Individuals who only get six hours of sleep per night are at a higher risk of dehydration than those that get eight to nine hours. So, if your schedule really doesn't permit you to get more sleep, these findings suggest that you ought to drink plenty of water first thing in the morning.
Researchers at Penn State analyzed sleep duration and hydration status in American and Chinese adults, using data from the National Health and Nutrition Examination Survey and the Chinese Kailuan Study, respectively. For both populations, adults who reported sleeping for six hours or less had significantly more concentrated urine and 16-59 percent higher odds of being inadequately hydrated compared to adults who slept eight hours a night, the authors reported in the journal Sleep.
This is an observational study, so no causal relationship can be established at this point. However, the observed association is significant, which should warrant attention.
According to the researchers, the reason why sleep duration may affect hydration comes down to a hormone called vasopressin. This hormone's job is to regulate hydration in the body and to prevent you from expelling diluted urine. When we drink alcohol the body sends a signal to the pituitary gland to block the creation of this antidiuretic hormone. Your kidneys then begin sending water directly to your bladder instead of reabsorbing the water into the body for use. As a result, cells do not get properly hydrated and you need to go pee every fifteen minutes.
The hormone is released both during the day and the night. However, its release cycles vary more during the night. People don't drink water while they sleep, so the body has to minimize water loss to remain sufficiently hydrated -- this is why you shouldn't feel thirsty during the night. Improper vasopressin generation and release, for instance, is also one of the main reasons why children wet their beds.
“Vasopressin is released both more quickly and later on in the sleep cycle,” lead author Asher Rosinger, assistant professor of biobehavioral health at Penn State, said in a statement. “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.”
Poor hydration can mess with the body's functions, negatively affecting mood, cognition, or physical performance. When dehydration becomes chronic, individuals risk serious problems such as urinary tract infections and kidney stones.
Rosinger and colleagues recommend people get enough sleep in order to stay hydrated. If that's just not possible, drink plenty of water in the morning to quickly start replenishing water content in your cells.