by Eric Trexler, Ph.D.

Study Reviewed: Effects of Different Caffeine Dosages on Maximal Physical Performance and Potential Side Effects in Low-Consumer Female Athletes: Morning vs. Evening Administration. Bougrine et al. (2024)

Key Points

• These researchers investigated the ergogenic potential of three different caffeine doses (3 mg/kg, 6 mg/kg, or a 0 mg/kg placebo) at two separate times of day (morning versus evening) in female handball athletes. 
• Caffeine was most effective at improving performance when ingested in the morning, and 6 mg/kg was generally more effective than 3 mg/kg. Evening caffeine ingestion did not significantly improve performance and was associated with more side effects, especially when the higher dose (6 mg/kg) was ingested.
• If you plan to obtain a meaningful ergogenic effect from caffeine, your best bet is to use it in the morning. Evening caffeine consumption may be less effective and comes with a much higher risk of sleep impairment.

In a new study from Bougrine and colleagues, participants completed six different lab visits for exercise testing. They completed three “morning” sessions (8:00 a.m. to 9:00 a.m.) and three “evening” sessions (6:00 p.m. to 7:00 p.m.). Within each condition of time (morning and evening), the participants completed three separate exercise visits: one with a placebo, one with a smaller dose of caffeine (3 mg/kg), and one with a larger dose of caffeine (6 mg/kg).

The purpose of the study was to investigate how different caffeine doses (3 mg/kg vs. 6 mg/kg) at different times of the day affected the maximal high-intensity physical performance and perceived exertion and potential side of female athletes. The battery of assessments included countermovement jump (CMJ) testing, modified agility testing (MATT), repeated sprint ability (RSA) testing, and rating of perceived exertion (RPE). The researchers also administered a caffeine side effects questionnaire at the conclusion of each testing visit.

Many late-afternoon and early-evening exercisers will undoubtedly time their last cup of coffee in close proximity to their workout or consume a caffeinated pre-workout supplement before heading to the gym. As such, it’s important to test whether or not late-day caffeine supplementation will improve performance to the same extent as early-morning caffeine supplementation.

The question of dosage is also extremely important for practical reasons. If you’re taking a large dose of caffeine, there’s a good chance that you’re going to detrimentally impact your sleep, which certainly isn’t a recipe for long-term fitness success. However, you could theoretically hope to consume the lowest effective dose of caffeine to enjoy a small boost in performance without dramatically impacting sleep quantity or quality.

Findings

In the placebo condition, evening CMJ performance was 2.6% greater than morning performance (p < 0.001). Caffeine supplementation closed the gap between morning and evening performance, with 3 mg/kg increasing morning performance by 2.5% (p < 0.001) and 6 mg/kg increasing morning performance by 3.8% (p < 0.001). However, evening caffeine ingestion did not significantly improve CMJ performance compared to placebo. Evening MATT performance was 6.4% better than morning performance in the placebo condition (p < 0.001). Ingestion of 6 mg/kg caffeine improved morning MATT performance by 4.5% compared to placebo (p < 0.001), but 3 mg/kg did not significantly improve morning MATT performance. Caffeine ingestion did not significantly improve MATT performance in the evening, once again showing that caffeine simply closed the gap between morning and evening performance rather than improving performance across the board. CMJ and MATT findings are presented in Figure 1.

Compared to the morning, evening mean RSA performance was 2.6% better in the placebo condition. Caffeine ingestion improved mean RSA by 1.1% (3 mg/kg, p < 0.05) and 3.5% (6 mg/kg, p < 0.001) compared to placebo when tested in the morning. However, evening caffeine ingestion did not significantly improve mean RSA. Evening performance was also better than morning performance for peak RSA in the placebo condition. The 6 mg/kg caffeine dose improved morning peak RSA by 2.8% compared to placebo (p < 0.001). However, the 3 mg/kg caffeine dose did not significantly improve morning performance, and neither caffeine dose improved peak RSA performance during evening testing sessions. Results for mean and peak RSA performance are presented in Figure 2.

Interpretation

The primary findings of this study are that 1) morning performance was typically worse than evening performance, 2) higher-dose (6 mg/kg) caffeine generally restored morning performance to a level similar to evening performance (that is, evening + placebo), and 3) caffeine did not meaningfully improve evening performance. It’s not particularly surprising that morning + placebo performance was worse than evening + placebo performance. Dr. Zourdos has written some great content covering the fact that shifting from evening training to morning training tends to temporarily impair performance. However, once you get accustomed to morning training, performance typically rebounds back to normal. It’s also not surprising that 6 mg/kg was generally more effective than 3 mg/kg; the effective dosing range of caffeine tends to be 3 to 6 mg/kg, but ergogenic effects get a little bit less consistent in the literature when you stick to the lower boundary of that range (3). In my opinion, the most interesting finding of this study is the differential impact of caffeine on morning versus evening performance.

