Regular physical activity is a cornerstone treatment for the prevention and management of multiple chronic diseases, including type 2 diabetes (T2DM) and obesity [1]. It elicits a myriad of physiological benefits across the life course, including favourable effects on body composition and markers of glycaemic control. That said, its application is often based on a one-size-fits-all approach, and there is considerable variability in how individuals respond.

Alongside investigations into the optimal volume and intensity of physical activity, the best time of day to be physically active remains an open question. For instance, recent randomised trials have indicated that undertaking late-afternoon or evening activity yields superior improvements in glucose control compared to morning activity [2, 3], culminating in the lowest incidence of mortality and cardiovascular morbidity in meta-analyses (which also includes those living with obesity and/or diabetes) [4]. Conversely, a recent meta-analysis suggested that, in individuals living with T2DM, morning exercise in the fasted state may be more beneficial for overall 24-h glucose levels than exercise undertaken at other times of the day [5].

In this issue of the journal, Stein et al. add to existing literature by investigating the cross-sectional associations between the timing of physical activity and obesity and T2DM risk in 61,116 German adults [6]. This large cohort study categorised 24-h accelerometer data into physical activity conducted in the day (06:00–23:59) and night (00:00–06:00), with the former being further categorised into morning (06:00–11:59), afternoon (12:00–17:59) or evening periods (18:00–23:59). Acceleration (mg), captured over a 7-day period was used as a proxy for total physical activity, reflecting activity over the whole day, rather than the more traditional bouts of moderate-to-vigorous activity. Consistent with previous evidence, results demonstrated that when comparing the highest vs. lowest quartiles within timing categories, morning, afternoon and evening physical activity were all associated with a lower risk of T2DM and obesity, with the risk reductions ranging from 20 to 64%. However, when daytime physical activity categories were directly compared, interaction analyses demonstrated that afternoon and evening yielded the strongest inverse associations with T2DM and obesity risk. In contrast, night-time physical activity was associated with a higher risk (up to 58%), potentially driven by the misalignment of behavioural and circadian rhythmicity, which is known to cause significant disruption to glycaemic control and thus be a contributing factor to T2DM [7].

In order to reiterate their findings, the authors also examined the impact of theoretically substituting physical activity from one time period (i.e., morning, afternoon or evening) to another. They demonstrated that replacing each standard deviation of morning activity with afternoon or evening activity was associated with a lower risk of obesity and T2DM. It is important to re-emphasise that although this analysis demonstrates that afternoon and evening activity provides additive health benefits in terms of T2DM and obesity risk, morning activity in itself is not detrimental.

It should be noted that establishing an association between physical activity and circadian rhythms is challenging. The considerable variability in human physiology is complicated further by potential confounders, some of which were included in the analysis. However, as noted by the authors, key lifestyle behaviours (e.g., meal composition and timing) were not available, somewhat limiting the interpretation and implementation of these cross-sectional findings. That said, this type of analytical approach, alongside other methodologies (e.g., compositional analysis) are fundamental for hypothesis generating research, with the aim of narrowing the gap between current knowledge around 24-h behaviours and clinical care. This will ultimately allow greater patient choice and more personalised, evidence-based intervention care.

Given the intricacy of physical behaviours across a 24-h period, there is a need for future research to continue exploring potential interactions (i.e., sleep and physical activity) and their association with key health outcomes (i.e., glucose) in controlled, experimental trials and prospective analyses. Although the authors adjusted for sleep duration, it is also necessary to consider other important components of sleep architecture (i.e., quality and timing) [8]. More specifically, chronotype, a diurnal characteristic that identifies individuals as having a preference for morning (i.e., go to bed early and get up early), evening (i.e., go to bed late and get up late) or neither (intermediate) has been linked with many chronic diseases, including T2DM and obesity [9, 10]. Future investigations examining whether the alignment between chronotype preference and physical activity timing modifies associations with health will further enhance our knowledge around the chronotherapeutic potential of physical activity.

In conclusion, physical activity improves overall health and the risk of T2DM and/or obesity, regardless of when it occurs. Thus, the overarching message should be to try to develop a physical activity routine to which individuals can adhere. When viewed through this lens, anytime is the right time to be physically active. However, in the era of precision medicine, we may want to consider individual rhythms, routines and 24-h physical behaviours in order to maximise metabolic benefits.