Owl or Lark? Resting Energy Expenditure Varies Throughout Day

Liam Davenport

November 13, 2018

The number of calories a person burns at rest varies depending on the time of day, US researchers have discovered, in findings that could explain why shift work or other sleep- and eating-related factors make people more likely to gain weight.

The team measured resting energy expenditure (REE) in 10 healthy volunteers who stayed in a sleep laboratory without any time cues, such as clocks or windows, for 30 days.

They followed either a standard 24-hour schedule or had their daily wake and sleep pattern adjusted by 4 hours every day, revealing that energy expenditure was highest in the afternoon/evening and lowest late at night. This means that, when at rest, people burn 10% more calories in the late afternoon and early evening than in the early morning hours.

The research was published online on November 8 in Current Biology.

Kirsi-Marja Zitting, PhD, Division of Sleep and Circadian Disorders at Brigham and Women's Hospital and Harvard Medical School, Boston, Massachusetts, who led the study, said they were not expecting to find such a difference in energy expenditure throughout the 24-hour cycle.

"The fact that doing the same thing at one time of day burned so many more calories than doing the same thing at a different time of day surprised us," she said in a press release from Cell Press, the publisher of Current Biology.

Coauthor Jeanne F. Duffy, MBA, PhD, also from Brigham and Women's Hospital, added: "It is not only what we eat, but when we eat — and rest — that impacts how much energy we burn or store as fat. Regularity of habits such as eating and sleeping is very important to overall health."

The researchers stress these findings mean that anyone conducting studies on energy expenditure in the future should control for any circadian phase misalignment, such as shift work, when interpreting results. This also means taking into account the diurnal preference of study participants: whether they are "owls" or "larks."

Measuring Resting Metabolic Rate at All Times of Day

The researchers note there is increasing evidence that irregular sleep–wake and fasting–feeding cycles, common in people working shifts, can disrupt the circadian rhythm, thus altering energy balance and potentially increasing the risk of obesity.

Daily energy expenditure, for the most part, occurs during rest, with REE accounting for up to 70% of all calories burned to support, among other functions, ventilation, circulation, temperature regulation, and brain activity.

Previous studies have reported mixed results when examining diurnal variations in REE, which the researchers suggest may be because of confounding meal effects.

To investigate further, they recruited 10 healthy adults who were free from any medical, sleep, or psychological disorders. Participants had not engaged in regular shift work in the previous 3 years and had not travelled across more than one time zone within the previous 3 months.

They were assigned to a recurrent circadian disruption (RCD) or control group and asked not to use drugs, alcohol, nicotine, or caffeine during a 21-day run-in period.

They were then admitted to an inpatient sleep laboratory free of time cues, including clocks, windows, phones, or the Internet, and maintained at a constant light level during wake episodes.

The participants underwent a 7-day induction period, during which they lived on a 24-hour schedule, adjusting to an 8- or 10-hour night-time sleep opportunity, and ate a strictly controlled diet.

Controls continued on these baseline conditions for the remaining 21 days of the study, while the RCD group underwent forced de-synchrony, which had the effect of adjusting the sleep and wake times by 4 hours, or traveling across four time zones, per day for 3 weeks.

"Because they were doing the equivalent of circling the globe every week, their body's internal clock could not keep up, and so it oscillated at its own pace," Duffy said. "This allowed us to measure metabolic rate at all different biological times of day."

To compensate for the adjustment, RCD participants were given a snack after each dinner containing an additional 16.7% of the 24-hour kilocalorie target.

One woman completed the study three times and one man completed it twice, and so the researchers had data on seven RCD episodes and six control episodes.

REE Lowest Late at Night, Highest in the Afternoon/Evening

The team found that REE was lowest in what they defined as circadian phase 0º, which corresponds to the lowest point in the endogenous circadian rhythm of core body temperature in the late biological night.

In contrast, REE was highest around 12 hours later at circadian phase 180º, which corresponds to the biological afternoon and evening, with a variance between the two phases of 55.2 kcal/day (P < .001).

"Thus, while awake and resting, the human body burns the fewest calories during the late biological night and the most calories during the biological afternoon and evening," the team says.

REE was found to have a significant effect on the day-to-day variability in body weight in both the RCD group (P = .004) and control group (P = .009).

However, that disappeared after further analysis in the RCD group, "suggesting that circadian timing is a stronger determinant of REE than small changes in weight."

The results also show that, similar to REE, the fasted respiratory quotient varied by circadian phase and was 2.5% lower in the biological evening compared with the biological morning in the RCD group (P = .019).

Fasting carbohydrate oxidation and lipid oxidation also varied by circadian phase, albeit in the opposite direction.

"This circadian variation in respiratory quotient and macronutrient utilization suggests that the body favors carbohydrate oxidation in the biological morning and lipid oxidation in the biological evening," the team notes.

Owl or Lark? Take Into Consideration When Conducting Studies

Zitting and colleagues write, "Our observations also highlight the importance of controlling for the effect of circadian phase (misalignment) when carrying out recordings of energy expenditure in clinical and research settings."

They continue, "Given the magnitude of REE change due to circadian phase alone, it is of utmost importance for research studies using calorimetry as a key outcome to assess energy expenditure to control for the circadian time at which calorimetry recordings are made."

For example, they found a difference of 129 kcal/day between the circadian peak and trough in REE, which they say is similar or greater in magnitude to that caused by acute sleep deprivation (one night of sleep deprivation lowered REE by 103 kcal/day) and chronic sleep restriction (five nights of 4 hours of time in bed lowered REE by 31 to 42 kcal/day).

"Circadian phase timing is influenced by many factors, including recent sleep–wake schedule...light exposure history, travel across time zones, history of night work or rotating shift work, age, sex, and diurnal preference of the participant."

Next, the team will examine how appetite and the body's response to food vary during the course of the day, and they will explore how the timing, duration, and regularity of sleep affect these responses.

The research was supported by the National Institute on Aging, Brigham and Women's Hospital, Harvard University, Finnish Cultural Foundation, and National Institutes of Health. Zitting and Duffy have reported no relevant financial relationships. Disclosures for the other authors are listed in the article.

Current Biol. Published online November 8, 2018. Full text

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