|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.|
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.
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).|
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.
|Figure 2: Sleep quality and perceived tiredness over the course of the 2x2 week RCT (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!
- 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.