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Everyone needs water to survive. Humans are approximately 60 percent water by weight, and going without water can cause death in as little as two days. While death by thirst is vanishingly rare in America, dehydration is common among all ages and walks of life. Researchers have estimated that 75 percent of Americans are chronically dehydrated, a deficit which can lead to a broad range of both physical and mental effects, including dramatic impacts to sleep. Yet, as another critical part of our well-being, sleep can also have its own effects on hydration levels.
In this article, we’ll examine the effects of dehydration, the impact of exercise, and the complex relationship between hydration and sleep. Keep reading to learn more about these important topics, as well as the best ways to avoid dehydration.
There are two forms of dehydration: acute and chronic. Acute dehydration can be moderate or severe and usually has a specific cause. Common causes of acute dehydration include:
Chronic dehydration, on the other hand, is usually far more subtle and is a reflection of a broader range of causes. As one example, approximately 19 percent of our water intake comes from food; this number is higher in places like Greece and South Korea which consume more fruits and vegetables, therefore hydrating more effectively.
Other potential causes include drinking less water and more recreational drinks (coffee, soft drinks, and alcohol), regularly exercising without replenishing fluids, living in a hot environment without increasing fluid intake, and other lifestyle-related causes. Sleep quality and quantity can be another cause of dehydration.
Regardless of the reasons, the end result is that your body regularly has less water than it needs to function correctly. Noticeable symptoms of dehydration vary from individual to individual, but some of the most common are:
|Mild Dehydration||Moderate Dehydration||Severe Dehydration|
|Thirst||Thirst||Severe thirst, or a lack of thirst|
|Dry mouth and lips||Dry and sticky mouth and lips||Irritability|
|Darker urine||Dry eyes||Confusion|
|Headache||Fatigue||Severely reduced urination|
|Constipation||Infrequent urination||Very dark urine|
|Muscle weakness||No sweating|
|Muscle cramping||Low blood pressure|
|Loss of skin elasticity||Rapid pulse|
|“Sunken” eyes||Cold hands and lips|
Severe dehydration is a medical emergency, but moderate and even mild dehydration has been shown to have both physical and mental consequences. Dehydration can have a dramatic impact on athletic performance, and is an area that has been well-researched.
Athletes may consider themselves more aware of their hydration needs, but this might be offering them false comfort.
Studies have shown that not only do athletes begin exercise dehydrated (over half of those studied were at least mildly so) but also that most athletes do not adequately rehydrate to make up for the fluids lost during exercise.
Dehydration affects every stage of athletic performance. Training, particularly building muscle, may be less effective. In competition, athletes have less strength, cognitive ability, and speed. Afterward, dehydration prolongs the recovery process, resulting in pain and potential injury.
Researchers generally express dehydration in athletics as a percentage of body weight lost as a result of losing fluids. Any amount of dehydration begins a cascade of effects in the body, including:
While there is some evidence that dehydration levels of less than 2 percent may impair performance, particularly in elite athletes, most experts agree that dehydration becomes a problem at levels of 2-3 percent. For example, runners who were 2 percent dehydrated were impaired by 6 percent in long races and by 3 percent in shorter races. Similarly, dehydrated rowers had 5 percent less power and took 22 seconds longer to complete a rowing simulation.
For the most part, athletic dehydration is not dangerous. However, it can contribute to the development of rhabdomyolysis (also known as rhabdo), an emergency which occurs when muscle fibers break down and release excessive amounts of the kidney-damaging protein myoglobin.
Sleep dehydrates the body through fluid loss (from breathing) and the absence of any additional fluids throughout the night. While this is a normal physiological process, researchers have discovered that the relationship between sleep and hydration is much more complicated — and potentially more serious — than that.
In the section below, we cover a range of ways in which sleep and hydration can affect one another.
While different people have different sleep needs, the federal government has set a standard of seven hours a night for young adults and eight hours a night for adults over the age of 21. However, only 71.6 percent of Americans reach this goal.
Sleep deprivation isn’t just unpleasant: it has severe physical and mental effects, and can even be dangerous (as in the case of drowsy driving). The list of effects is long, and grew longer in 2018 when researchers discovered that sleeping six or fewer hours a night is a significant risk factor for dehydration. Research subjects in both America and China showed that this level of sleep deprivation increased their risk of dehydration by 59 percent; given that the average American only sleeps for 6.1 hours a night, this study offers an exciting insight into why Americans tend to be chronically dehydrated.
Furthermore, the study also suggests that some symptoms of sleep deprivation (like difficulty thinking, confusion, fatigue, and headaches) may be due to or worsened by dehydration.
The current hypothesis is that sleep deprivation causes dehydration due to a disruption in the rhythms of a hormone called vasopressin. Released by the pituitary gland while we sleep, vasopressin increases fluid retention by telling the kidneys to produce less urine.
