When thinking of sleep and its relationship to fitness, many think about sleep as being recovery, in terms of energy. Wouldn't you say?
However, sleep is also critical to the building and repairing of muscles. In fact, human growth hormone is secreted during deep sleep. Sleep is, therefore, critical not only because you will have the energy to work out but also because it is critical to building lean muscle mass.
In addition, research has shown that those who are sleep deprived tend to lose more muscle mass than those who are not deprived of sleep. A study that is often cited in relation to this finding was published in The Annals of Internal Medicine (Nedeltcheva, Kilkus, Imperial, Schoeller, & Penev, 2010). Participants were placed on a calorie-restricted diet, with one group of participants' being provided with a sleep opportunity of only 5.5 hours and the other's being provided with a good sleep opportunity of 8.5 hours. The results revealed the following:
Sleep curtailment decreased the proportion of weight lost as fat by 55% (1.4 vs. 0.6 kg with 8.5 vs. 5.5 hours of sleep opportunity, respectively; P = 0.043) and increased the loss of fat-free body mass by 60% (1.5 vs. 2.4 kg; P = 0.002). This was accompanied by markers of enhanced neuroendocrine adaptation to caloric restriction, increased hunger, and a shift in relative substrate utilization toward oxidation of less fat. (Nedeltcheva et al., 2010)
https://www.acpjournals.org/doi/10.7326/0003-4819-153-7-201010050-00006?aimhp=
In 2017, a team conducted a research on 10 125 Chinese university students, ranging in age from 16 to 20. It found that "sleep quality was positively associated with muscle strength, and short sleep duration was associated with reduced muscle strength."(Chen, Cui, Chen, & Wu, 2017) The researchers also found a positive correlation between sleep quality and muscle strength in men, when comparing those who had slept for less than six hours, those who had slept between 7-8 hours, and those who had slept for more than eight hours.
Lesson for men: Basically, the better your sleep, the greater your muscle strength. Think about that the next time you are lifting weights or carrying heavy loads!
However, what about women? Well, Chen et al. did not find a significant difference between the three groups when looking at the women. Therefore, their results did not reflect the relationship found in an earlier study on elderly women, in which a relationship was found between sleep quality and reduced muscle mass as well as decreased grip strength (Buchmann, Spira, Norman, Demuth, Eckardt, & Steinhagen-Thiessen, 2016; https://pubmed.ncbi.nlm.nih.gov/27151463/)
Why would that be? Here's what Chen et al. (2017) put forward as a possible explanation:
Biological conditions unique to women, including menstrual cycles and female hormones, were suggested to be related to sleep pattern and sleep duration[32,33]. Whereas, several previous reports indicated that grip strength decreased during both the follicular and luteal phases[34]. In addition, delayed muscle recovery in female was found during menstrual phase[35]. It is considerable that menstrual cycles and female hormones might be an important mediator, which might influence the association between sleep duration and muscle strength in female.
Regardless, while mediating factors may exist for women and more research is needed in this area, evidence of a relationship between sleep duration and muscle mass definitely exists.
The key thing to remember is that if you neglect sleep, then you are not optimising your workout because just as you need good nutrition for muscle-building, so, too, do you need good sleep.
