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ATLANTA — If you prefer to go to bed and get up later — a sleep chronotype known as being a night owl — you may be at higher risk for type 2 diabetes and heart disease, a new study found.
Night owls were more sedentary, had lower aerobic fitness levels and burned less fat at rest and while active than early birds in the study. Night owls were also more likely to be insulin-resistant, meaning their muscles required more insulin to be able to get the energy they need, according to the study published Monday in the journal Experimental Physiology.
"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 about it like water from a water faucet: You turn the water on and a drop touches the sponge and is immediately absorbed," Malin said. "But if you're not exercising, engaging those muscles, it's like if that sponge was to sit for a couple days and get rock hard. A drop of water isn't going to make it soft again."
If sleep chronotype is affecting how our bodies use insulin and impacting metabolism, then being a night owl might be useful in predicting a person's risk for heart disease and type 2 diabetes, Malin added.
"The study adds to what we know," said 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 with the research.
"There is good evidence that being a late sleeper has been linked to a higher risk for metabolic and cardiovascular disease," said Zee, who is also a professor of neurology. "Several mechanisms have been proposed: sleep loss, circadian misalignment, eating later in the day and being exposed to less morning light and more evening light, which have all been shown to affect insulin sensitivity."
Body clock and chronotype
All humans have a circadian rhythm — an internal 24-hour body clock that regulates the release of the hormone melatonin to promote sleep and ceases production so that we wake. Our body clock also directs when we get hungry, when we feel most sluggish, and when we feel peppy enough to exercise, among many other bodily functions.
Traditionally, sunrise and nightfall regulated the human sleep-wake cycle. Daylight enters the eyes, travels to the brain and sets off a signal that suppresses melatonin production. When the sun goes down, the body clock turns melatonin production back on, and a few hours later sleep arrives.
Your personal sleep chronotype, thought to be inherited, may alter that natural rhythm. If you're an innate early bird, your circadian rhythm releases melatonin much earlier than the norm, energizing you to become most active in the morning. In night owls, however, the internal body clock secretes melatonin much later, making early mornings sluggish and pushing peak activity and alertness later into the afternoon and evening.
Sleep chronotype can have profound effects on productivity, school performance, social functioning and lifestyle habits, experts say. Early birds tend to perform better in school, and are more active throughout the day, which may partly explain why studies have found they have less risk of cardiovascular disease, Malin said.
Evening types may take more risks, use more tobacco, alcohol and caffeine, and are more likely to skip breakfast and eat more later in the day. In addition, research suggests "later chronotypes have higher body fat located more in the stomach or abdominal region, an area which many health professionals believe to be worse for our health," Malin said.
Fat or carbs?
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 levels were monitored for a week.
The research team determined each person's body mass, body composition and fitness level, and measured levels of insulin sensitivity. In addition, researchers looked at how each person's metabolism obtained most of their energy, either via fat or carbohydrates.
"Fat metabolism is important because we think if you can burn fat for energy that's going to help the muscle pick up the glucose in a more enduring fashion," Malin said.
Burning fat can promote endurance and more physical and mental activity throughout the day. Carbohydrates, on the other hand, are what the body uses for intense physical activity. Carbs are burned more quickly, which is why many athletes carb-load in advance of a race or marathon.
Results of the test showed early birds used more fat for energy at both rest and during exercise than night owls in the study, who used more carbohydrates as a source of fuel.
'Social jet lag'
There is a need for more research, Malin said, to confirm the findings and determine if the metabolic differences are due to the chronotype or a potential misalignment between a night owl's natural preference and the need to wake early due to the hours set by society for work and school.
People who are continuously out of synch with their innate body clock are said to be in "social jet lag."
"This extends beyond just diabetes or just heart disease," Malin said. "It may point to a bigger societal issue. How are we helping people who may be in misalignment? Are we as a society forcing people to behave in ways that might actually be putting them at risk?