Recontextualizing the ergogenic effect of caffeine

When I first read the findings of this study, my initial reaction was that we may need to reframe and recontextualize the ergogenic effect of caffeine. There is ample research supporting the performance-enhancing effects of pre-workout caffeine supplementation (2), but this research typically follows a very consistent playbook: caffeine is ingested on a single occasion, 30-60 minutes before exercise, in the morning after an overnight fast. That’s fine, but any line of research that is characterized by homogenous interventions and study designs is inherently susceptible to vulnerabilities. For example, evidence suggests that people tend to perform better in the late afternoon or early evening in comparison to the morning (until they adapt to regular morning exercise or develop a preference for it, that is). The present study suggests that early-morning caffeine supplementation, which reflects the vast majority of caffeine research, merely serves to overcome the performance-impairing neuromuscular deficit commonly experienced during early-morning exercise. More importantly, it’s not alone – the research isn’t entirely unanimous in this area, but similar findings have been reported in multiple studies from multiple independent research groups (4).

My message is not that caffeine is inherently bad, but rather that caffeine’s reputation as a rock-solid daily-use ergogenic aid with no downsides is incongruent with the available research. With this in mind, I think it’s worthwhile to reframe and recontextualize caffeine’s role in the fitness world. You could make a strong evidence-based case that caffeine’s best fitness-oriented use cases are 1) to facilitate the transition from afternoon or evening training to morning training by mitigating temporary performance decrements, 2) to mitigate potential performance decrements from training during caffeine withdrawal among habituated caffeine users, 3) to acutely preserve performance in a training session or competition occurring in an underslept state, or 4) to improve one’s subjective enjoyment of training sessions. This is quite different from the common assumption that caffeine will provide a perpetual, reliable enhancement of performance whenever you happen to use it. 

If you’re a late-afternoon or evening exerciser, the decision to avoid pre-workout caffeine is, in my opinion, a no-brainer, and the justification follows multiple lines of previously discussed evidence. While not completely unanimous, evidence suggests that caffeine is either less effective (or flatly ineffective) for performance when consumed in the late afternoon or evening (1). Even if you don’t find that evidence compelling, the best available evidence fails to show meaningful effects of regular pre-workout caffeine supplementation on chronic training adaptations (5). We also lack a ton of critical knowledge related to the time course of caffeine habituation and withdrawal, which means that early benefits (if observed) may fade fairly quickly (8). On top of all that, caffeine’s long half-life and well-documented potential for sleep impairment makes afternoon or evening caffeine consumption a pretty untenable gamble (7). In summary, I would encourage late-afternoon and evening exercisers to skip the caffeine for myriad reasons, many of which are fully unrelated to the presently reviewed study. These new findings simply provide further justification for the recommendation. If sleepiness or mental fatigue are hindering your workouts, you might be better off using another strategy, such as a short nap, a short mindfulness exercise, or another form of intentional rest before training.

Pitfalls of extrapolating caffeine mechanisms to specific dosing and timing protocols

Before wrapping up, I want to get on my soapbox for a minute. Adenosine is a neurotransmitter that contributes to sleepiness – as it builds up in your brain throughout the day, you feel increasing tiredness and “sleep pressure.” Caffeine is an adenosine receptor antagonist, meaning it binds to adenosine receptors to block the physiological effects of adenosine. Based purely on this simple understanding of caffeine’s primary mechanism of action, one could reasonably infer that caffeine is most potent in the evening, when brain adenosine levels tend to be considerably higher than the morning. However, this inference is somewhere between “not true” and “the opposite of the truth.” I bring this up because it speaks to a broader, highly relevant point – it has become fashionable to use our current (fairly basic) understanding of caffeine’s mechanisms to fabricate extremely specific caffeine dosing and timing protocols in the absence of direct evidence, and I don’t like it.

Perhaps the best example of this is the increasingly common recommendation to wait around 90 minutes after waking to consume your first caffeine of the day. This concept has never been directly tested in peer-reviewed research (to my knowledge), and is instead justified using a chain of mechanistic assumptions. As you sleep, adenosine is cleared from the brain. The majority of adenosine is cleared from the brain during sleep, but your body is still clearing out some of the adenosine when you wake up. Part of this process involves the “cortisol awakening response,” or the natural spike in cortisol levels that occurs shortly after waking up. The premise of strategically delaying morning caffeine intake is vaguely based on the concept that you will 1) interfere with morning cortisol release, 2) interfere with morning adenosine clearance, 3) blunt the effectiveness of your morning caffeine, 4) exacerbate the afternoon “energy crash,” and/or 5) somehow expedite caffeine habituation.

This sounds awfully scientific, and is often repeated by people with legitimate scientific credentials. However, as reviewed in a fairly recent publication (9), there is no evidence linking this strategy to endpoints anyone cares about. It is true that caffeine can modestly impact cortisol release patterns (10), particularly among caffeine-naive individuals, but there’s no research linking this delayed caffeine strategy to subjective feelings of wakefulness, afternoon energy crashes, caffeine tolerance, or any other practically relevant outcome. If this theory isn’t based on rigorous scientific evidence, you might wonder where it originated. As far as I can tell, this theory has been bouncing around biohacker communities for a while, but the most cited “origin” is a 2013 blog post (which doesn’t exist anymore) by a graduate student who never published any caffeine research. 

