Sleep has become a hot topic of late, particularly in regards to the athlete setting, as more and more practitioners and athletes look to gain further performance advantages over the competition.
Personally, sleep is a topic I hold dearly, due to my focus of study during my doctorate degree at Liverpool John Moores University (LJMU) and seems a fitting topic for a blog post category. We often hear of sleep being one of the “BIG TWO” for optimal athlete recovery and performance (the other of course being nutrition). Previously during my time at LJMU a number of great colleagues (whose primary research focus was exercise physiology and sport nutrition) often quipped that “Carbohydrates are King” within athlete performance. This simple yet effective slogan summarizes a number of research projects linking carbohydrates to increased exercise performance. During that time it got me thinking, well of course carbs are the King, I’m just surprised they never said that protein is the Queen (not as catchy I guess). So in an attempt to justify my own research, I decided that “Sleep is the (K)night in shining armor! ” (I was very proud of this pun at the time). As such, the concept as to why sleep is our knight in shining armor will be the topic of this blog post. Herein, I am going to discuss some important background information surrounding sleep and some general thoughts around the narrative of sleep in relation to athlete populations.
BACKGROUND ON SLEEP
It is no secret that sleep is important for each and every one of us. Afterall, it is well reported that the “average” person sleeps roughly ⅓ of their life (or at least for the most part this time is spent sleeping or attempting to sleep) per the National Sleep Foundation. The recurrent behavior of sleep suggests that it is a basic human need (simply put; as an adaptive species we wouldn’t do it if we didn’t need to!). Consequently this has led to numerous research studies to better understand the fundamentals of sleep. With sleep ideology being an interesting one, as we understand “The What” pretty well. Though this has only allowed us to scratch the surface on the complex theory surrounding hy humans sleep. Without turning this post into a full blown scientific review (i wrote one of those once before, for anyone who is interested, you can read that chapter within my phd thesis). For those who want the condensed version, below is a brief summary of the process and characteristics of sleep:
From the Figure above we can see that there are two proposed control processes of sleep. There is the Circadian component (visualized as the clock above, known as process C in the scientific literature) and then the Homeostatic component (visualized as the hourglass above, known as process S in the scientific literature (Borbely et al., 2016). Both these processes are key for the regulation of the sleep-wake cycle and alertness about the 24-h day.
Most individuals will likely be more familiar with the hourglass concept. For example; as an athlete, think of a long / hard two-a-day of training (or alternatively a hard day of work for coaches / others). You come home a little later, eat some dinner and likely slump on the couch. At that moment you would probably describe yourself as “drained” “tired” “exhausted” or some other derivative and really feel the need to rest and likely could fall asleep at any second. Conceptually the reason why, would be that during your day the additional physical/mental exertion has acted to “fill” the hourglass. The more this hourglass fills the more need there is to “empty it”, tiredness ensues and the pressure to sleep increases. Sleep acts as the counterbalance, tipping the hourglass and allowing it to empty. Consequently we wake up ready for the next day of action. Another way to view the concept of the hourglass is by staying awake later than normal. A good example of this would be a child fighting sleep as they try so hard to stay up past their typical bedtime, no matter how hard they battle they will eventually pass out. The longer we are awake the more we fill the hourglass and again the more pressure there is to sleep. While this is one simple explanation of why we sleep, this doesn’t consider the complexity of sleep regulation and is a good time to introduce the circadian clock component of sleep.
The circadian (circa: about, dies: days) process of sleep is best represented as a clock. Primarily, this process is controlled through our body’s master internal clock in the brain (the suprachiasmatic nucleus), which acts to regulate our alertness/sleepiness around the 24-hr day. This clock is influenced by environmental cues known as zeitgebers (time givers). One of the main influencers being sunlight (naturally distinguishing daytime and nighttime), which typically prompts sleep and wake (Borbely et al., 2016). In most people we are more alert during the hours of daylight, as the day continues, and get more tired as light diminishes, the sun sets and nighttime begins. With nighttime signifying time to sleep for most people (with the exception of night shift workers). Several other factors during the 24-hr day can also influence the timings of this internal clock, such as; genetics, meal timings, physical activity, social endeavors, temperature, changes in posture and of course other sources of light (think TV before bed etc…). Simply this system is regulated by each component intricately combining together in an attempt to maintain a scheduled rhythm of wakefulness and sleep.
