Want Better Blood Sugar Control? Here’s When You Should Exercise

This article is inspired by Gretchen Reynolds’ article in The Washington Post.

If you’ve been exercising to manage your blood sugar, especially if you have Type 2 diabetes, when you work out might matter almost as much as whether you work out.

A new review published in Trends in Endocrinology and Metabolism compared morning versus afternoon exercise in people with and without Type 2 diabetes. The results: for those with diabetes, the same workout done in the afternoon controlled blood sugar better than the identical session done in the morning.

In some cases, early morning exercise actually worsened blood sugar and insulin sensitivity in people with Type 2 diabetes, and those effects lasted for hours.

Why morning exercise can backfire

The problem is the “dawn phenomenon” — a natural cortisol spike that happens when you wake up. Cortisol gets you moving, but it also tells your liver to dump stored glucose into your bloodstream.

In people without diabetes, the pancreas releases insulin, which moves that glucose into muscle cells. But people with Type 2 diabetes are insulin resistant. Both blood sugar and insulin rise and stay high. Add a hard morning workout to the mix, and you make it worse.

Intense exercise raises cortisol further and increases your muscles’ demand for fuel, so the liver releases even more glucose. If you’re insulin resistant, that sugar has nowhere to go. It just sits there.
Afternoon or evening workouts avoid this problem. Exercise later in the day led to lower blood sugar that lasted up to 24 hours in study participants.

What to do

If you have Type 2 diabetes and work out in the morning, you don’t have to stop. Light activity like brisk walking doesn’t seem to cause the same blood sugar spike that hard exercise does.

But if you can move your workout to the afternoon or evening, the metabolic benefit could be real.

It’s also worth noting that exercise at any time still helps with diabetes. Movement improves insulin sensitivity, supports heart health, and helps regulate weight. Timing just optimizes the effect.

The circadian angle

What’s interesting here is that exercise acts as a “zeitgeber” — a cue that syncs your circadian rhythm.

Every cell in your body runs on a roughly 24-hour clock, coordinated by molecular timers that respond to light, food, and movement. In people with Type 2 diabetes, these clocks often fall out of sync. That disrupts everything from hormone release to energy metabolism.

Exercise helps reset those clocks, but only if the timing matches your body’s rhythm. If you work out when cortisol and blood sugar are already spiking, you might be sending mixed signals to a system that’s already confused.

What we don’t know

Most of the studies reviewed (1, 2, 3) involved middle-aged men. It’s unclear if women or older adults respond the same way. We also don’t know how timing affects other outcomes like heart disease risk, sleep, or longevity.

But the pattern is consistent: if you’re managing Type 2 diabetes and wondering when to work out, later in the day looks better.

For everyone else, the best time to exercise is whenever you’ll actually do it. Consistency will always beat perfection.