Exercising between 7 and 9 a.m. appears to help people lose weight more effectively. A team of researchers from the University of Virginia reached this conclusion after analyzing data from a large-scale study involving 5,285 middle-aged Americans.
The scientists found that early risers who do morning workouts have a lower body mass index (BMI) and smaller waist circumference compared with people who exercise at noon or in the evening.
Morning workouts are easier to stick with because evenings bring distractions like phone calls, emails, and meetings.
Clinical psychologist Rebecca Krukowski, the study’s lead author, says weight-loss success also depends on whether someone is a morning person or a night owl. People who naturally wake early shift their circadian rhythms earlier, which can improve sleep quality and, in turn, help with weight loss.
What the Researchers Discovered
They used data from the National Health and Nutrition Examination Survey (NHANES), which assesses the health of about 5,000 U.S. adults and children each year. The team focused on a three-year study that examined how exercise timing affected weight. During the study, each participant wore a hip-mounted accelerometer (a type of fitness tracker).
The volunteers were divided into three groups based on their exercise times: morning (7:00-9:00), daytime (11:00-1:00), and evening (5:00-8:00). The first group included 642 individuals, the second had 2,400, and the third consisted of 2,187.
Morning-group participants were 10 to 13 years older on average than people in the other groups. Most had higher levels of education, were non-smokers, and didn’t drink alcohol. They also followed healthier diets and ate less than the other participants.
The team collected BMI and waist circumference measurements for each group, then analyzed the relationship between those measures and the time of day people exercised.
The morning group had the lowest BMI at 25.9 kg/m², which is slightly above the healthy range of 18.5–24.9 kg/m².
Participants in the daytime and evening groups showed similar results, with a BMI of 27.6 in the daytime group and 27.2 in the evening group. Those numbers fall into the overweight range, according to the Daily Mail.
The average waist circumference for participants in the morning group was also the smallest at 91.5 centimeters. In contrast, the evening group averaged 95 centimeters, while the daytime group averaged 95.8 centimeters.
What Other Scientists Think
The results were published in the journal Obesity and echo earlier studies that linked exercise timing to weight outcomes.
For instance, a 12-week study last year of 100 adult volunteers at Skidmore College found that women aiming to lose weight benefited most from morning exercise.
A study this year from the Karolinska Institute reached similar conclusions, but it came from experiments with mice: animals that ‘exercised’ early in the day showed the highest metabolic rates.
