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Uncover the Top Predictors of Fatigue

Updated: Mar 15, 2021

Fatigue is a major problem in society, associated with a host of negative outcomes such as decreased cognitive performance and increased accident risk. If you experience tiredness regularly, then it might be worth trying to understand why. To help our readers, we ran several studies to learn what predicts fatigue and how it can be reduced. This article summarizes the results of these studies and draws some conclusions about what might cause tiredness. Our most interesting finding was that reducing anxiety (when a person is anxious) tends to reduce tiredness! So if you’re thinking “I’m tired; what I really need is to rest” we recommend taking a further moment to consider whether you’re feeling anxious. If you are, it may be that the anxiety is what actually should be addressed (e.g., by using an app like Mind Ease or taking some relaxing breaths), and that once you’re feeling less anxious the tiredness will lift too! For a more detailed breakdown of our study results, click here or keep reading to learn the summary of our findings!

Study 1: What factors are associated with tiredness?

The aim of our first study was to predict how tired the study participant was in that very moment based on many different variables about them. We measured current tiredness by developing a general fatigue score that aimed to get at four different aspects of fatigue and tiredness with these questions:

  • “How tired does your body feel right now?”

  • “How tired does your mind feel right now?”

  • “How slow and sluggish are you right now?”

  • “How awake do you feel right now?"

The study was observational, so the results only allow us to identify the factors that could help predict fatigue. We can’t say for sure that these variables cause fatigue.

Study 1: Results

Out of the 37 variables we tested for predicting fatigue, only a few of them appeared to have meaningful effects. But it's worth noting that some of the other variables that didn't have significant effects may also cause fatigue. For instance, having a fatigue-causing illness and how much the participant slept the previous night are well-known causes of tiredness despite only having a minor predictive role in our study. Additionally, time of day, how much sleep a person got the previous night, fatigue-causing illness, and age had meaningful effects in one of our analysis models. The top two variables that stood out in both of our analyses were typical tiredness and current anxiety. “Typical tiredness” reflects how tired or sleepy respondents said that they felt on a typical day, and “current anxiety” reflects a combined score on four questions asking how worried, tense, calm, and relaxed respondents were at that moment.

“Typical tiredness” might seem like a strange variable to predict fatigue, since to say “tiredness is predicted by typical tiredness” is the kind of tautological result that doesn’t really mean anything! However, it's important to note that the difference is that typical tiredness is how tired participants report they are on an average day, whereas what we're predicting in the study is how tired they report feeling right now. We were also concerned that this variable could be substantially correlated with a number of other variables, like the existence of a long-term fatigue-causing illness.

We were much more interested in the strong correlation between “current anxiety” and fatigue. Anxiety’s ability to predict tiredness is consistent with at least four different causal hypotheses:

  1. Higher tiredness levels cause people to feel more anxious (e.g, perhaps tiredness reduces resilience to stress)

  2. Higher present-moment anxiety levels cause people to feel more tired

  3. Anxiety over several hours could lead to a "burn out" effect which increases tiredness, and the correlation between in-the-moment anxiety and tiredness we saw simply reflects this burn out

  4. Current anxiety levels are caused by higher baseline anxiety, which leads to poorer sleep quality or quantity, which leads to higher tiredness (therefore, sleep is really what's contributing to tiredness)

Study 2: Does anxiety cause tiredness?

The aim of our second study was to use a randomized controlled trial to establish whether present-moment anxiety causes tiredness (hypothesis 2 from our list above); if we could randomize people to a condition which reduces their anxiety or a control condition that doesn’t, and tiredness drops in the former group, we can infer that a reduction in anxiety causes a reduction in tiredness instead of just being correlated with it. We used two present-moment anxiety reduction interventions from our Mind Ease’s app: progressive muscle relaxation and visualization (check out the app if you want to try them yourself!). The former exercise walks participants through a process of tensing then relaxing muscles in a stepwise fashion, while the latter guides people in imagining a very relaxing place. 200 participants - asked to take the study if they were anxious in the present moment -- were randomized to either the first intervention, the second intervention, or a control group in a 1:1:2 ratio.

Study 2: Results

Our results showed that fatigue dropped by 1.4 more points in the anxiety intervention group when compared to the control group (p = 5E-5), with anxiety dropping 3.3 points more than the control group (p = 2E-7). A simple linear regression found that anxiety would have to drop by around 3.6 points on the scales we used to reduce fatigue by 1 point out of 16. This means that our results appear to show that present-moment anxiety has a modest but real causal effect on present-moment tiredness! Our data is not consistent with the other three causal hypotheses we proposed causing tiredness independent of sleep time and sleep quality. However, that conclusion is less certain, given that those results are observational (not the result of a randomized experiment) so it's harder to establish causality in a reliable way. These studies have several weaknesses, which we detail in the full write-up of our results, but we’re excited to learn that there may be a causal connection between present-moment anxiety and present-moment fatigue. This means that, if you suffer from fatigue regularly, reducing your anxiety levels may help you combat these feelings of tiredness! You can read the full study here to learn the details behind the research methodology we used and to discover more about the factors that predict fatigue.


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