Some of us leap out of bed eager to get on with the day; for others, emerging from under the covers is left until the last possible minute—often because we have been awake until the small hours.
And most of us know that we either function better in the morning or the afternoon and adapt our working schedules to suit our ‘early bird’ or ‘night owl’ tendencies. But can our chronotype affect not only our functioning, but also our health?
Research studies have suggested that chronotype, and particularly chrononutrition—what times of day we eat — may indeed affect health, but the findings are not yet conclusive.
Now, a study published in Experimental Physiology has found that our sleep/wake cycles are associated with our body’s metabolism, with night owls having a reduced ability to use fat for energy. This may increase their risk of type 2 diabetes and heart disease.
Early and late chronotypes
A team of researchers from Rutgers University, NJ, and the University of Virginia, VA, divided a group of 51 adults into early or late chronotypes based on their answers to a questionnaire.
All participants were non-smokers, free of cardiovascular disease, cancers, and metabolic diseases, and were sedentary, exercising less than 60 minutes a week.
Using imaging techniques, researchers assessed the participants’ body mass and body composition. They also tested insulin sensitivity.
Participants wore an accelerometer on the right hip, during the day, for 7 days to record what times of day they were most active. Researchers compared this data with the chronotype from the questionnaire to determine whether chronotype was influencing activity patterns during the day.
After making them fast for 12 hours, researchers tested participants at rest and during exercise to assess what they were metabolizing to provide energy. They then took breath samples to calculate their fat and carbohydrate metabolism at rest and when exercising.
“Measuring metabolism during rest and exercise allowed us to see how changes in movement throughout the day could impact or relate to health.”
— Dr. Steven K. Malin, lead author, associate professor of kinesiology and health at Rutgers University, NJ
Comparing early birds and night owls
The researchers noted no significant differences in age, body mass, or metabolic syndrome between the groups. However, they did find differences in how energy sources were used by those with early and late chronotypes.
Early birds used more fat for energy than night owls. They were also more insulin sensitive— their cells used glucose more effectively, reducing blood sugar.
Night owls tended to be more insulin resistant, meaning they required more insulin to lower blood glucose levels, and they tended to use carbohydrates as an energy source rather than fat.
Insulin resistance indicates a greater risk of type 2 diabetes.
Dr. Malin told Medical News Today: “A key finding was that individuals with later chronotype were indeed less able to respond to insulin by promoting glucose uptake towards storage. And that observation related to how much fat was used for energy. This aligns with ideas that low fat metabolism relates to insulin resistance by either defects in the mitochondria […] or accumulation of fat metabolites that impair insulin action on tissues like muscle.”
Differences in muscle quality
The researchers found no significant difference in muscle mass between the two chronotypes, although muscle mass was slightly higher in those with late chronotypes.
They suggest that their finding of greater fat oxidation in early chronotype may be due to differences in skeletal muscle quality rather than quantity.
Dr. Malin explained: “Based on our work so far, our sense is something is dysregulated in skeletal muscle quality. […] The ability to use fat was directly related to maximal aerobic fitness, which is partly linked to mitochondrial function. Indeed, people of later chronotype had lower fitness when measured by VO2max [maximum amount of oxygen the body is capable of using during high-intensity activities], in addition to being more sedentary through the day.”
“That lower amount of fat metabolism was directly related to how well insulin promoted glucose uptake towards storage. That is critical because stored glucose, known as glycogen, helps fuel muscle for physical activity,” he added.