Vasopressin is released in higher levels towards the end of an ordinary night of sleep, so people who wake up before this happens will produce and excrete more urine than their peers.
Naturally, this also leads to more fluid loss.
There has been less research done on whether dehydration shortens sleep length, but one study has shown exciting results which demand further investigation. 32 students were placed on fluid restriction and enhancement schedules, which showed that dehydration was associated with worse sleep quality and abnormal sleep heart rates.
Though this study was limited in its scope, it’s clear that the sleep-dehydration relationship is likely to be reciprocal.
Despite the release of vasopressin, it is common for people to occasionally wake up during the night to urinate. When people wake up regularly, or wake up three or more times in one night, they are said to be suffering from nocturia, or nighttime urination. Self-treatment of nocturia can aggravate dehydration, and dehydration can, in turn, make nocturia worse.
Many people consider nocturia to be a normal part of aging, but it is usually due to one of two problems:
People who suffer from nocturia often forgo seeing a doctor in favor of attempting to manage the problem on their own. Unfortunately, this usually takes the form of restricting their fluid intake. Though this tactic can make a difference in the short term, it does not affect the underlying cause and can increase the risk of dehydration.
Dehydration in nocturia sufferers is of particular concern because rates of both nocturia and chronic dehydration are higher among the elderly. Up to 40% of elderly people may be regularly under-hydrated, and adults over the age of 65 are the most likely to need emergency care for dehydration.
Nocturia can also be worsened by dehydration, as low fluid levels lead to more concentrated urine. This then irritates the bladder, creating the need to go to the bathroom more often. (Concentration is also why urine darkens with dehydration.)
If you suffer from nocturia, speak to a doctor to learn your options. In addition to other forms of treatment, your doctor may suggest limiting your fluid intake after dinner. However, they will also ensure that you shift your drinking to earlier in the day to stay hydrated.
Although Americans now drink more water than soft drinks (a change from 1998, when we drank 25 percent more soda than water), the yearly average is still only 58 gallons of water to 44 gallons of soft drinks.
Multiple studies have now shown that sleep-deprived people are even more likely to forgo water in favor of sugary soft drinks and energy drinks, many of them caffeinated.
Researchers speculate that this is due to a vicious cycle of these drinks making it more difficult to sleep, which then leads to an increased craving for sweets, including sweet drinks.
All fluids “count” towards your intake for the day, but there is evidence that simple sugars like glucose and fructose (common in soft drinks) may worsen dehydration and its associated kidney damage.
The caffeine content of soft drinks and energy drinks is also a potential problem. As a diuretic, caffeine increases urine output and can make dehydration worse. Experts believe two to three cups of coffee a day should not affect hydration, but the high caffeine levels of energy drinks or the excessive consumption caused by sleep deprivation may be a concern.
Just as sleep and hydration have a close, complex connection, so do sleep and exercise. Though exercise can disrupt sleep in some cases, both anecdotal evidence and scientific research have determined that regular exercise has at least a modest positive effect on sleep quality and quantity.
What has been studied less is the interconnected relationship of sleep, exercise, and hydration. The research which has been done has primarily focused on other subjects, with insight into these three factors created mostly as a byproduct of the actual study.
For example, a 2010 study on heat acclimation examined the relationship of both thirst levels and sleep deprivation with exercise. Subjects exercising in a hot environment once a day for ten days were less thirsty post-exercise on the tenth day than the first, but on the eleventh day — after 28 hours of sleep deprivation — their post-exercise thirst levels spiked again.
The perceived effort of a given exercise also followed a similar pattern, declining over ten days and rebounding after sleep deprivation.
Due to their findings on heat acclimation, the researchers noted that the lowered post-exercise thirst over time may be partially due to the kidneys adapting to a hot environment. In turn, they theorized that the increased thirst on the eleventh day might be due to sleep deprivation interfering with this adaption. This would make sense, given the vasopressin response examined above.
As interest in the importance of both sleep and hydration increases, it seems likely that further research will be done on the interconnected relationship between sleep, hydration, and exercise. Until then, there is no question that staying hydrated and sleeping well both have positive effects on athletic performance.
Given that so many Americans are chronically under-hydrated, it’s important to know the benefits of proper hydration and the impact of dehydration. In this section, we will cover how proper hydration impacts both the body and mind.
Everything in your body needs water to function properly. Just some of the ways we use water include:
One particularly thirsty bodily process is digestion. To begin with, it takes 100ml of water to metabolize 100 calories. Producing gastric fluids takes more water, as does the production of mucus.
Water is also critical to pass stool through the body, and one common symptom of dehydration is constipation. Hydrated bowels are healthy bowels, and increased fiber intake — another key factor in bowel health — also requires additional water to process so as not to make constipation worse.
Our other excretory system, the urinary tract or renal system, is dependent on water to function. Not only can our kidneys be damaged by acute dehydration, but chronic dehydration also appears to cause kidney disease.
