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Does the time of day you train change what you get out of it?
Circadian biology is the scientific study of the body’s internal biological clock and circadian rhythms. These systems regulate many aspects of physiology, including sleep-wake cycles, hormone secretion, digestion, and immune responses. For example, strength and power output peak in the late afternoon, insulin sensitivity varies across the day, and disruptions to the body clock are associated with metabolic disease. So it is reasonable to ask whether resistance training at 6am delivers the same adaptations as resistance training at 6pm. A new randomized controlled trial from the University of Glasgow set out to answer exactly that.
The Study
The study enrolled 36 healthy young adults (average age 30, BMI 28) who were not regularly exercising, and randomized them into three groups: a morning exercise group training between 6am and 10am, an evening exercise group training between 4pm and 8pm, and a non-exercising control group. Both exercise groups performed the same program. eight resistance exercises, three times per week for six weeks, each set taken to volitional failure at 80% of one-rep max. Before and after the intervention, researchers measured insulin sensitivity via oral glucose tolerance test, vastus lateralis muscle, knee extensor maximal torque, grip strength, and one-rep max for upper and lower body. Continuous glucose data was also across several time windows.
Both exercise groups improved insulin sensitivity, muscle thickness, knee extensor torque, and upper body strength over the six weeks. The control group showed no such changes. In short, strength training improved muscle size and strength. When the researchers tested for a time-by-group interaction, none of the measures reached significance between the morning and evening groups. The improvements were essentially the same whether training happened early in the day or evening.
There has been a reasonable hypothesis in the literature that evening exercise might yield greater metabolic benefits, given that glucose tolerance is naturally lower in the afternoon and insulin-stimulated glucose uptake may be more impaired at that time of day. This study found no support for that idea. Both morning and evening groups improved their insulin sensitivity scores to a similar degree. On the muscular side, the vastus lateralis muscle thickness and knee extensor maximal torque were similar between morning and evening groups. The results align with previous findings from a systematic review, which found across eleven studies that knee extensor gains did not differ between morning and evening training, and across five studies that muscle mass gains were similar.
The glucose monitor data showed there were no significant differences between groups in mean glucose or glucose variability. The one exception was a reduction in glucose variability on post-exercise days over the course of the intervention, but this was a time effect only and did not differ between morning and evening groups. This result is fairly expected. Glucose dynamics in people with normal glucose control tend to be fairly stable and resistant to the time-of-day effects seen in populations with impaired glucose regulation.
Take-home for athletes
The practical message is a good one: train at whatever time works for your schedule and your life. The body will adapt to resistance exercise. For athletes who feel they should be doing morning sessions because of some metabolic or hormonal advantage, or who feel guilty skipping a planned early session and not getting it done later, this study supports practicality. Consistency matters more than time of day.
Limitations
Like most research, the study has several limitations that are worth noting. The participants were sedentary, healthy young adults. Adaptations over six weeks in previously untrained individuals are relatively large in percentage terms. In trained athletes, where adaptations are smaller, it is at least theoretically possible that time-of-day effects could emerge that were too small to observe in this population. This is not a reason to dismiss the findings, but a reason to hold appropriate context.
Second, no formal power calculation was conducted 🚩. This is a big drawback. The authors acknowledge this directly and describe the study as exploratory. The sample of 36 participants gives the study reasonable sensitivity to detect large differences but limited power to detect smaller ones. It is possible that real but modest time-of-day effects exist and were not detected.
Third, dietary intake was not controlled 🚩. As a nutrition expert, this is something I’d want the researchers to consider. Nutrition timing, especially protein and carbohydrate intake around training, can modulate resistance training adaptations. Without controlling what and when participants ate relative to their training sessions, the dietary context of morning versus evening exercise was left free to vary in ways that could have hidden true differences. Finally, six weeks is a short intervention. The study itself notes that adaptations were relatively modest over this period, which limits the ability to detect differential effects from timing.
Can the findings be extrapolated to endurance training?
With caution. The study exclusively used resistance exercise, and endurance training places different demands on metabolism, substrate utilization, and circadian-sensitive systems. There is some evidence in endurance contexts that time-of-day may have notable differences for individuals with type 2 diabetes (1,2). Whether the same result would hold for endurance training adaptations in healthy athletes is not something this study can answer. Many triathletes and cyclists include resistance training but struggle to fit it in at any consistent time. This suggests that wherever it slots into your week is likely fine.
Remember, no matter the time of day, Fuelin can help prepare you to train at your best. Focus on getting the training done and fueling the work required, not on the time you do it.
Megan
Study
Dighriri A, Lithgow H, Gabriel B, Altuwrqi M, Dunning E, Johnston L, Boyle JG, Logan G, Gray SR. The impact of the time of day on muscle and metabolic responses to resistance exercise in healthy adults: A randomised controlled trial. Exp Physiol. 2026 Mar;111(3):1109-1119. doi: 10.1113/EP093020. Epub 2025 Nov 13. PMID: 41231202; PMCID: PMC12949110.
Citations
Niu WC, Liu C, Liu K, Fang WJ, Liu XQ, Liang XL, Yuan HP, Jia HM, Peng HF, Jiang HW, Jia ZM. The effect of different times of day for exercise on blood glucose fluctuations. Prim Care Diabetes. 2024 Aug;18(4):427-434. doi: 10.1016/j.pcd.2024.06.004. Epub 2024 Jun 18. PMID: 38897914.
Savikj M, Gabriel BM, Alm PS, Smith J, Caidahl K, Björnholm M, Fritz T, Krook A, Zierath JR, Wallberg-Henriksson H. Afternoon exercise is more efficacious than morning exercise at improving blood glucose levels in individuals with type 2 diabetes: a randomised crossover trial. Diabetologia. 2019 Feb;62(2):233-237. doi: 10.1007/s00125-018-4767-z. Epub 2018 Nov 13. PMID: 30426166; PMCID: PMC6323076.