Key Points
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Calories are units of heat energy that measure the amount of energy you get from eating.
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Most adults require 1,600 to 3,000 calories daily, but fewer if they lose weight.
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You burn calories while sleeping but at a fairly slow pace.
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In order to burn more calories while sleeping, you need to boost your basal metabolic rate.
Calories: you've heard of them, you eat them, and if you're like most Americans you fear them, even though they seem to cluster in your favorite foods. What is a calorie, and what does it have to do with your health? Why do some foods have more than others, and how many do you really need?
This article answers all those questions and also looks at the relationship between calories and sleep. Your body is still at work and still burning calories while you sleep, but not in the same way as when you're awake. Find out more about how your eating and sleeping habits work together to keep you fit — or not.
What Are Calories?
Calories originated not in nutrition but as a scientific measurement of heat. In chemistry and physics, a calorie refers to the amount of heat necessary to raise the temperature of a gram of water by one degree Celsius (a "small calorie") or a kilogram of water by the same amount (a "large calorie"). Large calories are the kind in food measurements, which is why you sometimes see them called kilocalories or kcals.
This measurement reflects what your body does with the food you eat: converts it to energy. That's why your body is warm for as long as you're alive. When people talk about "burning" calories when you work out, they mean that conversion to heat, even though there isn't a literal fire inside you. The scientific term for that food-burning is "metabolism."
If you eat more food than you need for energy, your body stores the remainder as fat. Back in the days when food sources were a lot less reliable than they are now, these fat stores came in handy during lean times. Thus you also "burn" fat when you eat less food than your body needs.
What Are Examples of Calories in Foods?
All food calories come from one or more of three components: fat, protein, and carbohydrates. Protein and carbohydrates, in their pure form, have four calories per gram, while pure fat has nine calories per gram.
You don't normally eat them in their pure form, however. Most foods include a small quantity of all three and other ingredients that don't contribute calories but are nonetheless important: water, vitamins and minerals, and dietary fiber, a catchall term for plant matter that your body can't absorb.
Therefore, foods that are rich in fat (butter, cream, peanuts, and avocados, for example) are higher in calories by volume than foods that are mostly protein or carbohydrates (fish, chicken, bread, potatoes), which are higher in calories than foods that are high in water and fiber (celery, lettuce, oranges, grapes). Sugar is a carbohydrate, so high-sugar candies are similar in calories to starchy foods like bread and potatoes, although some kinds are also rich in fat.
There are multiple websites where you can look up the calories and other nutrition facts on specific foods, such as the U.S. Department of Agriculture's FoodData Central (which lists calories as "energy" in kcals). Packaged foods also list calories on their labels. The main thing to keep an eye on is what portion of food is in the listing — what is a "serving"? Once you understand that, you know how many calories you absorb when you eat it.
How Many Calories Do You Need a Day?
If you read the nutrition label on a package of prepared food, you'll see a footnote: "2,000 calories a day is used for general nutrition advice." That's because, in order to calculate the percentage of the recommended daily intake of various nutrients, the food manufacturers need a benchmark amount of food that a person eats in a day, and 2,000 calories is the benchmark the government uses.
That's a reasonable amount as an average, but there are actually huge variations in the number of calories each person needs; U.S. dietary guidelines range from 1,600 to 3,000 a day. Many factors influence this number.
Size
You've probably noticed that bigger people are usually bigger eaters, and science backs that up. Taller and more heavily built folks require more calories to maintain their weight than smaller people since they require more energy to move their bodies around.
Age
As a child, your smaller body required less food, but you needed more as you got bigger. Most people's calorie needs peak around college age, then gradually diminish as they age, and their metabolism slows down. By the time you hit your sixties, your daily calorie needs drop to about where they were in your early teens.
Activity
The more physically active you are, the more calories you burn. This doesn't have to involve a strenuous workout — playing a musical instrument burns a surprising number of calories due to all the small-muscle movements — but getting on your feet and moving around always burns more calories than sitting.
As a baseline, the FDA's guidelines consider you "moderately active" if you walk between one-and-a-half and three miles a day and "active" if you do three miles or more. The difference between those activity levels amounts to about 200 calories a day.
Of course, if you work out more strenuously than that, you burn quite a bit more.
Sex
Men generally burn more calories than women. This is partly due to their larger average size, but they also have higher metabolic rates than women (again, on average — there's a lot of individual variation).
Temperature
You've likely noticed that heat makes you feel languid while cold makes you want to move, and that's a sign of what's going on in your metabolism. After all, calories are units of heat, and your body has to work harder to heat itself when the air around you is cold. Even shivering burns calories!
Genes
Individuals have different innate metabolisms. Often you can see that in their energy level — people with higher metabolisms tend to be more restless and fidgety — and yes, fidgeting burns calories!
