“Insulin tells the muscles to be a sponge and absorb the glucose in the blood,” said senior study author Steven Malin, an associate professor in the Department of Kinesiology and Health. at Rutgers University in New Jersey.
“Think of it like water from a water tap. You turn on the water and a drop hits the sponge and it gets absorbed immediately,” Malin said. “But if you don’t exercise and tighten those muscles, it’s like that sponge would sit for a few days and become rock hard. A drop of water won’t make it soft again.”
If sleep chronotype affects how our bodies use insulin and affects metabolism, then a night owl could be helpful in predicting a person’s risk for heart disease and type 2 diabetes, Malin added.
“The study adds to what we know,” says Dr. Phyllis Zee, director of the Center for Circadian and Sleep Medicine at Northwestern University Feinberg School of Medicine in Chicago, who was not involved in the study.
“There is good evidence that a late sleeper is linked to a higher risk of metabolic and cardiovascular disease,” says Zee, who is also a professor of neurology. “Several mechanisms have been proposed: sleep loss, circadian alignment, eating later in the day, and exposure to less morning light and more evening light, which have been shown to affect insulin sensitivity.”
Body Clock and Chronotype
All humans have a circadian rhythm – an internal 24-hour biological clock that regulates the release of the hormone melatonin to promote sleep and stop production so we wake up. Our biological clock also determines when we get hungry, when we feel most sluggish and when we feel excited enough to exercise, among many other bodily functions.
Traditionally, sunrise and sunset regulated the human sleep-wake cycle. Daylight enters the eyes, travels to the brain and produces a signal that suppresses the production of melatonin. When the sun goes down, the biological clock turns on the production of melatonin again and a few hours later sleep comes.
Your personal sleep chronotype, thought to be hereditary, can alter that natural rhythm. If you’re a congenital early bird, your circadian rhythm releases melatonin much earlier than normal, giving you energy to get most active in the morning. In night owls, however, the internal body clock secretes melatonin much later, making the early mornings sluggish and driving peak activity and alertness later in the afternoon and evening.
Fat or carbohydrates?
Researchers classified 51 adults without heart disease or diabetes into morning or evening chronotypes, based on their natural sleep and wake preferences. During the study, the participants ate a controlled diet and fasted overnight while their activity level was monitored for a week.
The research team has determined body mass, body composition and fitness level of each person, and measured levels of insulin sensitivity. In addition, researchers looked at how each person’s metabolism got most of their energy, either through fat or carbohydrates.
“Fat metabolism is important because we think if you can burn fat for energy, that will help the muscle absorb the glucose in a more sustainable way,” Malin said.
Burning fat can promote endurance and increased physical and mental activity throughout the day. Carbohydrates, on the other hand, are what the body uses for intense physical activity. Carbohydrates are burned faster, which is why many athletes load up on carbohydrates prior to a race or marathon.
The results of the test showed that early risers used more fat for energy at rest and during exercise than night owls in the study, who used more carbohydrates for fuel.
More research is needed, Malin said, to confirm the findings and determine whether the metabolic differences are due to chronotype or a possible mismatch between a night owl’s natural preference and the need to wake up early because of the hours set by society for work and school.
People who are constantly out of sync with their innate biological clocks are said to be in ‘social jet lag’.
“This goes beyond just diabetes or just heart disease,” Malin said. “It could point to a bigger societal problem. How do we help people who may be misaligned? Are we, as a society, forcing people to behave in ways that could actually endanger them?