Is There an Optimal Time of Day for Exercise? A Commentary on When to Exercise for People Living With Type 1 or Type 2 Diabetes

Three key considerations should be acknowledged when translating any research to practical recommendations for individuals with prediabetes or diabetes. First, having the “free” time to exercise is a major consideration (6). Although it may be competing priorities rather than an actual lack of time creating this barrier, finding or prioritizing the time to exercise poses a challenge that must be overcome for many people with diabetes. Second, interactions between the effects of diabetes medications (i.e., insulin formulations, various oral antidiabetic agents, and noninsulin injectable therapies) and those of exercise must be considered. The type and timing of exercise may change the potency of a drug, result in hypoglycemia, increase ketone production, and/or influence how a person feels when medications are peaking (7,8). Finally, in people with diabetes, there appear to be subtle but unique metabolic responses to morning versus late-day exercise as a result of hormones, substrate mobilization, and the tissue metabolome/proteome profile (9). These and other considerations may need to be acknowledged by individuals with diabetes who are prescribed a new exercise regimen to help manage their glucose variability.

For children and adolescents with diabetes, physical activity is often spontaneous and spread out across each day, which makes glucose management around exercise challenging (10). On the other hand, sedentary jobs often limit adults living with diabetes to dedicated morning or afternoon exercise sessions, particularly on weekdays. One benefit of this lifestyle is the ability to pre-plan daily exercise timing. However, whether morning or afternoon exercise is preferable for individuals with prediabetes or diabetes to lower glucose levels remains a complex debate.

Given that any exercise seems to help modulate glucose levels in people with diabetes, the selection of morning versus afternoon exercise should accommodate when individuals feel more likely to want to exercise and thus adhere to a long-term routine. “Chronotype” is a term used to describe individuals’ preference for being a “morning person” or an “evening person” with respect to sleep patterns, physical activity patterns, work preferences, eating patterns, and energy levels (11). In general, individuals who are early chronotypes tend to do more physical activity in the morning and have greater overall daily energy expenditure than those who are late chronotypes (12). Early chronotypes also have less risk for cardiometabolic disease over their lifetime (12). In type 2 diabetes, having a late chronotype is associated with greater caloric intake at dinner, later bedtimes, later wakeup times, and higher A1C levels (13). These associations might suggest that, from a cardioprotective and glucose management perspective, completing morning exercise may be more beneficial than afternoon exercise for people with diabetes. In line with this notion, some evidence in healthy young men and women suggests that late chronotype individuals who can adopt early-day exercise sessions can shift their internal circadian clock to an earlier rhythm (i.e., become an early chronotype), thereby improving metabolism and health (14).

Aside from an individual’s chronotype, morning exercise may be preferred for many adults with diabetes for several other reasons. For example, individuals with later-day responsibilities such as work or family may prioritize their leisure activities by completing morning exercise. Morning exercise is associated with improved overall physical activity adherence and a lower daily caloric intake in individuals with obesity (15). One recent meta-analysis (16) demonstrated that in individuals with type 2 diabetes, morning exercise in the fasted state might be better for overall 24-hour glucose levels than exercise done at other times of the day. This improvement in glucose may suggest a greater utilization of endogenous carbohydrate and lipid stores and improved whole-body insulin sensitivity in the 24–48 hours after fasted morning exercise (17).

On the other hand, different studies have observed improvements in overall health and glycemia from afternoon exercise compared with morning exercise in individuals with diabetes, particularly if vigorous-intensity exercise is performed. In one highly cited study of men with type 2 diabetes, a single session of morning HIIT cycling exercise elevated continuous glucose monitoring (CGM)-measured glucose values by ∼10–15 mg/dL compared with no exercise or late-afternoon HIIT (18). The afternoon HIIT appeared to complement the innate circadian clock’s regulation of carbohydrate metabolism and promoted superior glycemic benefits with lower overnight and next-day glycemia. Improvements in overnight and next-day glycemia may have been attributable to an enhanced insulin sensitivity after the afternoon HIIT (18). Similarly, 12 weeks of supervised afternoon endurance exercise training in males with prediabetes or type 2 diabetes resulted in improvements in whole-body and adipose tissue insulin sensitivity compared with the same exercise done in the morning (19).

