Cross-cultural evidence from US and Chinese adults
Rosinger, et al. 2019, Sleep Journal
Introduction
Sleep duration has been associated with multiple health outcomes such as cardiovascular disease, obesity, diabetes and chronic kidney disease. But what’s gained less attention is how your sleep relates to hydration status in adults.
Optimal hydration is critical to physiological function and health and the consequences of being in a deficit, or dehydrated, are headaches, fatigue, irritability, dry mouth, visual acuity, and when sever can alter motor function.
Identifying and affecting daily behaviours that contribute to your hydration status may help you to improve your hydration status and, therefore, sleep performance. I think we can agree this is a simple and cheap win.
Hydration status is part of a tightly linked and regulated hormonal system of renin-angiotensin-aldosterone, which essentially regulates the concentration of your urine and will conserve body fluids.
Vasopressin is a hormone released depending on your hydration status and it’s function is to reduce your urine output which is important in the later part of your nights sleep to ensure you do not become dehydrated due to water loss at a time where you haven’t drunk any water for many hours. The timing of it’s release is linked to your circadian rhythm (CR), and is therefore an automatic action timed by your body when it thinks it should be well into your nights sleep.
Disruption to this CR and homeostatic action can affect insulin-sensitivity, hunger and satiety hormones, cardiovascular function and kidney function. There hasn’t been many studies in this area and therefore this is one of the first steps into investigating the link between hydration and sleep duration.
The aim of this paper is to examine how:
- Usual night-time sleep duration is associated with urine measures of hydration in US adults and a large group of Chinese adults.
- They hypothesize that both short AND long duration sleep will be associated with more concentrated urine and greater probability of inadequate hydration.
The will use two marker to test urine concentration
1 – Urine specific gravity (USG)
2 – Urine osmolarity (UOSM)
Method
American study
- A nationally representative population sample was used
- Used both in-person interviews alongside physical examination
- Collected appropriate data at 2-year intervals since 1999
- 50% men and 50% women
Chinese Study
- Ongoing multicentre cohort in Tangshan City, China
- The population used were selected to study risk factors of non-communicable diseases
- Chosen as they also collected sleep data
- 82.6% men and 17.4% women
Exclusion Criteria
- Incomplete or missing information from the previous data collection
- Pregnant women
- Diagnosed diabetes
- Diagnosed with weak or failing kidneys
Final sample size 8766, which is a large scale study.
Procedures
Urine samples collected and USG and UOSM data was collected, methods differed slightly and are shown below.
US study used digital hand-held refractometer
- USG values between 1.000 and 1.040 g/ml
- Values above 1.020 g/ml considered inadequate hydration
China study used
- A single mid-stream urine sample used and analysed using instrument colorimetric
method, within 2 hours of sample collection. All were run through a urine analyser
- Results rounded to the nearest 0.005g/ml for a minimum of 1.000 g/ml and a
maximum of 1.030 g/ml
Table 1 shows information about the populations studied. NHANES is the US study and Kailua id the Chinese study.
Main independent variable: Sleep duration
In the US study the participants were asked “how much sleep do you usually get at night on weekdays or workdays?”. This did not take into account daytime naps.
The China study participants were asked to report usual total hours of actual sleep during the night, to the half hour. They did not take into account daytime naps.
Categories were created for:
1 – Less than 6 hours of sleep
2 – 6 hours of sleep
3 – 7 hours of sleep
4 – 8 hours of sleep
5 – 9 or more hours of sleep
Cofounders
There are many factors that can interfere with results and are called confounding factors.
In the US study these were identified:
- Sex
- Race
- BMI
- Alcohol intake (via AMPM recall method)
- Caffeine intake (via AMPM recall method)
- Total water intake (via AMPM recall method)
- Physical activity (via Global Physical Activity Questionnaire)
- Time of the exam
- Diabetes status (via blood samples)
These were all identified and were addressed within the analysis to control for their influence on the results.