To be clear, I’m not taking a jab at this individual – I’m not in the business of policing graduate students’ blogs as they pursue passion projects or work on building their research communication skills. I will, however, take an infinite number of perpetual jabs at PhDs who popularize this idea without acknowledging the extreme lack of evidence supporting it. For some reason, caffeine is the topic where highly credentialed individuals seem to let their guard down and lose their interest in distinguishing between intuition, speculation, or blog posts and well-designed randomized trials, which would be extremely feasible to conduct. This is but one manifestation of a broader phenomenon: people think we know a lot more about the practical elements of caffeine use than we actually do, and are often shocked to discover we have little to no evidence addressing some pretty basic questions about practical application. This lack of evidence leaves wide-open gaps for oversimplified narratives built on mechanistic speculation – scientific as they may sound, they dramatically oversell their conclusions and overstate our current level of knowledge. As the present findings reinforce, we jump to caffeine-related conclusions at our own peril – we’re often better off leaning on empirical evidence to guide our caffeine decisions whenever possible.

Application and Takeaways

We still have much to learn about caffeine’s effects on performance across a wide range of scenarios and applications. You could make a strong evidence-based case that caffeine’s best fitness-oriented use cases are 1) to facilitate the transition from afternoon or evening training to morning training by mitigating temporary performance decrements, 2) to mitigate potential performance decrements from training during caffeine withdrawal among habituated caffeine users, 3) to acutely preserve performance in a training session or competition occurring in an underslept state, or 4) to improve one’s subjective enjoyment of training sessions. This is quite different from the common assumption that caffeine provides perpetual, reliable performance enhancement whenever you use it. If you train in the late afternoon or evening, I suggest avoiding pre-workout caffeine – there is some evidence indicating it won’t do much for your acute performance or chronic training adaptations, and considerably stronger evidence documenting a high likelihood of sleep disruption. In this scenario, the most likely outcome is a net effect that hinders progress more than it helps.

References

1. Bougrine H, Ammar A, Salem A, Trabelsi K, Żmijewski P, Jahrami H, et al. Effects of Different Caffeine Dosages on Maximal Physical Performance and Potential Side Effects in Low-Consumer Female Athletes: Morning vs. Evening Administration. Nutrients. 2024 Jul 11;16(14):2223.
2. Grgic J, Grgic I, Pickering C, Schoenfeld BJ, Bishop DJ, Pedisic Z. Wake up and smell the coffee: caffeine supplementation and exercise performance-an umbrella review of 21 published meta-analyses. Br J Sports Med. 2020 Jun;54(11):681–8.
3. Guest NS, VanDusseldorp TA, Nelson MT, Grgic J, Schoenfeld BJ, Jenkins NDM, et al. International society of sports nutrition position stand: caffeine and exercise performance. J Int Soc Sports Nutr. 2021 Jan 2;18(1):1.
4. Pickering C, Grgic J. Caffeine and Exercise: What Next? Sports Med. 2019 Jul;49(7):1007–30.
5. Giráldez-Costas V, Ruíz-Moreno C, González-García J, Lara B, Del Coso J, Salinero JJ. Pre-exercise Caffeine Intake Enhances Bench Press Strength Training Adaptations. Front Nutr. 2021 Jan 26;8:622564. 
6. Marcus GM, Rosenthal DG, Nah G, Vittinghoff E, Fang C, Ogomori K, et al. Acute Effects of Coffee Consumption on Health among Ambulatory Adults. N Engl J Med. 2023 Mar 23;388(12):1092–100.
7. Gardiner C, Weakley J, Burke LM, Roach GD, Sargent C, Maniar N, et al. The effect of caffeine on subsequent sleep: A systematic review and meta-analysis. Sleep Med Rev. 2023 Feb 6;69:101764.
8. Lara B, Ruiz-Moreno C, Salinero JJ, Del Coso J. Time course of tolerance to the performance benefits of caffeine. PLoS One. 2019 Jan 23;14(1):e0210275.
9. Antonio J, Newmire DE, Stout JR, Antonio B, Gibbons M, Lowery LM, Harper J, Willoughby D, Evans C, Anderson D, Goldstein E, Rojas J, Monsalves-Álvarez M, Forbes SC, Gomez Lopez J, Ziegenfuss T, Moulding BD, Candow D, Sagner M, Arent SM. Common questions and misconceptions about caffeine supplementation: what does the scientific evidence really show? J Int Soc Sports Nutr. 2024 Dec;21(1):2323919.
10. Lovallo WR, Whitsett TL, al’Absi M, Sung BH, Vincent AS, Wilson MF. Caffeine stimulation of cortisol secretion across the waking hours in relation to caffeine intake levels. Psychosom Med. 2005;67(5):734–9.