In this article, our expert Dr. Yu Sun Bin – reviews: Sleep and self-control: A systematic review and meta-analysis by Guarana et al 2021.
1) What was the high-level summary of the research?
The quality and duration of our sleep affects our levels of self-control i.e., our ability to control our impulses and overcome temptations.
2) How was the study undertaken?
This study was a systematic review, which means that the researchers comprehensively searched the scientific literature for studies on sleep and self-control to try and synthesise (bring all of the evidence together to summarise) what we currently know and to identify where more research might be needed.
The team of researchers then sorted through more than 1600 studies, focusing in on studies that tried to answer the question of whether sleep affects self-control. Specifically, the researchers used meta-analysis which means that they combined the numerical results from the studies to find an overall result. This overall result should be more reliable and more indicative of the true effect of sleep on self-control than the results of individual studies, which can be ‘noisy’ and throw up unusual findings by chance.
3) What did the study find?
The researchers found 56 studies on sleep and self-control.
Researchers tend to think of self-control in two ways: first, as a somewhat stable trait of a person (akin to personality), and second, as something much more fluid, like a state-of-mind which can change from moment to moment (like mood).
These two ways of thinking about self-control are important when it comes to understanding the results.
When the researchers examined the studies that considered sleep and self-control as stable traits, they found that people who get more sleep tend to have better self-control than people who get less sleep but that this was a weak relationship (correlation r=0.17*).
Similarly, people who have better quality sleep have better self-control than people with poorer quality sleep (r=0.26), this relationship is stronger than the one between sleep duration and self-control, but is still considered a weak to moderate strength relationship. This could be because there are other factors responsible for both better sleep and for self-control (e.g. shared genes); or could be due to reverse causality, that is because people with better self-control are also more organised and conscientious about their sleep habits, leading to better and more sleep on average. This result is less useful for us as individuals because we might not be able to change how much sleep we need personally or our personality.
When the researchers examined the studies that consider sleep and self-control as malleable states, both longer sleep duration and better sleep quality were again related to high levels of self-control (r=0.32, and r=0.35 respectively). This is the more important practical result, because it shows that when we have a good night’s sleep, we have a higher level of self-control the next day, compared to when we have a poor night’s sleep or not enough sleep.
Both of these relationships were stronger than the relationships we see for self-control as a trait, meaning that our self-control changes more in response to how our sleep changes from day-to-day, than differences in levels of self-control between people who naturally need different amounts of sleep.
It is important to note that the majority of studies relied on research participants to self-report their sleep which might lead to biases in the results. However, the researchers also found that the relationship between sleep and self-control was stronger when sleep was measured objectively, indicating that the results are not due to the way that people report or perceive their own sleep.
4) Is there any other research out there that supports these findings or contradicts it?
The broader scientific literature supports these findings. We know that when people are sleep-deprived, our ability to monitor our own thinking and behaviour is negatively affected. Lack of sleep particularly affects activity in the prefrontal cortex, which is the part of the brain responsible for our ability to be aware of our own thinking and to control conscious actions. Self-control involves being aware of our instincts (e.g. to have more cake) and to actively overcome those instincts (e.g. to tell ourselves we have had enough and to put away the cake for another day).
The results of this study also show that sleep quality is more important than sleep duration when it comes to self-control. This is supported by research showing that quality of sleep is generally more important than amount of sleep for wellbeing; shorter sleep durations (e.g., 6 hours a night) may not be a bad thing if it is accompanied by good sleep quality. This is because people can have a range of different sleep needs, with 7-9 hours of sleep opportunity recommended for working-aged adults.
5) How much weight should we give this research?
We should give the results of this study a lot of weight, because it systematically combines all the research that has been done so far on how sleep affects self-control. The main potential weakness of the study is that 32 of the 56 studies were correlational studies and these studies only show that sleep and self-control are correlated, not that sleep directly causes changes in self-control. With correlation studies, we cannot rule out the possibility of reverse causality i.e. that self-control causes better sleep. In the remaining 24 studies, researchers actively intervened to change the quality and duration of sleep in the research participants.
6) What does this mean for your work/research/industry?
This study opens up some interesting avenues for research. Having self-control is important for many aspects of life, including maintaining our health and for our success at work. It suggests that we should consider improving sleep as a way to improve people’s ability to self-regulate which in turn can enhance health and productivity. For health researchers, it suggests we should incorporate methods to improve sleep in programs that help people quit smoking or lose weight, as this could make them even more effective.
7) What’s the key takeaway for us to take from this research?
The key takeaway is that having good quality sleep (of a sufficient duration) is important for enhancing self-control.
8) Will you be doing anything differently because of this research?
Personally, I think the study confirms what we all know from experience – and gives me an extra reason to make sure I get a good night’s sleep every night!
*Note: Correlations are used to estimate the strength of a relationship between two variables, specifically the degree to which one variable changes when another variable changes. If a correlation is zero, there is no relationship between two variables. The closer the correlation is to 1 or -1, the stronger the correlation between them. The correlation statistic used here is Pearson’s correlation. Correlations of 0.1 or 0.2 are considered weak, correlations of 0.3 to 0.6 are considered moderate, and correlations above 0.6 are considered strong.
Dr Yu Sun Bin is an epidemiologist and public health researcher. Her particular research interests are on sleep and circadian rhythms and how these biological systems are reflected in behaviour, health, and disease.