The easiest example to relate to how the circadian clock influences sleep is when you go on vacation and cross several time zones. When we land at the destination, oftentimes on that first day we will experience wakefulness at hours we would typically sleep and be sleepy in hours we would typically be awake. I’m sure everyone who has travelled through time zones is familiar with the concept of jetlag. This concept is a good one to demonstrate the circadian component of sleep, as this is when the circadian process becomes disrupted (as the new time zone does not align with your typical circadian schedule at home and all the previous time givers that came before). It typically takes a couple of days for our brain/body to establish a new circadian rhythm in relation to the new time zone and then we start to feel back to our normal selves (There will be more on Travel & Sleep in a later post). The simplest way I think of the circadian process is as our typical daily routine. We normally have a set wake up time, set meal times, a time of the day we exercise, times where we get exposure to light (walking outside or from indoor lighting) and a scheduled time we go to bed (in most cases). Though a very simplistic way to think of it, all of the things in our typical schedule act to regulate the circadian processes and when we break routine, we may disrupt this (additionally this could affect the hourglass negatively too, as both processes work in tandem).
A quick mention should also be given to chronotype in relation to circadian rhythms. There are typically two main chronotypes described relating to sleep: the morningness type (lark) and the eveningness type (night owl). The Lark is an individual who typically prefers earlier bedtimes and earlier awakenings, and a night owl prefers the opposite, going to bed later and awakening later in the morning. There are some individuals who may be intermediate in a sense that they may have qualities of both types upon assessment. Typically an individual will gravitate to their default chronotype and it has been suggested that this is how individuals, like athletes, typically excel in their respective sports (Lastella et al., 2016; as their sport may align better with the athlete’s chronotype, i.e. morning chronotypes are mostly found in sports that typically have a morning schedule such as triathlon and cycling). In light of this, the next section will look to explore sleep in relation to athletes.
SLEEP & THE ATHLETE
Over the past 20 years we have seen an exponential growth in research relating to sleep and the athlete (as seen in the search image below; Pubmed, 2021 at the date this was posted). Athletes and those surrounding the athlete performance setting have acquired an increased interest in sleep (in an attempt to squeeze the best out of performance) and therefore this is reflected in attempts to further our understanding of sleep and athletic performance.
When I first started my literature search on the sleep of athletes, it seemed that there was one recurring theme; that athletes don’t get enough sleep and this is bad. Now bear in mind that athlete refers to an individual engaged in sporting activity at a high level and so that generalizes a lot of factors relating to participation in different sports, training and competitions. But despite the generalization, you might be wondering why do athletes sleep so bad? Well the simple answer is for the most part they don’t (at least from my experience), but maybe some do, it as always depends. One research paper I found intriguing back in 2012 was a new study (for the year) that was conducted by Leeder and Colleagues (2012). They observed a group of Olympic athletes in relation to non-athlete controls and in turn (Leeder et al., 2012) showed that sleep differences occurred ( ) when using wrist watch actigraphy (* see Side Note 1 for my thoughts on actigraphy).
Similar sleep durations have also been reported within other athletes have also been reported (See Table Below). This acted as a form of inspiration for my own research project that examined the differences between youth soccer players and non-athlete controls. We actually observed the athletes slept more than the non-athlete controls. Therefore adding a layer of complexity, again suggesting not every athlete sleeps badly. When purely comparing these studies, one might say it’s simply because of the differences in sports or the sleep technology used. In part I would agree this will account for some of the differences observed.