Another recent cross-sectional examination of exercise training adaptations in men and women with type 2 diabetes found superior improvements in A1C levels in those who did midday exercise compared with those who exercised in the morning (20). This study also found greater cardiorespiratory fitness improvements in those who did midday exercise. It is important to note, however, that data from the Look AHEAD trial suggest that, although males with type 2 diabetes did not seem to have a time of day–dependent improvement in cardiorespiratory fitness, females with type 2 diabetes may have greater benefits with afternoon moderate to vigorous physical activity (21). Presently, other studies examining lower-intensity exercise, including walking (22) or mixed forms of exercise training (23), show no superiority of afternoon or evening exercise versus morning exercise in type 2 diabetes.

For individuals with type 1 diabetes, additional factors should be considered while participating in morning or afternoon exercise because time of day influences the acute glucose response. Late-day exercise in type 1 diabetes promotes a greater drop in glucose during exercise compared with fasted morning exercise and increases the risk for nocturnal hypoglycemia (24,25). Participating in fasted morning endurance exercise appears to be associated with a lower risk for exercise-associated hypoglycemia in the 36 hours after an exercise session and better overall glycemic time in range compared with performing the same type of exercise before dinner (26).

In contrast to a morning endurance exercise regimen, undertaking high-intensity morning exercise (i.e., heavy resistance training or HIIT) while fasting can promote hyperglycemia in type 1 diabetes, particularly in individuals with dramatic increases in circulating lactate levels (27,28). Indeed, the rise in glucose can be so dramatic in some individuals that they require a full or partial insulin bolus correction dose using their own unique correction factor to help limit hyperglycemia before the first meal or snack of the day (29,30). Thus, morning HIIT without insulin bolus correction is less likely than late-day HIIT to result in hypoglycemia during the next 24 hours (24). Plasma cortisol levels tend to be higher in the morning than in the afternoon, and this may help to explain why glucose levels tend not to fall with fasted exercise (31).

On the other hand, regular morning resistance exercise tends to increase glycemia and result in more glucose variability in recovery, whereas afternoon resistance exercise is associated with better glucose stability in recovery (32). The acute glucose responses related to morning and afternoon exercise should be made clear to individuals with type 1 diabetes, and mitigating actions such as using CGM, carbohydrate snacking, and/or reducing insulin doses can be implemented to help ensure exercise safety (33).

For individuals with gestational or type 2 diabetes, premeal exercise may lower premeal glucose concentrations and increase postmeal insulin sensitivity, thereby improving overall glycemia (34). However, a recent systemic review suggests that postmeal mild to moderate activity may be the more favorable approach for limiting postprandial glucose excursions, at least in individuals with prediabetes or type 2 diabetes (35). Postmeal exercise has an acute glucose-lowering effect, as muscle contractions enhance skeletal muscle glucose uptake (35,36). Because of the acute nature of this mechanism, completing activity after each meal is likely more beneficial than only completing activity after one meal in people with prediabetes or type 2 diabetes (36,37).

For individuals with type 1 diabetes, performing light to moderate postmeal exercise (e.g., walking for 15–30 minutes) may be beneficial from a glucose-lowering perspective (38–40). However, if postmeal exercise extends beyond 30 minutes, hypoglycemia can often occur (41,42). On the other hand, if high amounts of protein or fat are consumed in a meal, the time to peak blood glucose can be delayed, leading to rebound hyperglycemia after exercise (43). Thus, it may be better to postpone the activity start time by 30–60 minutes after meals that have a delayed postprandial glucose excursion because of their fat and protein content (e.g., North American pizza). Moreover, after a mealtime insulin bolus, the amount of prandial insulin in circulation is elevated, and postmeal aerobic endurance exercise can promote a rapid drop in glucose level, so extra care should be taken to monitor glucose closely and have a snack ready if glucose trends toward hypoglycemia.

For this reason, it may be preferable to complete a planned endurance exercise session before meals for individuals with type 1 diabetes because circulating bolus insulin levels are often the lowest then (3). Endurance exercising before meals can also help to minimize the need for pre-exercise carbohydrate snacking to prevent exercise-associated hypoglycemia, but some carbohydrate during the activity might be needed to avoid hypoglycemia (44). HIIT and resistance exercise are associated with a lower hypoglycemia risk than endurance exercise in type 1 diabetes and are generally fine from a glucose perspective, whether performed before or after meals (45). Regardless of the timing of this exercise, CGM should be encouraged for individuals with type 1 diabetes, in whom glucose responses to exercise are highly individualized (33).

