A primary goal of my blogs is to reduce barriers to consistent exercise and physical activity. A common barrier to exercising regularly is the perception of lack of time during the preferred period of the day. As well, people with a current preference to exercise at a particular time of day (a chronotype) often ask whether their exercise time preference is beneficial or detrimental to their health and performance goals. So, is one time of day better than another, or can we swap around our exercise schedule to better fit the time available without compromising the benefits?

Physiological functions influenced by the time of day that have been previously been reported to affect physiological adaptations from exercise have included diurnal differences in core body temperature, cardiovascular function, respiratory control, hormonal responses, as well as subjective alertness. Previous research has also been completed to assess the effects of exercising at certain times of the day on particular health outcomes such as cardiovascular disease, certain types of cancer and obesity. However, the studies have been observational and measured association rather than causation. Bruggisser et al. recently published a meta-analysis of 22 longitudinal intervention trials to better quantify causation of the time of day of exercise on health or performance.  A summary of the findings is listed below.

Methods:

• 35% of participants were female.
• Participants with an evening chronotype were underrepresented with only 1% of all participants classified as definitely evening chronotype and none as having a moderately evening chronotype.
• Mean clock time of 07:58 ± 36 min (range 07:30–09:00) was compared to a mean clock time of 17:19 ± 61 min (range 14:30–19:00).
• Endurance or aerobic training studies included running, cycling, swimming, or a combination of various types of aerobic exercise.
• Strength training studies included interventions focusing on the lower body, upper body and/or full body training.
• “The anthropometric parameters were measured using a body impedance analyzer, dual-energy X-ray absorptiometry, or a simple digital scale”.
• “Aerobic endurance was assessed using VO2max with varying protocols and types of exercise (e.g., cycle ergometer versus treadmill ergometer) and distance during a nine-minute running test or six-minute walking test”.
• “The muscle groups tested in the studies on strength performance were the extensors and flexors of the knee and elbow” or simply the quadriceps/hamstrings and triceps/biceps using isometric, isokinetic, and repetition maximum testing. 
• Lower body power was measured by jump height which “was measured by a squat jump and a countermovement jump”.
• “For the blood analyses, venous and capillary blood were used to determine the main parameters glucose, insulin, HbA1c, testosterone, cortisol, total cholesterol, high-density lipoprotein, low-density lipoprotein, and triglycerides”.

Results:

• “No consistent overall finding as well as little statistical evidence for a time-of-day specific influence of training on the listed anthropometric, performance-related, and health-related outcome measures”. “With regard to the long-term effect, it is impossible to define conclusively whether physical activity at a certain time of day has a greater benefit on health or performance compared to another time of day”.
• Many original studies concluded superiority of exercise training in the late afternoon and evening over morning training but this meta-analysis determined that was not generalizable or consistent.
• “No general indication of an advantage for a specific time of day, but rather the results indicated an advantage when time of day of training and testing coincided”. This was especially clear for jumping and strength performance.
• In 2 of the 7 studies assessing health outcomes, evening exercise was seen to be superior to morning exercise compared to the other studies while showing equal benefit for improving resting systolic blood pressure, triglycerides, and fasting glucose. 
• 3 out of 4 studies assessing jump performance showed no significant differences of time-of-day training.
• Strength assessment showed a small non-significant effect favouring evening exercise.
• The two studies that investigated the effect of training at a certain time of day on VO2max (the gold standard for measuring aerobic fitness) suggested improvements were independent of the time of day at which the training or testing was performed.
• However, “time of day is not irrelevant and warrants further investigation with a rigorous methodological approach”.

Interesting but speculative mechanisms for or against training at certain times.

• Endogenous plasma glucose concentrations and whole-body insulin sensitivity is at its best in the morning.
• Faster recovery rates of systolic blood pressures occur in the late evening than at night and in the morning.
• Experimental studies increasing or decreasing core body temperature before performance tests indicate that core body temperature do not have consistent influences on performance.
• “Potential underlying mechanisms might be the muscle clock, central clock influences, rhythms of cardiopulmonary function, autonomic nervous system output on muscles, environmental temperature, sleep homeostasis, or nutritional status”. However “most of the studies have inadequate control of confounding factors like diet and sleep”. “Timing and composition of ingested food affect the circadian systems”.
• Waking early in the morning to exercise or shortening your sleep to exercise in the morning might negatively influence blood pressure and insulin sensitivity. 
• Few studies have directly tested and measured these proposed mechanisms.

The underlying mechanisms causing within-day variations in peak performance remain unclear but include the convergence of “individual chronotype, sleep, food and caffeine intake, environmental conditions, and the endogenous circadian system”. There appears to be little evidence for or against exercising at a certain time of day when aiming to improve performance, body composition, cardiovascular or cardiometabolic health outcomes. For athletes with upcoming performance testing or competition, training around that time appears to be beneficial. The benefit of exercising for health outcomes at a particular time of the day is therefore only certain when that time allows consistent preferred attendance. 

Written by Tom Murphey, DPT.

“Unfortunately for our community, scientific scaremongering is common, easy to believe and hard to heal. Research is often messy, and strong stances or beliefs can be both erroneous and dishonest. I aim to produce honest reviews of some high-quality research to provide informed insight so you can make up your own mind on the science you value.”

References.

Bruggisser, F., Knaier, R., Roth, R. et al. Best Time of Day for Strength and Endurance Training to Improve Health and Performance? A Systematic Review with Meta-analysis. Sports Med – Open 9, 34 (2023). https://doi.org/10.1186/s40798-023-00577-5