In the China study they collected confounding factors via a questionnaire
- Sex
- Age
- Physical activity
- Alcohol consumption
- Diabetes (via fasting blood glucose test or history of diagnosed diabetes)
Results
US study results: Urine-specific gravity (see figure 1 below)
- Sleep duration and USG showed a mild U-shaped relations
- Suggesting that short amounts of sleep tended may be linked to higher urine osmolarity (a sign of dehydration)
- This UGS decrease until 8 to 9 hours of sleep where UGS increases again.
- Compared to 8 hours sleep, 6 hours of sleep had significantly higher concentrations of urine
- Adults with 6 hours of sleep had a 1.59 times more chance of having high USG compared to those that sleep 8 hours
- Very short sleepers (less than 6 hours) and 9 hours or more sleep did not have significantly more chance of inadequate hydration, but this could be due to less of the population fitting into these categories so they had less of these people to test.
Figure 1 shows the distribution of the participants across sleep hours (X-axis) and USG (Y-axis)
US study results: Urine Osmolality (see figure 2 below)
- Mild U-shaped relationship observed again between sleep duration and UOSM
- Those sleeping 6 hours had significantly higher UOSM than those sleeping 8 hours and adjusted for the confounding factors
- No other sleep duration varied significantly from those that slept 8 hours
- Those with 6 hours sleep had a 1.16 times chance of being inadequately hydrated.
Figure 2 shows the distribution of the participants across sleep hours (X-axis) and UOSM (Y-axis).
China study results: Urine-specific gravity
- Similar to US study when comparing 6 and 7 hours to those sleeping 8 hours
- Main difference to US study was that those with very short (under 6 hours) and very long (over 9 hours) sleep durations actually reduced the chances of them inadequately hydrated
Sensitivity analysis
- Results between sleep duration and USG, UOSM and inadequate hydration were consistent across the different durations of sleep.
- Results were consistently significant
- Adults with 5-6 hours of sleep had 17-32% significantly higher odds of inadequate hydration (using USG and UOSM measures)
- US study – 6 hours of sleep = 1.94 times more likely to inadequately hydrated than those sleeping 8 hours.
Discussion points
1 - Short and not long sleep was associated with more concentrated urine and higher chances of inadequate hydration
2 - Adults sleeping for 6 hours has significantly more concentrated and 16-59% higher chance of being inadequately hydrated when compared to those that slept for 8 hours on a regular basis
3 - Longer sleepers (9 hours or more) did not exhibit worse hydration status that normal sleepers
4 - Mild dehydration has health implications such as cognitive, mood, and physiological impairments, and chronic dehydration can lead to increased risk of kidney stones and urinary tract infections
5 - Most adults reported 6-8 hours of sleep per night (80-88% of the populations studied)
6 - Short sleep durations are linked to higher risk of kidney disorders, higher kidney filtration rates and elevated albumin-creatine clearance
7 - These can lead to dysregulation of body water through, the hormone, vasopressin release
8 - This study supports the thought that short sleep reduces kidney health
9 - Vasopressin should be release in the later part of sleep to reduce urine output. If sleep is cut short and therefore the later part of your sleep is short or absent (a short nights sleep), then the vasopressin is not released and more water is lost, increasing risk of dehydration
10 - It is possible that confounding factors are at play, and that those that sleep less may drink less fluid on a daily basis, or have life habits that dehydrate them
11 - This means that we cannot 100% guarantee a causative link of short sleep = dehydration as these other factors weren’t included in the study. But there is a clear link between short sleep and inadequate hydration that we should be aware of, and potentially try to affect.
Increasing sleep may help protect kidneys from problems later in life
Limitations
- Causal relationship cannot be made completely due to point 11 in Discussion Points section
- Reported sleep is the usual amount of sleep and not necessarily the amount of sleep achieved last night, which may be more accurate for how you feel today
- Hard to make strong conclusions about the sleeping less than 6 hours and those sleeping more than 9 hours as there were not as many of those people in the study, as the are at the extremes of sleep duration
- China study had 83% men and US study at 50% men, more women would be required for this to be more robustly linked to women in general
- Only one measure was taken, would be good to see how hydration status changes for an individual over time
- China study used a method which is less accurate than the US study
Link to study https://pubmed.ncbi.nlm.nih.gov/30395316/
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