Keeping an irregular schedule may change how many calories you burn
Timing is everything. Even how many calories someone burns while resting can depend on the time of day, new data show. These findings add to evidence that when people eat and sleep may be as important as what they eat for maintaining proper health.
People burn about 129 more calories when resting in the afternoon and evening than they do resting in the early morning. Morning is better for burning carbohydrates — sugars and starch. The body is more likely to burn fats in the evening.
Calories burned at rest fuel breathing, blood circulation and brain activity. They also help maintain body temperature. But there had been conflicting data on whether people at rest burn calories at a fairly constant rate. Some data suggested the rate might rise and fall in a daily — or circadian (Sur-KAY-dee-un) — rhythm. Metabolism is the rate at which the body burns calories to fuel activities.
The new study shows that a body’s resting metabolism is governed by circadian clocks.
Jeanne Duffy is a neuroscientist. She works at Brigham and Women’s Hospital in Boston, Mass. She and her colleagues followed seven people. They remained in windowless rooms for three weeks. That kept them from getting any clues to the time of day. Each night, the seven went to bed four hours later than the night before. The schedule change allowed the researchers to study the natural body rhythms of each subject without outside influences (such as sunlight and darkness).
Study recruits all showed clear rhythms for when they burned calories. But the timing of the highest and lowest rates could vary from person to person. For instance, calorie burning at rest peaked on average around 5 p.m. But some people peaked earlier, at around 2 p.m. Others peaked around 8 p.m. Calorie burning was lowest around 5 a.m. (but ranged from about 2 a.m. to 8 a.m., depending on the person).
That variability is normal for circadian rhythms, Duffy says. After all, some people are morning people. Others may be night owls. The timing of their daily rhythms reflects those differences.
“Regularity is really important,” Duffy says. Irregular schedules interrupt circadian rhythms. That in turn can throw off your metabolism, which can cause people to burn fewer calories. Studies have already shown that shift work (working at night) and chronic sleep loss can lead to weight gain and health problems. It doesn’t matter exactly what time people get up or eat, Duffy says. The most important thing is that they keep to a regular schedule. This includes the weekends — no matter how tempting it is to sleep in.
Duffy’s team shared its findings November 8 in Current Biology.
average (in science) A term for the arithmetic mean, which is the sum of a group of numbers that is then divided by the size of the group.
biology The study of living things. The scientists who study them are known as biologists.
calorie The amount of energy needed to raise the temperature of 1 gram of water by 1 degree Celsius. It is typically used as a measurement of the energy contained in some defined amount of food.
carbohydrates Any of a large group of compounds occurring in foods and living tissues, including sugars, starch and cellulose. They contain hydrogen and oxygen in the same ratio as water (2:1) and typically can be broken down in an animal’s body to release energy.
chronic A condition, such as an illness (or its symptoms, including pain), that lasts for a long time.
circadian rhythm Biological functions such as body temperature and sleeping/waking times that operate on a roughly 24-hour cycle.
circulation (adj. circulatory) A term that refers to the pumping of some fluid repeatedly throughout a system of vessels. (in medicine) The pumping of blood through the arteries and smaller types of vessels (and from there into other organs and tissues).
colleague Someone who works with another; a co-worker or team member.
constant Continuous or uninterrupted.
fat A natural oily or greasy substance occurring in plants and in animal bodies, especially when deposited as a layer under the skin or around certain organs. Fat’s primary role is as an energy reserve. Fat also is a vital nutrient, though it can be harmful if consumed in excessive amounts.
fuel Any material that will release energy during a controlled chemical or nuclear reaction.
metabolism (adj. metabolic) The set of life-sustaining chemical reactions that take place inside cells and bigger structures, such as organs. These reactions enable organisms to grow, reproduce, move and otherwise respond to their environments.
neuroscientist Someone who studies the structure or function of the brain and other parts of the nervous system.