Francois et al. (46) first used the term “exercise snacking” to describe a protocol of three short, discrete exercise sessions, one before each meal, to help control postprandial glucose levels in individuals with prediabetes. Each exercise snack session was only 19 minutes in total length. Sessions consisted of a 5-minute walking warmup, six 1-minute sets of inclined walking on a treadmill (at ∼90% of the individual’s maximal heart rate) with a 1-minute walking or mild resistance exercise recovery between sets, and a 3-minute walking cooldown. These exercise snacks helped to flatten postmeal glucose levels and lowered mean glucose levels by ∼10 mg/dL over 24 hours compared with a no-exercise day (46). Similar results were found by Takaishi et al. (47) using a 6-minute moderate-intensity stair-climbing intervention done 90 minutes after a meal in men with impaired glucose tolerance. Together, these studies demonstrate the potential benefits to glycemic control of exercise snacks performed throughout the day.

One interesting epidemiological study shows that dog ownership is associated with a greater level of total weekly physical activity; lower BMI, blood pressure, and cholesterol levels; less depression; and a lower risk for the development of type 2 diabetes compared with not owning a dog (48). It may be that two or more short (15- to 30-minute) walks during the day, with or without a dog, could improve glycemia in both type 1 and type 2 diabetes.

For example, in individuals with impaired glucose tolerance, walking for 15 minutes after each meal was associated with improved post-dinner glycemic levels compared with one 45-minute walking session that occurred earlier in the day (37). Notably, both three short walks and one longer walk improved glycemia compared with not walking at all (37). Another study involving Hispanic/Latino adults with prediabetes or type 2 diabetes found that achieving higher step counts later in the day was associated with improved A1C and BMI (49). These studies demonstrate the potential benefit on glycemia of short activity bursts throughout the day, even if they are done at a lower intensity, such as with walking.

Given the number of potential variables that can influence the relationship between exercise time of day and overall health and glycemia in individuals with diabetes, it may not be surprising that current research findings are not always consistent between studies. Nevertheless, some commonalities in the current literature indicate where future research may be focused.

First and foremost, consistent exercise generally improves overall health and glycemia in individuals with diabetes, regardless of when it occurs or its intensity. Thus, from a practical perspective, the overarching conclusion for people with diabetes is to try to develop an exercise routine to which they can adhere. When viewed through this lens, anytime is the most optimal time to exercise, and individuals should consider their individual rhythms and routines to develop a program that will best position them for success.

There is some evidence that people with type 1 diabetes may experience more tangible health benefits, and with less hypoglycemia, by adhering to mild to moderate endurance training if they tend to exercise earlier in the day. In contrast, individuals with type 1 diabetes who exercise in the afternoon may find that HIIT or resistance training is more beneficial than endurance exercise in managing glucose variability. However, these results are not as apparent in individuals with prediabetes and type 2 diabetes.

Another common thread in the current research is that, although premeal exercise may help insulin sensitivity, postmeal exercise may offer the largest reductions in postprandial glucose for individuals with prediabetes or type 2 diabetes. On a related note, individuals may find a postmeal exercise “snack” or “burst” (i.e., a brief workout or stair climbing after each meal) to be an effective way to manage glucose levels throughout the day.

For people with type 1 diabetes, the variables influencing glycemia are somewhat more complex. Postmeal endurance exercise is generally beneficial from a glucose-lowering perspective, but it is important to consider a heightened risk of hypoglycemia. Thus, the duration and intensity of endurance activity should be adjusted accordingly in type 1 diabetes. Conversely, if hypoglycemia is a concern, premeal endurance exercise or HIIT or resistance training performed at any point pre- or postmeal may be beneficial for individuals with type 1 diabetes. Regardless of exercise timing, CGM and carbohydrate snacking based on glucose trends is recommended in people with type 1 diabetes.

M.C.R. serves on advisory boards for Eli Lilly, Indigo, Insulet, Supersapiens, and Zucara Therapeutics and has given industry-supported lectures for Eli Lilly, Insulet, Novo Nordisk, and Sanofi. No other potential conflicts of interest relevant to this article were reported.

M.C.R. researched the topic and drafted the manuscript. L.V.T. and S.R.P. researched the topic and edited the manuscript. M.C.R. is the guarantor of this work and, as such, takes responsibility for the integrity of the information provided in this review.