In my last blog I wrote briefly about why it’s important for all of us to try to maintain relatively stable blood sugar levels. This goal is especially relevant to people who have diabetes and hope to minimize the risk of developing problems with their cardiovascular systems, eyes, kidneys, and nerves.

Like diet and sleep, physical activity is a key determinant of how blood sugar fluctuates each day. Researchers studying how nutrition affects blood sugar regulation have generally focused on what to eat and how much to eat. Similarly, scientists exploring whether physical activity influences blood sugar control have mostly fixated on exercise modality and workload. Meanwhile, research on the importance of the time of day of diet and physical activity interventions has largely been neglected.

Fortunately, this is changing. Timing’s time has come, and some of the best scientists are starting to identify how critical when we eat and when we move might be for our health. So, I was very pleased to see a group of scientists led by Harriet Wallberg-Henriksson from the Karolinska Institute in Stockholm recently publish work on this topic. Their findings are especially relevant to those of us who wish to improve our blood sugar regulation.

 

What is the best time of day to exercise to improve blood sugar regulation in people who have type-two diabetes?

There’s a strong body of evidence showing that high-intensity interval training (HIIT*) is a time-efficient and safe way to improve numerous aspects of cardiometabolic health, including blood sugar regulation. However, we haven’t known much about whether it’s better for people intent on improving their blood sugar regulation to do HIIT in the morning or afternoon. To address this gap in our knowledge, here’s what the researchers did:

Eleven middle-aged men who have type-two diabetes that they manage through diet and/or using metformin took part. They underwent two, two-week periods of HIIT with two-weeks of no training between.

During both training periods, the men did three HIIT sessions per week (on Monday, Wednesday, and Friday). The HIIT sessions proceeded as follows: After a seven-minute warm up, the participants did six repetitions of one minute of cycling at the highest load at which they could pedal at 75 RPM. Between repetitions, the men actively recovered by cycling at 75 RPM with minimal added load.

Now, the only difference between the training periods was that whereas in one of the periods the participants did the HIIT in the morning (at 8 AM), in the other they did the HIIT in the afternoon (at 4 PM).

To track their blood sugar, the men wore continuous glucose monitors. The researchers used data from the monitors to calculate participants’ average blood sugar levels every hour.

So, did the timing of HIIT have any effects?

 

Advantage afternoon HIIT

Only afternoon HIIT reduced blood sugar levels on exercise days. Morning HIIT actually increased blood sugar levels on exercise days**. And the magnitudes of these effects are clinically meaningful.

Want to optimize your blood sugar regulation? If so, you may be better off doing HIIT in the afternoon than in the morning Click To Tweet

The findings are consistent with a previous study of people with type-one diabetes. This prior work showed that, compared to afternoon exercise, morning exercise was less likely to lead to blood sugar levels later dropping too low.

Nevertheless, there’s not a strong consensus about the best time to do HIIT yet. Research in which people who have type-two diabetes completed morning HIIT 90 minutes after a standardized meal showed that the exercise bout improved blood sugar regulation. However, I’d put more stock in the findings of the latest study because in it blood sugar was measured over a longer period, during which the participants did multiple bouts of exercise.

 

Why might the timing of exercise affect blood sugar regulation?

Honestly, I don’t think we really know the answer to this question. As I discuss in the Circadian OS program (available to Pro members), our bodies’ clocks optimize our biology according to the time of day. Various processes peak in the afternoon, including core body temperature and metabolic rate. And our muscle cells have their very own clocks that time various processes in anticipation of physical activity at certain hours each day. Partly as a result of the effects of these clocks, it could be that exercise that relies heavily on carbohydrate use, such as HIIT, sprinting, and weight training, is best placed in the afternoon.

Given these changes in core body temperature and skeletal muscle metabolism, it’s no surprise that many studies have shown that we’re typically strongest and most powerful in the afternoon. I therefore wonder if participants completed more work during the afternoon HIIT sessions. Were this the case, it’s plausible that one consequence would be a lowering of blood sugar – higher exercise performance may lead to better metabolic regulation.

Another possibility is that it’s the timing relationship between physical activity and diet that’s important, as some studies of other animals have shown. Were this the case, the negative effects of morning HIIT might be redeemable by appropriately adjusting diet timing.

And yet another possibility is simply that participants lost a little sleep in order to arrive at the lab on time during the morning training period. This isn’t discussed in the paper but makes sense to me, for sleep loss consistently worsens blood sugar regulation.

We should also remember that our bodies (and our bodies’ clocks) are adaptable. Interestingly, the average blood sugar levels were numerically lower in the second week of morning HIIT than the first. So, it may be that the participants were adapting to the morning exercise, and perhaps the difference between the conditions would have been nullified had the training continued for longer.

 

Key takeaways

Based on what we know so far, if your goal is to optimize your blood sugar regulation then you may benefit from saving your HIIT for the afternoon. My guess is that this is also true for other forms of vigorous exercise, such as lifting weights or sprinting.

Nonetheless, if your schedule only allows you to exercise in the morning, please don’t read this and think you should stop exercising! Many other studies have shown that well-executed morning exercise training produces lots of health benefits, after all.

 

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*The goal of HIIT is to accumulate a substantial amount of high-intensity exercise by interspersing brief bouts of near-maximal efforts with periods of active or passive recovery.

** To assess whether the training affected other aspects of metabolic regulation, the participants also gave blood samples at baseline and at the end of each training period. Afternoon HIIT increased concentrations of parathyroid hormone and thyroid-stimulating hormone, and reduced T₄ levels. Morning exercise increased thyroid-stimulating hormone concentration only. I’m not sure we can make inferences about what the consequences of these minor changes might be.

 

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