When You Exercise Matters: How Time of Day Affects Blood Sugar During Intense Activity

Researchers at a Canadian institution investigated how the time of day affects blood glucose levels during intense, short bursts of cycling exercise in adults with Type 1 diabetes. They recruited 10 adults (ages 22–26, on average) who had lived with Type 1 diabetes for about 10 years.

Each participant completed two identical workout sessions on different days: one in the morning at 8 a.m. and another in the evening at 8 p.m. Both sessions took place 60 minutes after eating a standardized meal. The workouts involved maximal sprints on a stationary bike—the kind of short, all-out effort that demands a lot from the body in a brief time.

Evening exercise produced a noticeably larger drop in blood glucose compared to morning exercise. After evening workouts, glucose fell by about 3.56 mmol/L on average, while morning workouts caused a drop of about 2.01 mmol/L—roughly 44% less.

The research team also measured cortisol, a hormone the body naturally releases throughout the day. Cortisol levels were significantly higher in the morning (128.59 ng/mL) than in the evening (67.79 ng/mL). This difference in cortisol may explain why morning exercise was gentler on blood glucose levels.

Cortisol is one of the body's stress hormones, and it naturally peaks in the early morning hours. One key function of cortisol is to raise blood glucose—a protective mechanism that helps fight drops in blood sugar. Because cortisol is higher in the morning, it may act as a buffer, partially offsetting the glucose-lowering effect of exercise.

The researchers suggest this natural hormonal difference could mean that morning exercise is less likely to cause sharp blood sugar drops, while evening exercise—when cortisol is lower—may bring glucose down more steeply. For people managing Type 1 diabetes, this adds another layer to consider when planning workouts.

This research was small, involving only 10 adults, and looked at just one type of exercise: brief, intense sprints. Results may differ for steady-state cardio, strength training, or longer workouts. Because the group was also mostly sedentary and relatively young, findings might not apply equally to all people with Type 1 diabetes.

Individual responses to exercise vary considerably—what holds true on average for a group may not match your personal experience. If you're interested in understanding how your blood glucose responds to exercise at different times of day, working with your healthcare team to monitor your own patterns remains valuable.

Source: Canadian journal of diabetes. Evidence type: PubMed indexed literature. Type1Cure is an information and intelligence hub, not a medical advice service. This article summarizes published research and does not provide diagnosis, treatment, or personal medical guidance. Always talk to your own care team before changing anything about your Type 1 diabetes management.