26,000 Samples of Morning Urine Reveal the Effects of Sleep Deprivation 

A morning pee contains a surprising amount of information. A light yellow is probably a good sign, while a darker yellow may tell a tale of dehydration. For those who tend to be on the darker end of the spectrum, new research suggests that the problem may not lie solely in neglecting to drink enough liquids. According to a paper published Monday in the journal Sleep, it might also have something to do with how many hours of sleep you manage to get.

In the short term, depriving the body of water can generally worsen one’s mood or even cause pounding headaches, says Asher Rosinger, Ph.D., assistant professor of biobehavioral health and anthropology at Penn State. Not taking care to actively drink water during the day is usually the driver behind this, though the paper he first-authored, which analyzed over 26,000 subjects gleaned from two national surveys in the US and China, found that people who slept less than eight hours tended to have certain markers of dehydration lingering in their urine samples.

Rosinger tells Inverse that he believes sleep-dependent dehydration comes down to the release of an important hormone, called vasopressin.

“[Vasopressin] increases towards the late sleep period as a way to conserve body water. It does that to help avoid dehydration,” he says. “So as we were reading that finding in the literature, we started thinking that, as people don’t get enough sleep, they might miss that late sleep period and disrupt their body water homeostasis.”

Typically when the brain’s pituitary gland receives signals that water is running low, it can release vasopressin, which allows the water that usually resides in urine to actually be pulled back into the body. Some research, however — for example, a study on night shift workers — suggests that the body is bound to certain patterns of vasopressin release beyond this primary mechanism that help us stay hydrated, particularly during sleep when the body releases more of the hormone.

Rosinger’s study didn’t actually measure vasopressin levels, but he was able to approximate dehydration by examining biomarkers like urine gravity and osmolarity (which measured how much water is in pee compared to other components) from 26,142 subjects in the United States and China. He and his co-authors then compared these numbers to subjects’ reported sleep time. Here they noticed a pattern: People who slept for six hours a day tended to have highly concentrated pee (indicating dehydration) compared to those who slept for eight hours.

This may be slightly confusing. Because vasopressin helps the body pull water back from urine to manage dehydration — thus concentrating pee — you might expect that seeing concentrated pee could indicate that the body is actually releasing vasopressin in these sleep-deprived people. This is why Rosinger makes it clear in the paper that he suspects that poor sleep habits might make individuals more vulnerable to dehydration over time by messing with their natural vasopressin rhythm. In a sense, by waking up early, we’re throwing a wrench in a natural cycle that could have consequences even during waking hours.

His pattern does demonstrate this with some power — given the numbers behind his study. He also found that people who slept for eight hours a day tended to not have these same issues with dehydration.

“We were able to replicate this across two of the world’s largest cultures in the US and China,” Rosinger says. “To be able to see the same results in both US adults and Chinese adults, really strengthens these findings.”

But still, it’s too early to assume a causal relationship, which is why he’s doing an additional study on this now. It’s possible, Rosinger says, that simply being dehydrated might lead someone to sleep less over time. In follow-up work, he’ll experimentally restrict how much sleep people get in order to see if dehydration is causing sleep loss, or if sleep loss is causing dehydration.

The most likely answer, at least given his findings right now, is that it’s probably a bit of both: creating a cycle of sleep loss and dehydration that feed into one another.

“This could actually be something that’s quite cyclical,” he says. “It could be that someone’s sleep is affecting their hydration status, and if that person is dehydrated it could affect their sleep as well. It could be interesting to explore.”