Try as they might to adjust to a schedule that has them toiling while others sleep, swing-shift workers do not fool their bodies into burning calories and using nutrients the way people who obey their internal circadian clocks do, says a new study.
The result -- a metabolism that burns fewer calories during sleep and slows down at night despite work demands -- could help explain why those who work overnight shifts are more likely to be obese and to develop type 2 diabetes.
In a study conducted by University of Colorado sleep specialists, 14 lean, healthy adults agreed to turn their days upside-down progressively over a six-day period. Checking into a University of Colorado clinical research facility, the volunteers were all fed a diet sufficient to maintain their weight. They simulated a sleep-wake schedule that started with two regular working days and then made a transition to three night shifts in a row.
Published Monday in the journal PNAS, the research reported that as subjects commenced their simulated "swing-shift schedule," their bodies first responded by revving higher: for the first 24-hour period that their sleep-wake schedule was transposed, they burned 10 calories more than they had during an ordinary baseline day.
At no point in the six-day experiment did subjects adapt to a snooze-by-day, work-by-night schedule by boosting production of the sleep-friendly hormone metatonin in the day and curtailing it at night.
But their bodies quickly adapted to the reversal by turning their thermostats down. Compared to the baseline readings taken on their arrival (a day when they were awake by day and asleep eight hours at night), the subjects burned 52 fewer calories on day 2 of their swing-shift schedule, and 59 fewer calories on day 3 of that schedule.
The lower energy expenditure came both during daytime sleep and during night-time wakefulness: their sleep was more fragmented; and their energy expenditure at night, when they were awake but their bodies were accustomed to sleeping, remained suppressed.
As their upside-down schedules entered days 2 and 3, their bodies became more efficient at using fat, but less efficient at using carbohydrates and protein. Despite reduced levels of the "satiety hormone" leptin on days 2 and 3, subjects reported lower levels of hunger. And the "thermic effect" of a late-night dinner -- the revving that signals the body has taken on a new infusion of fuel -- was significantly more muted than their baseline response to an end-of-day dinner.
The study lacked a control group, and it lasted only six days, which hardly allows researchers to predict how a worker will adapt to a schedule that consistently upends sleep-wake cycles or to one that does so inconsistently.
But it does suggest that even a brief "circadian misalignment" puts the metabolism into a lower gear. Even if the reversal of night and day activities is occasional, burning 50 to 60 calories a day fewer (without adjusting one's diet accordingly) every once in awhile is a formula for weight gain. And the study may explain why, despite evidence suggesting that night workers eat no more than those on a "normal" sleep-wake schedule, they more often struggle with obesity.
The researchers also point out that shift work is wearing, and that hardly bodes well for a worker's exercise schedule. As "increased exhaustion and fatigue levels" mount, a shift worker's physical activity levels are bound to decline, they suggested, further upsetting their metabolism.