How much sleep do we need? Much of the literature suggests a good 7-9 hours of shut-eye is needed in order to properly rest, recover and recuperate.

There’s no doubting the importance of sleep and pretty much anyone - bar maybe a new parent - could tell you what a good night’s sleep feels like.

But, if you’re doing sleep right, then it’s effectively 7-9 hours where you're not consuming any fluid. Surely you dehydrate to some degree in that time?

Do we get dehydrated when we sleep?

This was the question put to us recently and the logic appears sound - not drinking overnight should mean waking up dehydrated to some extent come the morning, right? 

It sounds especially reasonable when you consider that during the day we're encouraged to drink up to 2 litres of fluid to maintain a good state of hydration... but our bodies are clever and they run on their own internal clocks.

In short, this internal clock prevents us from becoming dehydrated overnight.

You may be familiar with the term 'circadian rhythm'. This is the name given to an internal process that regulates our sleep-wake cycle and which repeats roughly every 24 hours. Many bodily functions follow their own circadian rhythm some examples being core body temperature, metabolism and hormone secretion.

Back in 2010, the circadian rhythm of the body's hormone for conserving water (the antidiuretic hormone; ADH - also known as vasopressin just for extra confusion) was investigated and found to increase overnight (Trudel & Bourque, 2010).

This increase in ADH secretion encourages the kidneys to reabsorb more water and reduce the amount lost to urine production.

This makes perfect sense when you consider the bigger picture. Unable to drink overnight, the body maintains its fluid balance by limiting its water loss.

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Now, don’t get me wrong, you do lose some fluid overnight. This is unpreventable despite ADH’s best efforts. This is because a small amount of water is lost by evaporation during breathing and insensible perspiration. What ADH does do though is ensure you don’t dehydrate to a degree which leaves you parched and reaching for a glass of water first thing in the morning.

Why do I wake up thirsty?

Whilst this is all well and good, we've all experienced instances where we've woken up thirsty, so what's all that about? Under normal circumstances, we do not/should not be waking up in a dehydrated state. But, if a person is waking up feeling this way (dry mouth, thirsty etc.) then there may be something else at play. 

I've listed a few possible causes below:

1. Let’s get the obvious one out of the way, a hangover!

Alcohol is a diuretic which means when consumed, even in relatively small amounts, it increases urine production and throws off the body's fluid balance.

How does it cause us to pass more water in our urine? By suppressing the secretion of our good friend ADH, of course. Less water-saving hormone = less water! And less water = more thirst!

2. How you sleep makes a difference

Remember those water losses via breathing I just spoke about? Well, snoring or breathing through your mouth during sleep will mean your respiratory water losses are much greater.

3. Night sweats

Waking up in a pool of sweat is an obvious way to throw off the body’s fluid balance. Night sweats can occur for a number of reasons; the most common ones being overheating (bedroom too warm, duvet too thick) or the work of an underlying illness or medical condition. In women of a certain age, menopause can also be a cause.

4. Exercising too late - i.e. starting the night dehydrated!

This may only be a problem if you don’t rehydrate sufficiently before hitting the hay. There’s not much to say here, with some PH electrolytes on hand this one is easy to avoid. That, or start exercising earlier. 

5. A side-effect of the medication you’re taking

A number of medications have been linked to increasing the risk of dehydration at night, including antidepressants and decongestants. The latter are particularly prone to causing a dry mouth in the morning.

They’re designed to dry out the nasal passage but in reality can dry everything out. If you’re consistently waking up dehydrated, it’s worth checking the side-effects of the medication you’re taking.

Some of the above are avoidable (yes, I do mean that glass of red) and some less so. But rest assured that for most people, on most occasions, the body prevents serious dehydration overnight.

The role of evolution in sleep and preventing dehydration

We’re meant to sleep; when we’re young it’s vital for our growth and development and as we mature it’s pivotal to our well-being, productivity and, much, much more.

From an evolutionary perspective, we would have needed it to survive. With this in mind,the body has figured out a way to prevent us dehydrating overnight.

Further reading