Should I Workout in the Morning or at Night?

The timing of exercise has been a topic of interest in the field of health and fitness for decades. While it's generally accepted that exercise is beneficial regardless of when it's performed, there has been considerable debate as to whether exercise at different times of the day may have differential impacts on health outcomes, including weight loss. To date, there have been several studies designed to investigate the effects of exercise performed at different times of the day on weight loss.

One 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. Another 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. 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 the morning exercise 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.

Despite some studies suggesting that morning or evening exercise may have advantages for weight loss, the literature doesn't definitively show that exercise timing influences weight loss. However, what seems to be important for 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, as 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.

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.

Exercise at different times of day may also influence weight loss by affecting appetite and energy intake. While appetite wasn't specifically measured in some studies, 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 or found no change 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.

The maintenance of some of the health improvements at the 3-month and 6-month follow-up time points in one study supports 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. 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 timing may have a differential impact on energy expenditure, energy intake, and sleep, with all these factors ultimately affecting body weight regulation." Therefore, when it comes to exercise and weight loss, the timing of exercise may not be as important as exercising consistently at the same time of day. The benefits of temporally consistent exercise are independent of the time of day at which exercise is performed, and successful weight loss maintainers tend to exercise at a more consistent time of day rather than have inconsistent patterns. This may be related to habit formation, which is crucial for long-term adherence.

That being said, exercising at different times of the day may influence weight loss by affecting appetite and energy intake, which can impact body weight regulation. Additionally, the body's circadian system is involved in a number of physiological processes related to metabolism and physical performance, and exercising at different times of the day may specifically target improvements in some of these parameters. 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.

However, much less is known about the cardiometabolic benefits of morning and evening exercise and whether one time is better for achieving health benefits. More research is needed to fully understand the impact of exercise timing on weight loss and overall health.

The most important aspect of exercise for weight loss is consistency, regardless of the time of day at which exercise is performed. While exercising at different times of the day may have some impact on weight loss and overall health, more research is needed to fully understand this relationship. Ultimately, it's important to find the most opportune time to exercise for each individual and stick to it for long-term adherence and success.