An afternoon nap may help people lose weight as we burn 10 per cent more calories resting then than during the night, suggests a study. Sleep researchers have discovered that people burn around 10 per cent more most calories while resting in the late afternoon or early evening than in the early hours of the morning.
The study, published in the journal Current Biology, shows that the number of calories people burn while resting varies, depending on the time of day. Scientists say that the discovery could explain the role of the body clock and its disruption through night and shift work in making people more likely to put on weight.
They say it also has implication for people who snack all day – but stay awake late surfing the internet. Research Fellow Dr Kirsi-Marja Zitting, of the Division of Sleep and Circadian Disorders at Brigham and Women’s Hospital and Harvard Medical School in the US said: “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.
“There is now emerging evidence that an irregular sleep-wake and fasting-feeding cycle, common in people working night or rotating shifts, can lead to disrupted circadian timing, which in turn may alter energy balance and lead to increased obesity risk. The positive energy balance associated with nightwork – which leads to weight gain and obesity over time – may be caused by increased energy intake, changes in the timing and/or frequency of food intake, or decreased energy expenditure.
“Fasted resting energy expenditure, or REE, also known as resting metabolic rate, or RMR, is the largest component of daily energy expenditure and a major determinant of changes in weight. REE accounts for 60 to 70 per cent of all calories burned at rest each day to support basic physiological functions such as ventilation, circulation, temperature regulation and brain activity. “
To determine changes over the course of the day in metabolism apart from the effects of activity, sleep-wake cycle, and diet, seven people were placed in isolation in a sleeping lab with no clues as to time of day. They had assigned times to go to bed and wake up and each night, those times were adjusted four hours later, the equivalent of travelling westward across four time zones each day for three weeks.
Study co-author Associate Professor Jeanne Duffy added: “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. This allowed us to measure metabolic rate at all different biological times of day.”
The data showed that resting energy expenditure is lowest at the circadian phase the researchers designated as ~0°, corresponding to the dip in core body temperature in the late biological night. Energy expenditure was highest at circadian phase ~180°, about 12 hours later, in the biological afternoon into evening.
It found participants’ respiratory quotient, which reflects macronutrient utilisation, varies by circadian phase, too. This measure was lowest in the evening and highest in the biological morning. Dr Zitting said: “This is the first characterisation of a circadian profile in fasted resting energy expenditure and fasted respiratory quotient – with rhythmic profiles in both carbohydrate and lipid oxidation, decoupled from effects of activity, sleep-wake cycle, and diet in humans.
“These results may have important implications for understanding weight gain and obesity among night-shift workers. They may also have relevance to non-shift workers, There is evidence that many individuals keep irregular schedules, including a large portion who have social jet lag.
“When taken together with evidence that many US adults eat throughout their entire wake episodes, the circadian variation in metabolic functions we have observed may impact weight gain more broadly. “
Prof Duffy 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 research team said that future studies will look at how appetite and the body’s response to food varies with the time of day.