Calorie calculators online allow you to estimate your ideal caloric intake, but they can only measure some of those factors, not all. The most accurate reading of your metabolic rate requires a calorimeter, which is available at some healthcare and weight-loss clinics.
How Many Calories Do You Need To Eat a Day To Lose Weight?
If you got here because you're googling about calories, you're probably wondering how to lose weight, right? The simple answer is "eat fewer calories than you burn," but it's a little more complicated than that. As a registered dietician, Samantha Cassetty, said, "You might want to lose weight, but that doesn't mean your body wants you to lose weight."
In other words, if you start eating dramatically less food, your body could easily go into crisis mode and slow down your metabolism. It's normal, in fact, for people's metabolisms to "reset" after they lose weight, which is one reason it's so hard to keep weight off and even harder to lose it again. Even if you're impatient to take off some pounds, you don't want to restrict calories too much.
As a rule, health experts advise calculating how many calories you need per day to maintain weight, then trimming 500. Assuming you keep the same activity level, that adds up to a pound lost per week. Increasing activity increases the burn rate, but it also often makes you hungrier because you need food energy for exercise.
Different bodies respond differently to diet and exercise changes. This is why some people you know seem to lose weight easily while others struggle. "There are people who live in larger bodies and that's where their body wants to be," said Cassetty. "So it would be really hard and restrictive to reduce calories further than where they're comfortably at."
Because of that, consult a professional to design the right weight-loss program for you. In addition to metabolism, all sorts of personal factors prevent one-size-fits-all solutions to obesity.
How Many Calories Do You Burn Sleeping?
As you go about your day, your metabolism speeds up or slows down depending on what you're doing — and even what you're feeling. Anything that increases heart rate, from jogging to anxiety, boosts your metabolism. The rate at which you burn calories when you aren't doing anything is the basal metabolic rate (BMR).
When you sleep, you're usually about 15 percent below your waking BMR. There's some variation depending on your sleep stage; at the deepest stage, your metabolism is the slowest, while in REM (active dreaming) sleep, your metabolism is closer to its waking state. Also, if you have a condition like sleepwalking, where you move around more than normal during sleep, you burn more calories than the average person.
On average, people burn between 40 and 55 calories an hour when they're asleep. That's not much — about the equivalent of a small orange every hour. This might lead you to believe that people who sleep less are leaner than those who clock the recommended seven to nine hours per night, but that is not the case.
In fact, there's a correlation between poor sleep and obesity. The causation probably runs both ways, but there's evidence that lack of sleep makes you hungrier, perhaps because your body is going into stress mode (recall the earlier point about fat preparing you for hard times). Lack of sleep also gives you less energy during the day, so you make up for a more active night with a more sluggish day.
Is There a Way To Burn More Calories While Sleeping?
The most important way to burn more calories during sleep is to increase your BMR. While your BMR is innate to some extent, increasing it through regular exercise is possible. The key point is that exercise must increase your heart rate and breathing — activities like running, swimming, aerobics, dancing, etc. Do these for at least 45 minutes at least thrice a week, and your BMR should increase.
There's also evidence that sleeping in cooler temperatures increases metabolism. One study found that subjects who spent a month sleeping in a 66-degree room with only sheets for covering showed a 10 percent increase in fat metabolic activity and improved sensitivity to insulin (which lowers the risk of diabetes). Sleeping in that temperature range also improves sleep quality for most people, which leads to the next point.
Another way to burn more calories while sleeping is to ensure that you get enough REM sleep. REM is the last sleep cycle stage, so it most likely ends short if something disrupts the cycle. Therefore, peaceful and uninterrupted sleep is the key to REM.
How do you get it? That's really the entire theme of Snooze because there are so many things that can derail good sleep: disorders, health problems, bedding, stress, noise, work schedules, and on and on. However, there are some general principles to follow to improve your chances of sweet dreams.
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Exercise, but keep it at least two hours before bedtime.
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Turn off unneeded lights and electronic devices in the evenings.
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Avoid large meals and alcoholic drinks within two hours of bedtime.
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Avoid caffeine after lunch.
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Establish a routine of relaxing evening habits, perhaps with the help of a sleep app.
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Try to go to bed and wake up at the same times every day.
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Get some sun during the day to help maintain your internal circadian rhythm.
If your sleep is still lacking, consult a healthcare professional to see if you have a sleep disorder.
Calories and Sleep: You Need Both To Live
In a society where you can never be too rich or too thin, calories and sleep often seem to get in the way. You need both of them to stay in good health — just not too much.
The right balance of eating and sleeping is crucial to maintaining your health and keeping the energy to do everything you want to accomplish. With a proper diet and good sleep, your quality of life improves no matter your body weight.
Want more tips for better sleep? Follow Snooze.