In addition to the sleep issues covered above, proper hydration plays other roles in sleep health.
Some sleep effects are surprising. The risk of a painful, sleep-killing attack of gout (crystalized uric acid crystals in the joints, particularly the big toe) rises at night and the early morning. This is possibly due to dehydration, as it prevents the flushing out of uric acid in urine.
Water is also necessary for the body to detoxify itself as it does during the night. During sleep, cerebrospinal fluid helps clear out toxins in the brain. Dehydration has been shown to affect the amount and concentration of this cerebrospinal fluid, potentially causing some of the mental effects of sleep deprivation and possibly even raising the risk of neurological disorders.
Some other sleep issues caused by dehydration include:
Just as hydration affects the objective functioning of your brain, so too does it affect how you experience your mental health and ability to think clearly.
Many studies have shown that hydration levels have a clear impact on cognitive (thinking) ability. In one, dehydrated students performed worse than properly hydrated students in four out of five tests. In another, dehydrated drivers were shown to be impaired to the level of sleep deprivation, or a 0.08 percent blood alcohol level.
These and other studies support the scientific understanding that dehydration has the greatest effect on tasks requiring complex thought, prolonged attention, and coordination. As with physical performance, the worst of these symptoms begin at a dehydration level of 2 percent body weight loss.
One interesting note is that women seem to be affected more by dehydration’s effects. There are many theories as to why this might be, with most centering on the hormonal differences between the sexes.
Dehydration doesn’t just upset your electrolyte balance: it can also disrupt the balance of serotonin and dopamine in the brain. These natural chemicals help control your mood, and when they are out of balance we can feel anxious, depressed, and exhausted.
Even mild dehydration of 1.5 percent has been shown to provoke mood changes like a lack of motivation (including the motivation to exercise) and increased levels of tension and stress.
As with cognitive issues, dehydration-related mood problems also appear to be worse among women.
The researchers’ acknowledgment of exercise motivation being impacted by dehydration also reflects how our mood can have its own impact on our physical well-being. We know, for example, that anxiety and stress worsen sleep problems like insomnia. As happens so often with the body, disrupting one part will affect the whole in a multitude of ways.
As we’ve explored above, proper hydration also has a role to play in sleep health. In this section, you’ll learn about what to drink, how much to drink, and when to drink to stay hydrated, rested, and healthy.
While there is no average requirement for fluid intake, a good starting point is the adequate water intake (AI) chart:
|Age||Female Fluid AI||Male Fluid AI|
|2-3 years old||1300 ml||1300 ml|
|4-8 years old||1700 ml||1700 ml|
|9-13 years old||2100 ml||2400 ml|
|14-18 years old||2300 ml||3300 ml|
|18+ years old||2700 ml||3700 ml|
By the time you begin to feel thirsty, you have likely already lost approximately 1 to 2 percent of your weight through fluid loss.
This doesn’t necessarily pose a problem if you’re otherwise hydrating enough, but research does show that a 2 percent fluid loss can provoke many of the problems associated with dehydration. Furthermore, few people have a drink immediately upon feeling thirsty.
While the AI chart above does take into account a moderate level of exercise, it can be useful to know how much fluid to drink while exercising. The chart below has great starting points for how much to drink, but tailor it to your own needs:
|Type of Exercise||How Much to Drink|
|Light exercise||Normal amounts, or when thirsty.|
|Heavy exercise||16-32 oz every hour.|
|Distance walking/running||Weigh yourself before and after a test run, hydrating normally. In the future continue drinking as you did in the test run, plus 16 oz per pound lost in the test.|
|Hot weather exercise||5-10 oz every 15 minutes. Consider using isotonic drinks to recover electrolytes lost through sweating.|
Although hot weather is more of a dehydration danger than cold temperatures, rates of winter dehydration are still a concern due to reduced thirst, dry air, and heavy clothing. Ensure you’re drinking enough fluids no matter the weather.
Finally, be aware of the dangers of drinking too much. Called hyponatremia or water intoxication, this potentially lethal condition is caused by drinking too much water and diluting your electrolyte levels. While serious, it is rare and most common in endurance athletes.
Alcohol, like caffeine, is a diuretic. This effect increases with age, and dehydration worsens alcohol-related impairment. Moderate alcohol intake should not impact hydration or sleep, but pay attention to your body’s response and cut down on your intake if necessary. Avoid using alcohol as a sleep aid, as it is ultimately counterproductive.
Hydration and sleep are both critical to the health of our bodies and minds. Together they form a complicated relationship with many implications for our well-being, but by hydrating correctly and focusing on our sleep health, it is possible to avoid complications.
We hope this article has shed light on the sleep-hydration connection, as well as the importance of hydration and the best ways of achieving it. If you struggle with either hydration or sleep, speak to your doctor for advice and further information.