A new study by the University of Colorado shows that the longer people are awake during the time their biological clock is telling them to sleep the worse their sensitivity to insulin, which is a precursor to diabetes. The study showed that a disruption, or circadian misalignment of a person’s internal clock induced by five-hour “short-sleep” schedules, resulted in morning wakefulness during the biological night when melatonin levels were still high, said CU-Boulder Professor Kenneth Wright, lead study author.
Melatonin is a sleep-promoting hormone produced naturally by the pineal gland and controlled by the brain’s master clock, while insulin, produced in the pancreas, regulates blood sugar. “We found the longer you are awake during the biological night, the worse your insulin sensitivity is,” said Wright, who directs CU-Boulder’s Sleep and Chronobiology Laboratory. “This is important because impaired insulin sensitivity can lead to both pre-diabetes and type 2 diabetes.”
While the study participants all ate healthy diets during several days and nights of baseline measurements, they were allowed to eat the types and amounts of foods they wanted during the short-night sleep testing periods in order for the researchers to better understand insulin changes, said Wright. “We found the body is not prepared for food intake during the biological night, nor is it prepared to be physically active,” said Wright. “We are supposed to be resting and recovering during that time.”
The study included co-authors from CU-Boulder and from the University of Colorado Anschutz Medical Campus in Aurora, Colorado. The team tested two groups of eight healthy men and women in their early 20s, all of whom were given both oral and intravenous tests in which they drank or were infused with a sugary liquid, after which their baseline insulin levels were tested. Insulin helps fat and muscle cells absorb glucose from the bloodstream, lowering blood-sugar levels. Impaired insulin sensitivity occurs when those cells cannot easily absorb glucose, causing the pancreas to produce more insulin.
During the sleep disruption part of the study one group had just five hours of available sleep time each night for five days – simulating a workweek with too little sleep – before transitioning into five days with nine hours of available sleep time. The other group did the opposite, starting with nine hours of available sleep time for the first five nights, then switching to just five hours per night for the next five nights.
The team found that the simulated five-day workweek of five hours of sleep per night resulted in a 20% reduced oral and intravenous insulin sensitivity in otherwise healthy people. It took three consecutive nights of nine hours of available sleep time to restore oral insulin sensitivity in test subjects, Wright said.
“Our study and other previous studies have shown sleep loss reduces sensitivity to insulin,” said Wright. “But we also found in this study that the test subjects with sleep loss had to release a lot more insulin to keep their blood sugar levels normal.”
Wright said further testing could reveal a lot more about insulin sensitivity. “These were young and very healthy people we studied,! said Wright. “We don’t know how much this might affect middle-aged or elderly people. One good follow-up study would be to see if improving the sleep of older people could improve their metabolic health.”
Short sleep duration and circadian misalignment are hypothesized to causally contribute to health problems including obesity, diabetes, metabolic syndrome, heart disease, mood disorders, cognitive impairment, and accidents [ 1–7 ]. Here, we investigated the influence of morning circadian misalignment induced by an imposed short night-time sleep schedule on impaired insulin sensitivity, a precursor to diabetes. Imposed short sleep duration resulted in morning wakefulness occurring during the biological night (i.e., circadian misalignment)—a time when endogenous melatonin levels were still high indicating the internal circadian clock was still promoting sleep and related functions. We show the longer melatonin levels remained high after wake time, insulin sensitivity worsened. Overall, we find a simulated 5-day work week of 5-hr-per-night sleep opportunities and ad libitum food intake resulted in ∼20% reduced oral and intravenous insulin sensitivity in otherwise healthy men and women. Reduced insulin sensitivity was compensated by an increased insulin response to glucose, which may reflect an initial physiological adaptation to maintain normal blood sugar [ 8 ] levels during sleep loss. Furthermore, we find that transitioning from the imposed short sleep schedule to 9-hr sleep opportunities for 3 days restored oral insulin sensitivity to baseline, but 5 days with 9-hr sleep opportunities was insufficient to restore intravenous insulin sensitivity to baseline. These findings indicate morning wakefulness and eating during the biological night is a novel mechanism by which short sleep duration contributes to metabolic dysregulation and suggests food intake during the biological night may contribute to other health problems associated with short sleep duration.