All the biochemical processes your body does daily make up your metabolism. These processes consist of basic bodily functions -- such as pumping blood -- physical activity and digestion. The energy used to fuel these processes is measured as calories, which you get from food. Your metabolism naturally slows during the night, especially because you're most likely sleeping and not actively exercising, doing chores or eating food. Night is a time that your body rests so it has a chance to repair tissue, improve cognitive function and restore energy levels.
Components of Metabolism
The total number of calories your metabolism burns daily is known as the total daily energy expenditure, or TDEE, and is composed of three primary parts: your basal metabolic rate, physical activity and digestion. Your basal metabolic rate, or BMR, is the energy, or sum of calories, you use to simply exist. If you were to lie in bed all day, you'd still burn these calories to breathe, to allow your organs to function and to build or repair tissue. Although BMR is often described as how many calories you burn per day, the rate can vary minute by minute throughout the day. BMR comprises about 60 percent of your metabolism.
Your BMR is partially determined by genetics and your size, but is also influenced by your body composition. The more muscle you have, the higher your metabolic rate because muscle takes more calories to maintain than fat. This explains why men tend to have a higher metabolic rate than women. As you age and naturally lose muscle, your metabolism slows slightly. Severely restricting calories also affects the rate at which your BMR burns, because your body slows down to conserve energy.
In addition to BMR, your TDEE includes activity, whether exercise or non-exercise -- from running to washing the dishes or showering. About 30 percent of your metabolism comes from this activity.
Digestion makes up the final 10 percent of your metabolism. This is the energy you use to digest food and to process the nutrients.
Sleep and Metabolism
Nighttime can affect all aspects of your metabolism. When you sleep, your BMR slows down. During deep sleep, also called slow wave sleep, your metabolic rate is at its lowest. In a study published in "Metabolism" in 2009, researchers measured a consistent drop in metabolism during the first half of participants' sleep -- as much as a 35 percent decrease.
Rapid-eye movement sleep, or REM sleep, which is characterized by vivid dreams, typically occurs during the later portion of your sleep cycle. Electroencephalograms, or brain wave readings, taken during REM sleep show that your brain is working at a rate nearly as high as when you are awake. The study in "Metabolism" found that this brain activity leads to subtle fluctuations in an otherwise depressed metabolism. Even with these slight increases during REM sleep, your BMR is still -- on average -- about 10 to 15 percent reduced overnight.
Other aspects of your metabolism slow down considerably at night too. Unless you have a night job, you're not boosting your metabolism with physical activity or digestion.
Why Sleep Slows Metabolism
Scientists aren't certain why your metabolism experiences a slowdown overnight. Changes in body temperature, reduction in muscular activity and basic circadian rhythms -- based on a 24-hour sunlight cycle -- are likely causes. Sleep and the consequent metabolic slowdown could be a natural way the body conserves energy -- a holdover from when humans had to search for food and often faced shortages.
Sleep is also a serious time of restoration, when the body builds muscle, repairs tissue, and recharges and enhances brain function. Your metabolism may slow down so your body can focus on these actions.
Implications of a Slower Nighttime Metabolism
You may be concerned that calories eaten at night will cause you to put on fat, when compared to calories eaten during the day. Many diet plans suggest you do not eat after 7 p.m. because it could lead to weight gain. This isn't because your metabolism has naturally slowed, but because foods eaten late at night are typically not the healthiest options. You've likely consumed dinner and are seeking out snacks to munch mindlessly in front of the television or computer.
Calories you consume in the evening don't have any more impact than they do during the day. Your total calorie intake all day counts toward weight loss or weight gain. If you consume more calories than you burn though your TDEE over the course of a day, it leads to weight gain -- regardless of what time you consume those calories.
The same is true for chores or physical activity done at night. A person burns the same number of calories for activity of the same intensity and duration, regardless of what time it occurs. If a 180-pound man runs 30 minutes at a 10-minute-per-mile pace before breakfast or after dinner, he burns 444 calories. The exercise affects the metabolism in the same way, regardless of when it occurs.