Table 1. Brief overview of sleep studies conducted in athlete populations | |||
Study (sleep monitoring method) | Participants / Sport | Sleep Duration (h ± sd) | Sleep Efficiency (% ± sd) |
Lastella et al., 2012 (Self Reported Questionnaire) | Marathon Runners (Female N=63; Male N=40) | 5.51 ± 1.25 (Pre-Competition) !! | NA |
Lastella et al., 2014 (Actigraphy) | Elite Athletes (Individual Sports; Male N=57, Female N=9. Team Sports; Male N=104, Female =20) | 6.7 ± 0.9 (Cycling) ! 7.0 ± 0.7 (Mountain Bike) * 7.1 ± 1.0 (Racewalker) * 6.4 ± 1.5 (Swimming) ! 6.1 ± 0.9 (Triathlon) ! 6.7 ± 1.2 (AFL) ! 7.5 ± 1.0 (Basketball) * 6.9 ± 1.5 (Rugby U) ! 6.9 ± 1.5 (Soccer) ! | 86.5 ± 5.5 83.7 ± 5.4 91.1 ± 5.7 84.4 ± 6.4 83.8 ± 4.1 85.0 ± 4.9 88.1 ± 3.8 87.3 ± 5.2 86.7 ± 4.2 |
Leeder et al., 2012 (Actigraphy) | Elite GB National Olympic Sport Athletes (N=46) | 6.55 ± 0.43 (Average) ! 6.58 ± 0.23 (Canoeing) ! 7.05 ± 0.47 (Diving) ** 6.25 ± 0.50 (Rowing) ! 7.06 ± 0.38 (Speed Skating) * | 80.6 ± 6.4 81.8 ± 4.3 80.9 ± 5.3 82.5 ± 8.3 77.2 ± 7.1 |
Miller et al., 2017 (Actigraphy) | Elite Team Sports Male Athletes (N=51) | 6.8 ± 1.2 (AFL) ! 7.2 ± 1.6 (Rugby U) * 6.7 ± 2.0 (Soccer) ! | NA |
Richmond et al., 2007 (Actigraphy) | Elite AFL Male Athletes (N=19) | 8.85 ± 0.06 (Baseline) ** 9.68 ± NA (Home Game) ** 9.53 ± NA (Away Game) ** | 92 93 92 |
Roach et al., 2013 (Actigraphy) | Elite Male Australian National Youth Soccer Players U-17 (N=14) Elite Male Bolivian Youth Soccer Players U-20 (N=12) | 7.0 ± 0.5 (Sea Level) * 6.5 ± 0.7 (Altitude) ! 5.7 ± 0.7 (Sea Level) ! 6.4 ± 1.0 (Altitude) ! | 80 ± 3 75 ± 5 78 ± 6 80 ± 6 |
Robey et al., 2014 (Actigraphy) | Elite Male Australian Youth Soccer Players U19 (N=12) | 7.13 ± 0.39 (Home) * 7.35 ± 0.56 (Train) * 7.42 ± 1.01 (Train+CWI) * | 89.4 ± 5.8 90.4 ± 4.5 88.9 ± 8.1 |
Whitworth-Turner et al., 2017 (Wireless EEG Monitor) | Elite Youth Soccer Players U20 (N=12) | 8.1 ± 0.55 (Train) ** | 93 ± 3 |
! Below NSF Recommended Sleep duration * Within NSF Recommended Sleep duration, But below Athlete recommended 8h ** Above NSF & Athlete Recommended Sleep duration | |||
Let us consider some observations from the studies above. Firstly of the studies selected the majority on average are either below or at the range of 7hrs sleep recommended by the national sleep sleep foundation NSF (again this could be in part to do with measurement error of the device used to monitor sleep). Within the Leeder & colleagues study for example we see a between subject standard deviation (on average) of 43 minutes for sleep duration. This means each individual athlete varies about ~43 minutes in total sleep duration in comparison to their peers. Lastella & Colleagues showed roughly 80 minutes variation on average. Robey & Colleagues showed roughly 40 minutes. This means that some of the athletes are almost definitely observed to sleep less than recommended durations than the NSF (<7-8hrs sleep) and therefore likely require some attention. We showed a variation of 33 minutes between the youth soccer players using an alternative sleep monitoring device in comparison to the aforementioned studies. What was interesting about this study is that each of the youth soccer players lived in the same residency, which eliminated to an extent, factors such as different home environments and travel to the training facility (which may account for some of the variation observed in the other studies). So even in the same environment under the same sporting constraints individuals still differ from one another when it comes to the amount of sleep they attain. At this point you may be thinking this is an obvious point and we didn’t need research to tell us this. But as simple as this sounds it is often a point that is missed as we may clump athletes as “good or bad sleepers” based on aggregated group data and then embark on a quest to improve the sleep of individuals who may not necessarily need it.
This leads to another interesting observation. The intraindividual variation of the youth soccer players (this means how much one individual varied from night to night) was around 1 hour of sleep duration from the 6 nights observed. Therefore, suggesting that some factor or combination of factors is affecting sleep either positively or negatively for specific individuals throughout a typical training week (which excluded competition). This duration could be considered a significant amount of sleep gain or sleep loss (particularly when it comes to maintaining a habitual sleeping pattern). This is where I believe more attention should be focused. Again to some of you this may be stating the obvious. Those individuals who have the most variation in regards to their sleeping patterns, often have the greater number of reasons regarding why? – But it doesn’t always mean they are a bad sleeper either, that’s the beauty of sleep ideology – likely more questions – likely more monitoring – likely some fresh new insights, but it may not always be problematic.
