Recent research has shown that both inadequate and excessive sleep – defined as durations of less than seven hours and more than nine hours per night, respectively – are associated with significant disruptions to our bodies' self-maintenance mechanisms. And as any morning lark working late shifts or night owl with an early commute can tell you, we humans turn into chronically fatigued, high-strung, forgetful hot-messes when these homeostatic processes get out of whack. Oh, and we also have greater risks of cancer, heart attack, and metabolic diseases.   

Now, a team from Pennsylvania State University have found evidence that short sleep duration may also lead to dehydration by interfering with the brain’s release of the water retention hormone vasopressin. This discovery could help explain why people who don’t get enough sleep are more likely to suffer from hydration-impacted health problems, such as insulin sensitivity, hunger/satiety signaling, and cardiovascular and kidney functioning.

In their paper, published in the journal Sleep, research lead Asher Rosinger and his colleagues analyzed sleep duration and urine concentration information from 20,119 adults who participated in one of three large health studies: two cohorts of the American National Health and Nutrition Examination Survey (NHANES) and one cohort of the Chinese Kailuan Study. Subjects with confirmed or suspected diabetes, kidney problems, those taking diuretic medications, and pregnant women were excluded.

Among both US and Chinese participants, the authors noted a significant association between six hours of sleep and a higher urine specific gravity, a measurement that means the urine is highly concentrated, and thus the person is dehydrated. (For Kailuan participants, urine samples were collected in the morning, but for NHANES participants, urine was collected at unspecified times during physical exams.) Compared to those who reported sleeping eight hours a night, six-hour sleepers were 16 to 59 percent more likely to be dehydrated.

Rosinger’s team speculate that this trend was driven by vasopressin, also known as antidiuretic hormone (ADH), because past studies have shown that circulating vasopressin – introduced into the bloodstream from the posterior pituitary gland – peaks in the last few hours of the sleep cycle.

“So, if you’re waking up earlier, you might miss that window in which more of the hormone is released, causing a disruption in the body’s hydration,” Rosinger said in a statement.

However, he and his fellow authors are quick to point out that their study only reveals an association, not a causal relationship. Moreover, “[i]t is unclear from our data what is driving this association, whether shorter sleep duration is affecting vasopressin release, which affects kidney concentration of urine, or whether shorter sleep duration is also associated with other health factors uncontrolled for that may affect hydration status,” they wrote. “For example, short sleep duration may be associated with hydration-related behaviors, e.g., drinking less fluid, as part of a feedback loop.

Though the physiological mechanism needs to be confirmed, the team report that their findings support those of past research that links short sleep duration with higher risk of chronic kidney disease and higher kidney filtration rates. While we wait to learn more, Rosinger has some helpful advice that it won’t hurt to follow: "This study suggests that if you’re not getting enough sleep, and you feel bad or tired the next day, drink extra water,."