.6 pounds, but the group with less sleep increased the loss of fat-free body mass by 60% and decreased weight lost as fat by 55% [29]. A recent study by Ness et al. also demonstrated that restricting sleep to 5 hours per night for 5 nights reduced insulin sensitivity without a compensatory increase in acute insulin response to glucose, and this was not reversed after 2 nights of recovery sleep for 10 hours [30]. The combination of these three studies highlight how sleep can significantly affect diet and recovery. Furthermore, participants in the study by Nedeltcheva who were restricted to 5.5 hours of sleep per night showed increases in hunger and higher concentrations of acylated ghrelin [29]. Have you ever been on a diet and given in to that cookie or pizza craving? Ghrelin might have been a contributing factor since it stimulates hunger and food intake [29]. This is an important consideration for rehabilitation because the foods you reach for, and subsequently eat, may play a role in inflammation and your sensitivity to pain [31]. Sleep well. Eat well. Recover well. What are tips to improve sleep?

Okay, so now you know you should be sleeping. What are ways to actually help you sleep? Hopefully this goes without saying, but this is not medical advice. If you have a condition affecting your sleep quantity or quality, please seek a licensed healthcare provider. With that being said, here are some tips from the National Sleep Foundation. Stick to a sleep schedule. Whether it’s a weekday or weekend, try to wake up and go to sleep at regular times. Create a habit for yourself and your body. Avoid alcohol or cigarettes prior to sleeping. Although alcohol can help you fall asleep, it can negatively affect sleep quality and quantity. Similarly, nicotine in cigarettes can impair your ability to fall asleep and stay asleep. Avoid caffeine 8 hours before bed. Caffeine takes a long time to clear from your system so it’s probably best to avoid energy drinks, coffee, or soda after your post lunch pick-me-up. Avoid large meals and excess fluid intake 2-3 hours before snoozing. Avoid naps late in the day. If you have a hard time falling asleep at night, it may be useful to eliminate your midday naps. Create the right environment. Make sure your room is cool, dark, and free from electronics. Minimize cell phone, computer, and TV usage 1-2 hours before bed. Develop a relaxing routine. A hot shower, writing things down in a journal, or reading a book might help you relax and wind down. Spend time in the sun. Aim for at least 30-60 minutes, especially in the morning. Save your bed for sleep and sex only. If you can’t sleep at night, don’t just stay in bed. Find something to help you relax in a different part of your home. Exercise daily (more on that in the next section!) What is the relationship between sleep and exercise?

Without a doubt, exercise improves sleep quality and duration. Although the exact mechanisms are unknown, it has been suggested that the “beneficial effects of regular exercise on sleep may be explained by multiple pathways with the interaction of circadian rhythm, metabolic, immune, thermoregulatory, vascular, mood and endocrine effects” [32]. Whether exercise is acute (single bout or less than 1 week in duration) or regular (greater than 1 week in duration), it seems to positively impact slow wave sleep (deep sleep), total sleep time, sleep onset latency (how long it takes to fall asleep), awakenings after sleep onset (how often you wake up), sleep efficiency, and symptoms associated with obstructive sleep apnea [33-38]. As far as the parameters go, higher intensity, frequency, and duration on a regular basis may be more effective [35,39]. However, there is likely a dose-response relationship with too little providing a negligible effect and too much potentially hampering sleep as we might see in elite athletes, although travel, time zone changes, performance anxiety, and other factors may play a role [39,40]. Aerobic and anaerobic exercise, including resistance training, demonstrate benefits [35,36]. The effects of exercise are not going to be completely uniform between individuals as it relates to sleep, as they will be moderated by fitness levels, age, exercise selection, sport, and other factors [37,40]. It was a long held belief that exercising before bed would negatively impact sleep. Yet, on average, it doesn’t seem to matter that much as long as you aren’t performing vigorous exercise (average HR > 76% of HRmax, oxygen consumption > 63% of VO2 peak, or >69% of 1RM) 2 hours or less before bed [41]. Our recommendation would be to exercise regularly using a method of your preference at a time of day that doesn’t negatively affect your sleep. Some people can get away with exercising 1 hour before bed while others might need to exercise in the afternoon to sleep soundly that night. Find what works well for you. Should I use a sleep tracker?

Quite frankly, they don’t seem to serve much function. Sleep trackers can provide accurate and reliable data with regards to your total sleep duration or how much time you spend in bed, although you probably don’t need an app to tell you that [42,43]. However, if your watch or phone keeps you accountable to sleep for 7-9 hours per night, we’re all for it. On the other hand, sleep trackers don’t offer any reliable information as it relates to your sleep quality [42,43]. The research also shows that being assigned sleep quality (i.e. being told your quality was good or bad) can actually significantly impact your cognitive functioning [44]. So if your sleep tracker is telling you that you slept poorly even though you feel great, we’d recommend listening to your body. What’s the best way to sleep?

We saved this question for last because it is the one shrouded by the most controversy and misinformation - what sleeping position is optimal for pain and function? Unsurprisingly, most people tend to actually sleep in the position of their preference [45]. The favored sleeping posture seems to be sidelying, with our arms and legs in a semi-fetal position [45-47]. As we age, we assume right sidelying more than left, and we begin to minimize the amount of time spent on our stomachs [46]. From the perspective of obstructive sleep apnea, side sleeping is the recommended sleep position as it may normalize the apnea-hypopnea index [48]. Additionally, obstructive events are more severe and frequent in supine [49]. For everyone else? There is no one best sleeping posture. Although we all have a positional preference or two, we move 2-5 times per hour or 32-33 times per night while adopting 7-8 different sleeping postures because there are technically hundreds of options for us to choose from [45,46]. There is also absolutely no correlation between side, prone, or supine sleeping, with or without your arms overhead, and cervical, scapular, thoracic, and/or arm pain or risk for shoulder injury [47,50]. Sleep positions matter sometimes, but it’s completely based on comfort. If a pillow between your legs or under your knees reduces your back pain enough to let you sleep, by all means, keep doing it. And if you just had a surgery like a rotator cuff repair, it might be advisable to stay off that side for awhile. Otherwise, just find what works best for you. Conclusion Is one night of bad sleep automatically going to send you into a downward spiral? No, of course not. We just wanted to highlight the importance of prioritizing sleep as it relates to rehabilitation and your overall health. “If sleep does not serve an absolutely vital function, then it is the biggest mistake the evolutionary process has ever made.” - Allan Rechtschaffen