Describing sleep is an important step for identifying those individuals who may not be optimizing their sleep process. However, description typically only offers the insight to the WHAT, not the WHY. Guiding this process should be an understanding of the factors that can impact sleep, so this can lead to more in-depth analysis as to what may be the reasons why for that specific individual.
FACTORS AFFECTING SLEEP
You don’t have to try very hard to find some research surrounding the factors that may affect the sleep of athletes on PUBMED or Google Scholar. There is a nice recent narrative review conducted by Walsh et al., (2021), which contains a good summary of some of the key details pertaining to sleep in athletes and the factors affecting sleep (whereby they split these factors that disrupt sleep into sporting and non-sporting contexts). Recognizing this work and the work of others, primarily I like to categorize these factors as two primary groups; Behavioral and Physiological. The sub-categories being internal and external (which can include both sporting and non-sporting contexts). An overview of some of the primary factors affecting athletes can be seen in Figure 3. Some of these factors could be considered both behavioral and physiological (as denoted by example). For example, The choice to drink a coffee could be considered a behavioral one, but the effects of caffeine can be considered a physiological one.
UNDERSTANDING THE BEHAVIORAL FACTORS:
When first looking at the behavioral factors there are two key subsets. I like to describe these as external and internal (though other names can exist). The external factors refer to those that influence the individual indirectly and are not controlled by the individual personally. The internal factors are those more so made through choices of the individual and as such are controllable by the individual. Within these sub categories there are both sporting and non-sporting factors which can also be categorized (Walsh et al., 2021).
One example of a sport related external behavioral factor we see is the timing of training. Given that this is typically scheduled by the sport coach or performance coach, the athlete may have no control (or little say) on this imposed schedule. Particularly one scenario that gains a lot of talk is early morning training. Whereby an athlete potentially has to wake up at 4-5am to make their 6am training session. Aside from the numerous twitter rants we see about impacting athlete recovery with early training like this, it has been documented that scheduling of training is an important influence on sleep duration (Sargent et al., 2014; Whitworth-Turner et al., 2018). I do, however, quickly recognize that for some individuals like our morning lark chronotypes (as described earlier) this may actually be preferential to them and they wouldn’t have any problems being awake and ready to train at that time anyways (individual sport athletes like swimmers or running based sports spring to mind; Lastella et al., 2016). I also recognize that in some environments (like the collegiate setting) scheduling is at the mercy of facility availability, academic class commitments, study hall and sports calendar and therefore there may not be enough hours in the day to simply schedule a normal training time. However, this factor is easily accounted for within the right setting.
Though the timing of training may also get particularly tricky in team sports settings (whereby you may have a large varied group of individuals with their own preferred wake up schedule and distance that they have to travel to get to the training facility, which may impact what time they have to wake up). However, within an ideal scenario, with all things considered, the later the training session can begin, the more likely that every individual can get enough time in bed to achieve their desired sleep durations (Whitworth-Turner et al., 2018). For those who are comfortable with early training let them have at it. Though I do like to think of it as if there is no need to go earlier then why choose to and give everyone ample opportunity to sleep. This can sometimes be met with resistance (for example: a coach may suggest that the later you start training the more likely the athletes will go out the night before). It’s a catch 22 situation, on one side does earlier training dissuade those athletes from going out or does it just mean even less time to sleep when they do choose to go out anyway. Simply in such a scenario educating them around sleep and why the training is later and trying to establish trust is better than some enforced restrictions in the hope they go to bed earlier (in my opinion and you don’t screw the genuine nightowls on your team).
A non-sport related external behavior factor could be something family related. Think here of a new born baby. This is a sure way for any new parent to have their sleeping patterns interrupted. Opportunities to sleep become lessened due to the additional care required for the infant. Not every athlete will have kids. But for those who do, it’s important to understand that their sleep in this case may not be impacted by choice, but is impacted by circumstance. Therefore the education around this would also change, it no longer becomes here how you sleep more at night, but here is how you create opportunities to sleep and still perform (e.g. optimizing napping or the use of ergogenic aids). This becomes tricky as it is harder to control external behavioral factors, but there still are strategies to limit the impact even if this is out of the person’s control.
