Caffeine, Exercise and Your Sleep: The Link Could be Sign. Different From What You Expect - Sleep Better W/ Caffeine?

Coffee and exercise both effect sleep, but their effects don't simply add up. The study at hand does yet suggest that your preworkout coffee won't ruin your sleep.
I have to admit, the following are not results of peer-reviewed research, but with a 2x2 week design, participants being randomized to exercise (4 workouts per week) or be sedentary and to consume caffeine or placebo prior to exercise or rest, it looks methodologically complex, but sound and, more importantly, interesting enough to make it into the SuppVersity news ... I mean, it's about coffee ;-)

With that being said, the scientists, who were hopefully less biased than I am, required their subjects to refrain from any extra regular physical activity and or coffee / caffeinated beverage consumption outside of the conditioning/treatment sessions, in which they didn't drink coffee, but 350-mL of Gatorade with or without a rel. low dose of 3mg/kg caffeine.
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The authors' data analysis involved the usual mixed analysis of variance with treatment (placebo or caffeine) and condition (exercise or sedentary) as between subjects factors. In addition, time as the repeated measure, and the subjects' usual caffeine intake and BMI were included as covariates.
Figure 1: Mean sleep duration (h) in the different arms / phases of the 2x2 week RCT (O'Brien. 2016).
As the data in Figure 1 tells you, the statistical analysis yielded an independent main effect of
condition (sedentary/exercise) on the number of hours the subjects actually slept (this is not "time spent in bed"). In that, it did matter, whether the subjects worked with or without caffeine, but both, the effects of exercise (SED vs. EX | Figure 1), and as those of caffeine (see PLA vs. CAF | Figure 1) are not exactly what you probably expected:
  • Effects of exercise - Subjects who exercised in the lab self-reported less time (hours) sleeping [F(1,18) = 4.5, p = 0.049] compared to sedentary. In that, there was a trend for an independent effect of treatment (placebo/caffeine) on hours slept (p = 0.08),
  • Effects of caffeine - Subjects who received placebo self-reported less time (hours) sleeping compared those who received caffeine (that was not what you'd expect based on previous evidence). In that, there were no interactions by usual caffeine intake.
Now, one's sleep duration is only one out of several parameters that will determine whether or not you rise and shine refreshed; plus, since all subjects had average sleep times in the "green zone" of 6.5-8h per night, they were all sleeping enough - irrespective of exercise and/or caffeine. The parameter of actual interest is thus the subjects' subjective sleep quality and its relationship to their perceived tiredness in the AM / over 24h, which were both assessed with questionnaires in the study at hand.
Figure 2: Sleep quality and perceived tiredness over the course of the 2x2 week RCT (O'Brien. 2016).
For the former, i.e. the subjects' sleep quality, the data in Figure 2 signifies that here was a significant time x treatment x condition interaction on overall sleep quality [F(11,198) = 1.92; p = 0.038]. In that,  the subjects' sleep quality decreased over time in subjects who exercised compared to condition controls (sedentary). In contrast to what you'd expect, though, it were not the subjects who worked out and consumed caffeine who had the lowest sleep quality, but those "who exercised and received placebo [who] had the lowest overall average sleep quality" (O'Brien. 2016).

What may come as a surprise is that this decline in sleep quality had no effect on the subjects' perceived tiredness (Figure 2, right), which showed a main effect of time for ‘Body Feels Tired’ [F(11, 154) = 2.1; p = 0.026], but no treatment (placebo/caffeine) or condition (sedentary/exercise) interactions - which is unquestionably odd. About as odd, as the misleading statement that "[p]oorest sleep quality ratings associated with caffeine and exercise" (O'Brien. 2016) from the scientists' own summary of the results. Now, don't get me wrong. This statement is correct, but only if we are talking about the individual effects of exercise / sedentary and caffeine / placebo, on their own. The way O'Brien et al. phrased it, does however appear to suggest that the subjects' sleep was worst during the exercise + caffeine trials... Now, that, in turn, is what you probably expected the study to show, but another brief glance at the data in Figure 2 (left) confirms: caffeine did not mess with the subjects' sleep quality. In fact, the group with the most stable sleep quality are the sedentary coffee drinkers . eventually, you could thus argue that they had the best sleep quality!
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Bottom line: As the authors point out, "[e]xercise and caffeine did not have the hypothesized results on sleep quality and duration" (O'Brien. 2016). Instead of improving the sleep quality of the subjects, as it has been observed previously in both, middle-aged and older adults (Yang. 2012) and young healthy sleepers (Flausino. 2012), exercise clearly reduced the young subjects' sleep quality in the study at hand. As O'Brien et al. point out, this may have been a function of the novelty of the exercise and subsequent "physical discomfort that disrupted sleep quality and duration" of the previously untrained subjects in the study at hand, so that the results would change over time / be different if the study had used trained individuals.

Another important subject characteristic that may have "messed" with the results were the sujects' individual habitual caffeine consumption (100mg/day on average). Even though their habitual intakes were low, the fact that caffeine did not, as it did in previous studies, per se mess with the subjects' sleep quality, but rather improved it, could, as O'Brien et al. suggest, be due to "[w]ithdrawal reversal" of which the scientist argue that it appears to be "the primary action mechanism of caffeine [in the study at hand]" (O'Brien. 2016). Practically speaking, this would mean that "[r]eversing [the] negative state [of being on caffeine withdrawal] through caffeine administration improved [not decreased the subjects'] sleep quality and duration" (O'Brien. 2016 | my emphasis). How realistic this assumption is does yet appear questionable, with std. deviations of <50mg/day, the subjects don't seem to be caffeine junkies and with a dosage of only 3mg/kg per day (all subjects were normal weight, so that's probably in the 200-300mg range) switching from a caffeine to a no-caffeine group in the 2nd of the 2x2 week phases doesn't appear to be likely to induce significant "caffeine withdrawal", either. I am thus doubly curious to see the (hopefully) full dataset, when this intriguing study is eventually published (also because the the caption of Figure 1 in the "FT" says that there was no interaction with habitual caffeine intake for sleep duration, at least). In the mean time, I'd suggest you simply listen to your body. The effects of exercise and caffeine on one's sleep are, after all, just as so many things, highly individual | Comment!
  • O’Brien, E, et al. "Caffeine and Exercise Affect Sleep Duration, Quality and Perceived Tiredness." Department of Exercise and Nutrition Sciences---University at Buffalo, Buffalo, NY (Poster presentation).
  • Yang, Pei-Yu, et al. "Exercise training improves sleep quality in middle-aged and older adults with sleep problems: a systematic review." Journal of physiotherapy 58.3 (2012): 157-163.
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