Is there a best time of day to exercise for weight loss? Original paper

Physical activity is effective for weight loss and preventing weight gain. Typical exercise prescriptions include suggested frequency, intensity, type, and duration of activity. However, the time of day when exercise is performed could be another important variable for optimizing the benefits of exercise.

Unfortunately, most weight-loss studies don’t control for the time of day that participants exercise, and few even report this variable. Since both the morning and evening are popular times for exercise, it’s worth investigating whether one of these windows is best for weight loss.

In this randomized controlled trial, 88 adults with overweight or obesity (average age of 40, average BMI of 30, 78% women, 22% men) completed 12 months of aerobic exercise training in the morning (AMEx, between 6 a.m and 9 a.m.), in the evening (PMEx, between 4 p.m and 7 p.m.), or a nonexercise control intervention.

The exercise groups were prescribed the goal of accumulating more than 250 minutes of exercise per week. All 5 exercise sessions were supervised during weeks 1–4, which decreased to 4, 3, and 2 supervised sessions during weeks 5–6, 7–8, and 9–12, respectively.

The primary study outcomes were body weight, fat mass, fat-free mass, and body fat percentage. The secondary outcomes included cardiorespiratory fitness (VO₂peak), blood pressure, blood lipids (HDL, LDL, total cholesterol, triglycerides), blood glucose, and resting metabolic rate (RMR). Participant compliance, adherence, perceived exertion, and enjoyment were also assessed as secondary outcomes. All outcomes were assessed at baseline, 6 weeks, 12 weeks, and after a 3-month and 6-month unsupervised follow-up period.

The participants lost weight after AMEx (−2.7 kg/6.0 lb) and PMEx (−3.1 kg/6.8 lb). Weight loss continued in the AMEx group (−3.9 kg/8.6 lb) and PMEx group (−4.5 kg/9.9 lb) at the 3-month follow-up time point and in the PMEx group (−4.7 kg/10.4 lb) at the 6-month follow-up time point. Fat mass, fat-free mass, and body fat percentage also decreased in the AMEx and PMEx groups at 12 weeks.

VO₂peak increased in the AMEx (+4.7 mL/kg/min) and PMEx (+4.2 mL/kg/min) groups. Diastolic blood pressure decreased in the AMEx (−3 mmHg) and PMEx (−5 mmHg) groups, and systolic blood pressure decreased in the PMEx group (−7 mmHg). Blood glucose decreased in the PMEx group, and total cholesterol decreased in the AMEx group, but there were no changes in HDL, LDL, or triglycerides in any of the groups.

RMR did not change in any of the groups, though energy intake decreased in the AMEx and PMEx groups. Compliance, adherence, perceived exertion, and enjoyment were similar between groups.

Changes in body composition in the morning and evening exercise groups

Changes in VO₂peak in the morning and evening exercise groups

Although there were no statistically significant differences between the AMEx and PMEx groups for the study outcomes, a greater percentage of participants achieved clinically significant improvements in weight loss (more than 5%) and VO₂peak (more than 3.5 mL/kg/min) after PMEx (33% and 75%, respectively) compared to AMEx (19% and 69%, respectively). While the implications of these findings aren’t immediately relevant, they may indicate a more consistent, global improvement from a.m. exercise compared to p.m. exercise.

To date, there have been 3 studies designed to investigate the effects of exercise performed at different times of day on weight loss.

One 3-month study in postmenopausal women found that walking in the evening reduced body fat to a greater extent than morning walking, though both groups improved their aerobic fitness to a similar extent.^[1] A 6-week study in women with overweight found that aerobic exercise performed in the morning (but not the evening) reduced weight, body mass index, and abdominal circumference.^[2] However, this finding may have been confounded by the fact that the morning exercise group also spontaneously reduced their energy intake more than the evening exercise group. In support of this finding, another study in adults with overweight and obesity reported that participants who were randomized to morning exercise lost more weight than a group randomized to evening exercise.^[3]

Along with the findings of this study, the literature doesn’t definitively show that exercise timing influences weight loss. Even though some studies favor morning exercise and some evening exercise, there doesn’t appear to be a single optimal time to exercise.

However, what does seem to be important when it comes to successful weight loss and weight loss maintenance is performing exercise at the same time of day, whether that’s in the morning, afternoon, or evening. This may be related to habit formation. Successful weight loss maintainers tend to exercise at a more consistent time of day rather than have inconsistent patterns, and they also engage in more moderate-to-vigorous physical activity overall.^[4][5] These benefits of temporally consistent exercise are independent of the time of day at which exercise is performed. Thus, when prescribing exercise, it may be important to dictate a particular time of day when exercise should occur, but this time should be individualized to each person’s preferences and life schedule. The best advice seems to be to find the most opportune time to exercise (for you), and stick to it!

Although exercise timing is important for enhancing long-term adherence, there are also biological mechanisms proposed to underlie the benefits of time-of-day-specific exercise.

The body’s circadian system is involved in a number of physiological processes related to metabolism and physical performance. For example, the body’s sensitivity to the hormone insulin may be greater in the morning, whereas fat metabolism may be enhanced in the later afternoon and evening. In theory, exercising at different times of the day, corresponding to the peaks and troughs in different hormones and metabolic processes, could specifically target improvements in some of these parameters, like fat metabolism or insulin sensitivity. Circadian timing has also been applied in athletic performance contexts, where it’s been shown that peak physical performance tends to correspond to the late afternoon, when body temperature reaches its maximum.^[6] Much less is known about the cardiometabolic benefits of morning and evening exercise and whether one time is better for achieving health benefits.

Exercise at different times of day may also influence weight loss by affecting appetite and energy intake. Though appetite wasn’t specifically measured in this study, the participants did spontaneously reduce their energy intake by about 500–600 calories per day (caloric intake increased in the control group by about 100 calories/day), which could indicate that their appetite was reduced. However, because they were enrolled in a weight-loss study, this knowledge could have also influenced their choice, conscious or not, to reduce their energy intake. Similar studies have either not measured^[1][3] or found no change^[2] in appetite during an exercise and weight-loss intervention. This is supported by results from acute studies that find that a single moderate-intensity exercise session performed in the morning or the evening doesn’t affect appetite or 24-hour energy intake in women with overweight.^[7]

The maintenance of some of the health improvements at the 3-month and 6-month follow-up time points in this study support the benefits of consistent exercise timing for long-term adherence. Participant adherence wasn’t directly measured during the follow-up period, but the fact that participants maintained their weight loss and improvements in VO₂peak for up to 6 months after the study suggests that exercise habits were maintained to some degree. In fact, the participants in the AMEx and PMEx groups continued to lose weight at the 3-month follow-up time point, a trend that continued at the 6-month follow-up time point in the PMEx group. The maintenance of the cardiometabolic health improvements is very promising, especially when considering that the participants were not monitored and had no contact with study staff in the 6 months after the study’s completion.

A recent review on the topic of exercise timing and weight loss elegantly concluded that “exercise at any time of day is important for overall health but the timing of exercise may have a differential impact on energy expenditure, energy intake, and sleep, with all these factors ultimately affecting body weight regulation.”^[8] This underscores the importance of making exercise (at any time) a priority. However, if someone has the freedom to choose, it may be worth experimenting with exercising at different times of day in order to see how the time change affects preferences, appetite, sleep, and health-related outcomes. Everyone will respond differently to exercise and exercise timing, so what’s important is not finding the best time to exercise, but the best time for you to exercise.