The study led by the University of Colorado Boulder analyzed fourteen healthy adults. For the first two days, the participants followed a normal schedule sleeping at night and staying awake during the day.
    
They then transitioned to a three-day shift work schedule when their routines were reversed.
    
"When people are on a shift work-type schedule, their daily energy expenditure is reduced and unless they were to reduce their food intake, this by itself could lead to weight gain," said Kenneth Wright, director of CU-Boulder's Sleep and Chronobiology Laboratory and senior author of the paper.
    
During the experiment, participants' meals were carefully controlled, and they were given the amount of food they would normally need to eat at home to maintain their current weight.
    
When the participants transitioned to the shift work schedule, the timing of their meals changed but the total amount of calories remained the same.
    
The participants also were given the same eight-hour sleep opportunity regardless of whether those hours were scheduled during the day or night.
    
The researchers found that total daily energy used by participants decreased when they were put on a shift work schedule.
    
The reduction is probably linked to the mismatch between the person's activities and their circadian clocks, Wright said.
    
Humans have evolved to be awake and eat when it is light outside and sleep when it is dark. In large part, the human circadian clock is set by exposure to sunlight.     

People's circadian clocks can shift over time even radically with the use of artificial lights if they are not exposed to the sun.
    
But because shift workers typically switch back to a daytime schedule on their days off, their biological clocks do not flip to fit their night shift schedules.
    
"Shift work goes against our fundamental biology," said Wright, also an associate professor of integrative physiology.
    
"Shift work requires our biological day to occur at night and our biological night to occur during the day and that's very difficult to achieve because the sun is such a powerful cue.
    
The research team was surprised to find that the study participants burned more fat when they slept during the day compared to when they slept at night.
    
It is not clear why this happens, but Wright said it is possible the extra fat-burning is triggered by the transition day between a daytime schedule and a nighttime schedule.
    
The study was published in the journal PNAS.

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