Water Works

by Dr. Michael Breus, PhD

Water is an essential macronutrient, and hydration plays a key role in keeping the body in physiological balance or homeostasis. Maintaining healthy hydration throughout the day and night is critical for powering our body battery at a cellular level, boosting mental and physical energy, promoting emotional regulation and positive mood and allowing us to engage our full cognitive capabilities. Balanced hydration also enables sound, restorative sleep.

There’s a complex, two-way relationship between sleep and hydration. Both dehydration and overhydration interfere with sleep at night. And lack of sleep can impede the body’s natural ability to maintain healthy fluid balance. Understanding how hydration and sleep interact remains a vital, underutilized tool in health and wellness—one that can be harnessed to improve nightly rest, daily performance and mood, and overall health.

How hydration affects sleep

The mental and physical effects of dehydration create obstacles for sound sleep. Research has shown that dehydration decreases alertness and focus, depresses mood and increases sleepiness—while making restful sleep harder to achieve. Tiredness, fatigue and irritability interfere with daily sleep-wake cycles and make it difficult to get up and going in the morning and stay active throughout the day. Headache, dry mouth, muscle fatigue and cramping, excessive thirst make it harder to fall asleep and to stay asleep throughout the night.

Overhydration also contributes to trouble sleeping. Nocturia—the need to wake from sleep to urinate—interrupts sleep cycles and deprives us of time in slow-wave sleep and REM sleep, the deepest and most physically and mentally restorative phases of sleep. Overhydration that leads to frequent nighttime urination can also meaningfully reduce nightly sleep amounts, particularly when we have trouble falling back asleep after going to the bathroom.

Getting up more than once to urinate during the night may be a sign you’re overhydrating at the end of the day. Frequent nighttime urination can also be a sign of one of several sleep disorders, including sleep apnea, insomnia and restless leg syndrome. And there are medical conditions, including overactive bladder, urinary tract infection and diabetes that are associated with frequent nocturia, and with excessive daytime urination. It is always important to discuss nocturia with your physician, to identify and address its underlying cause.

How sleep affects hydration

Many people are familiar with the impact of hydration issues on sleep. The effects of sleep on healthy hydration are less commonly understood. Sleep’s impact on fluid balance has been relatively under- examined by scientists, but in recent years more attention has been given to this important relationship.

Sleep exerts a direct influence over hydration. We lose roughly a full liter of water over the course of a night’s rest, through our breathing. During sleep, we’re not able to replenish our fluid loss and as a result, we all wake at least somewhat dehydrated. People who snore and have other forms of sleep-disordered breathing, including obstructive sleep apnea, will lose more than a liter overnight. Sweating at night also contributes to overnight water loss and morning dehydration. The menopausal transition, health conditions and medication use, environmental factors such as a too-warm bedroom and over- dressing for sleep are some of the common issues that create night sweating.

Not getting enough sleep may increase overnight dehydration, according to recent research. A large study conducted by scientists in the U.S. and China found that people who slept six hours or less a night were significantly more likely to be dehydrated in the morning than people who slept eight hours.

This study did not investigate or determine the reason why short sleep raises risks for dehydration. But scientists think the likely explanation involves the hormone vasopressin, which functions as an anti- diuretic and plays an important role in maintaining the body’s water balance throughout the day and night. Like most hormones, vasopressin has a daily circadian rhythm, with production and release of the hormone rising and falling in a cycle throughout the 24-hour day.

To help the body retain water overnight, vasopressin is released abundantly during the later phase of sleep. Going without a full night of sleep cuts into this vasopressin surge, which may exacerbate morning dehydration.

How to keep sleep and hydration in sync

There are several steps you can take to help ensure sleep and hydration support and reinforce one another, on a daily (and nightly) basis.

Know your chronotype and stick to a consistent sleep routine

Regularly getting a full night’s rest is all-around critical for mental, physical and emotional health—and it’s one important way to ensure your body can exercise its natural capacity for maintaining hydration overnight. Setting a sleep-wake schedule that aligns with your individual chronotype is the most powerful tool you have in creating consistent sleep routines that meet your individual needs for rest.

Drink a tall glass of water first thing in the morning

I recommend having a 12- to 16-ounce glass of room temperature water as soon as you wake up, to replenish water lost overnight. (If you can drink your water near a window to soak up the early morning sun, all the better—it will energize you, help you become alert more quickly and strengthen your body’s circadian clock.)

On the other end of the day, limit fluid intake in the 90 minutes before bedtime, to avoid nighttime overhydration.

Wait 90 minutes before you consume any caffeine

Holding off on your cup of morning coffee or tea? It’s worth it for a few reasons. Caffeine is a diuretic. In the morning, we need to replenish our bodies with water, not deplete them.

What’s more, the human body has a built-in system for becoming alert in the morning, as circadian rhythms go immediately to work to lift energy and get rid of morning grogginess. First thing in the morning, stimulant hormones— including cortisol, insulin and adrenaline—are on a steep rise. This initial waking boost of cortisol and other hormones is powerful, and it renders caffeine useless, in terms of elevating alertness. Any caffeine you consume right upon waking will have a negligible effect on your alertness. What it does do is increase your tolerance for caffeine, so you’ll need more overall to experience caffeine’s effects, regardless of the time of day.

Limit alcohol to three hours before bedtime

Alcohol is a big-time dehydrator—and it’s also a major disruptor to sound sleep. In addition to curtailing drinking within three hours of bedtime, drink a full glass of water for every alcoholic drink you consume, to help maintain balanced hydration overnight.

When you exercise, replace sweat with water

Exercise is a powerful sleep promoter, and daily exercise can significantly improve your nightly rest. Be sure to replenish the fluids lost to exercise, to avoid dehydration during the day and at night.


Michael Breus, PhD, is a double board-certified sleep specialist, founder of TheSleepDoctor.com, author of three books on sleep, including the latest, Energize! with Stacey Griffith, and has been in practice for 23 years.

You may also like