This leads into the next focus which is internal behavioral factors (which for the most part should be the easiest to control). Anyone who has worked within sport probably knows the internal behavioral factors of athletes oh so well. If there is one thing we know about sleep, it’s that typically it’s an individual’s behavioral choices that tend to be the most disruptive. Whether that’s staying up late playing video games, watching tv, or socialising (to name a few). For an athlete sleep scheduling is often an afterthought (unfortunately). But it’s fairly easy to understand why, being in an environment that necessitates high performance functioning all day, everyday, sometimes year round, an athlete is still only human and therefore activities away from sport need to be prioritised too (which means sleep might be the thing that has to give a little). It must start first understanding the athlete, and then educating them towards their own sleep habits (controlling the controllable). This is why athlete education around sleep has become more mainstream. For example, one study by O’Donnell & Driller (2017) investigated the effects of one sleep hygiene education session within elite female netball athletes. Though most of the positive effects were small, it would appear that the athletes attempted to go to sleep earlier, wake up later and therefore get a little extra total sleep time (Side Note*). This would indicate a positive impact of sleep education (albeit a small one).
I like this study as it attempted to take a strategy and measure its effectiveness. We move away from describing sleep and try to make an impact. It also shows a glimpse into the importance of internal behavior change. If athletes are more aware, they may be more empowered to make changes and prioritise things like sleep (like changing their routine to prioritise an earlier bedtime). Again this takes the sleep of athletes in a positive direction. However, even if sleep is improved, we may not fully know if it positively or negatively affects sport performance.
UNDERSTANDING THE PHYSIOLOGICAL FACTORS
The physiological factors for the most part I believe are far less understood when it comes to an athlete’s sleep (applied sleep research is already pretty hard to do, without the complexity that is added by layers of physiology related to exercise and sport). Conceptually I still think of the physiological factors as external and internal. There could be some semantics around this classification (for example we cannot control the timing of the sun, which is an external source of light, but that light would have an internal impact on the rhythm related to melatonin secretion).
An important external physiological factor is temperature. If a room is too hot or cold then this may interrupt sleep. Therefore optimizing the environment (recommended 60 – 68℉) is also an important step in sleep hygiene education. This can also be coupled with an internal physiological factor that can be considered (body temperature). Thermoregulatory rhythms typically match that of the circadian process previously shown above. That is, our core temperature typically decreases later in the evening and is higher during the daytime, which is one of the sensory processes that promotes differences between wake and sleep behavior. Previously we have used a practical strategy to accelerate this temperature rhythm and improve sleep latency of youth soccer players (Whitworth-Turner et al., 2017). Using a hot shower timed before bedtime, we are able to manipulate skin temperature (and consequently heat loss) to promote potential reductions in core temperature and in turn promote a sleep rhythm. This can be a simple yet effective strategy that we all can do prior to bedtime. The relationship between external temperature and internal temperature is one that we must consider when understanding sleep
There has been suggestion from myself and others (Knufinke et al., 2018; Whitworth-Turner et al., 2018) that things like exercise training (external physiological factors like volume and intensity) may impact sleep. But also we have shown that in some athletes, sleep isn’t affected by the type of exercise intensity when performed in the evening . When we circle back to that hourglass concept and the homeostatic pressure to sleep, one may suggest that increasing energy expenditure (i.e. through increased exercise training) would increase the need for sleep. (Thomas et al., 2020). However, this is only one process of many that can influence sleep and therefore is likely a reason why we see mixed results in the research when considering how exercise training, volume, intensity and even timing affects sleep. Simply put, we need more controlled research in this area to begin to understand this more clearly, but we have an inclination that it plays some factor – how much weighting though remains to be determined.
SUMMARY
It is important that we acknowledge how sleep is regulated and how behavioral and physiological factors may affect sleep. We can then begin to control some of these factors in an attempt to identify what impacts sleep most in individuals such as athletes. That way we can begin to create strategies to help improve sleep. Meanwhile, as is mostly the case in science, more research studies are needed. We certainly have a better understanding of sleep now than we ever did in the past two decades. Much of what we have learnt has been translated to expert consensus statements, which help guide our understanding of sleep. As we continue to expand our knowledge of sleep, the more we may see optimized sleep strategies for improved performance in all populations. For now we still must trust in the principle that sleep is important and should be a priority. Despite not necessarily knowing the intricacy of why sleep is so important and impactful.
References
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