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Why do we sleep? How do we measure it? How much is normal and what are the effects if you don't get enough?!


Our understanding of the science of sleep mirrors our understanding of the human brain: we are only just scratching the surface in what we currently know. Yet we know enough to know “how we sleep” affects every aspect of our lives and performance on the sporting field.


It took for me to have children (who didn’t sleep at night!!), to really appreciate the impact of poor quality sleep on cognitive function, immune ability and physical performance! This article will examine the impact that sleep deficit has on performance and injury, with a future article discussing how to improve sleep.

Why do we sleep?

It was once thought that the brain simply “shuts-down” during sleep, yet we know that there are much more complex activities happening. One way to explain what occurs is “brain housekeeping”: the brain performs checks, repairs and cleaning toxins that build up during the day [1]. In addition to this, good sleep promotes tissue repair, immune function and hormonal and metabolic re-balancing [2, 3].


How do we measure sleep? 
There are continued advancements in technology for quantifying sleep quality. Currently the gold standard is a sleep study conducted in the lab via polysomnography which measures multiple variables including brain waves (EEG), breathing, heart rate and rhythm, muscle activity, eye movement and blood oxygen saturation. 

One could argue, however, that sleeping in a lab connected to multiple machines might not give an accurate result when compared to the individuals “natural” environment [4]. Enter the portable sleep monitoring devices. With the rise of popularity in these devices (FitBit, Oura ring, ReadiBand) and phone apps, the question has to be raised on their accuracy. Most devices are similar in that they are based on the same type of sensor (accelerometer and/or noise). Ultimately, sleep is best tracked through polysomnography, but these devices offer much more accessible and convenient options for casual users to track their activity. Multiple studies [5-8] conclude that wrist monitors can be quite accurate when it comes to estimating information such as total time asleep, sleep percentage, and how long after sleep waking occurs.


It has been my personal experience with sleep monitoring on both myself and my patients, that if you wake in the morning and you feel tired, you haven’t had ideal sleep quality/quantity. Whilst it’s nice to be able to put an objective measure on everything, sometimes subjective measures can be just as useful, as previously discussed in Saw et al’s study on training response [9]. At a basic level, recording when you went to bed, when you woke, and subjectively measuring how rested you feel in the morning can give you a good indication of your sleep quality.

How much is normal?
The National Sleep Foundation’s recommendation for young adults (18-25 years) and adults (26-64 years) is between 7-9 hours. From the research, we see that athletes commonly average below 7 hours [10]. A recent questionnaire based study found that 50% of highly trained and elite athletes were identified as “poor” sleepers, with 28% experiencing daytime sleepiness [11].
For athletic individuals, sleep is paramount when considering the training response, which requires a balance between stress, fatigue and recovery[12]. Therefore athletes should look to promote recovery which would then decrease their stress/fatigue state. Sleep is surprisingly often overlooked by athletes themselves as a cause of fatigue [13]. Poor sleep quantity and/or poor quality appears to exist in many athletic populations, which can be due to a combination of scheduling of training sessions and competition, travel fatigue, and impaired sleep-onset due to increased arousal post competition.

Effects on injury and performance
We previously briefly mentioned the benefits of sleep, but what about the detrimental effects of decreased sleep?



  • Milewski et al’s study on adolescent athletes found those who sleep on average less than 8 hours per night have 1.7 times greater risk of being injured than those who slept more than 8 hours [14]. 

  • In another study on elite adolescent athletes, von Rosen et al found that sleeping more than 8 hours during a week reduced risk of injury by 61% [15].



  • A range of metabolic and immunologic process are negatively affected by poor sleep [16].

  • Reduced sleep quantity was associated with increased incidence of illness within the next 7 days in competitive male AFL players [17]

  • Those who sleep less than 5 hours are 4.5 times more likely to suffer a cold vs those who sleep 7 hours [18].



  • Reductions in motor and cognitive performance, reaction times, and mood state/emotional stability are often observed in sleep-deprived athletes [19].

  • Poor sleep quality, particularly during high training loads and competition periods, has been identified as a marker of under recovery and an early sign of overreaching [20]. 

  • Brandt et al’s study on Brazilian athletes revealed a relationship between poor sleep quality and lost matches during a competition[21].

  • Silva et al found a correlation between sleep duration and competition performance in elite gymnasts [22].

  • Napping improves running performance in those endurance runners that had less than 7 hours sleep [23].


This information should hopefully highlight to you the importance of sleep, and perhaps get you examining your own sleeping quality and quantity!


In our next installment, we will discuss what can be done in those with sub-optimal sleeping habits..... click here

References available on request

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