Why Does Water Taste Bad In The Morning After Sitting Out?

Taking a refreshing gulp of water is a great way to start the morning. But hydration can turn to horror when the glass on your nightstand seems to be filled with a weird-tasting, stale liquid instead of thirst-quenching water. Why does water taste bad in the morning after sitting out?

Water tastes bad in the morning after sitting out mainly because of carbon dioxide dissolving into it, making it taste acidic and stale. If the water tastes sweet, your mouth is likely acidic. Water sitting out overnight is generally safe to drink.

There is also chlorine evaporating from it, and particles in the air making other chemical and bacterial changes to the water’s composition.

Keeping a water glass or bottle at your bedside seems like a good idea, especially if you wake up with a dry mouth. But the odd flavor can be worrying. Let’s look at why water tastes bad after sitting out overnight and whether it is dangerous to drink.

Reasons Why Water Tastes Bad After Sitting Out

Tap water doesn’t consist of pure hydrogen and oxygen (H2O). Some minerals and other molecules react with the environment, changing how water tastes.

The main reason water tastes bad after sitting out overnight is that chemical changes occur in the water, altering its flavor. What are these changes that make water seem stale?

1. Carbonic Acid Makes Water Taste Bad

Water will taste different in the morning if you leave it exposed because the carbon dioxide in the air dissolves in the water, causing a chemical reaction.

The air around us contains various gases, including oxygen and carbon dioxide. Carbon dioxide reacts with the water in your glass, creating carbonic acid.

Carbonic acid has a bitter flavor, which is what you taste when you sip water in the morning. 

Although carbonic acid makes the water unappealing to drink, it is harmless. Our bodies convert the acid back to carbon dioxide, which we then breathe out.

2. A Lack Of Chlorine Makes Water Taste Bad

As carbon dioxide enters the water, another chemical reaction takes place between the air and the chlorine in the water.

Our tap water often contains chlorine, which water treatment plants add to water to destroy bacteria and make it suitable for drinking. Many people add larger amounts of chlorine to their swimming pools for the same reason.

However, when exposed to air, the chlorine in the water evaporates, changing the chemical makeup of the liquid. (Getting rid of chlorine is why you let tap water sit out before putting it in a fish tank.)

We are used to the flavor of chlorine, so water that has been sitting out will taste stale.

3. Dirt Makes Water Taste Bad

The water may also taste horrible if your glass isn’t as clean as it should be.

Your water may contain one of the following, which will influence the flavor:

  • dish soap from washing up or the dishwasher
  • dust from a glass standing on a shelf for a few days
  • dirt from the environment (e.g., if you’ve left a window open)
  • bacteria from your mouth or dirty hands.

Introducing extra microbes means more things to affect the taste.

4. Warmth Makes Water Taste Bad

The temperature of your water intensifies the flavors, so you will taste them even more strongly if the water is warm.

Cold water seems more refreshing than warm water because it suppresses your sense of taste, so you enjoy the cold, wet sensation rather than the flavor of the water. Cold beer tasting much less bitter than warm beer is the same perception by our taste buds.

Once the water gets warm, you can taste the water and all the flavors you don’t typically experience. To see more about how temperature affects the taste of drinks, see my post on why cold drinks taste better.

Why Does Water Taste Sweet In The Morning?

While many people find that water sitting out tastes unpleasant in the morning, others find that the water tastes sweet. There are three reasons why water may taste sweet.

1. An Acidic Mouth Makes Water Taste Sweet

When you wake up, your water may taste deliciously sweet. The reason isn’t the water but the conditions in your mouth.

Overnight, oral secretions from saliva can dry out your mouth, creating an acidic environment. The contrast between the taste in your mouth and the refreshing sip makes the water taste unusually sweet.

Check out my post on why water tastes so good when you are thirsty.

Conditions like acid reflux can also cause an acidic taste in your mouth. If you suffer from chronic GERD (gastroesophageal reflux), this experience can be common. To avoid this situation, finish eating for the day at least four hours before bed.

2. Diabetes Makes Water Taste Sweet

Another case of the oral environment making water taste sweet is if you have diabetes.

Diabetes is a disease of the pancreas, making it difficult or even impossible for your body to metabolize sugar. 

However, this sign of unstable blood sugar levels will persist, not only appearing when you drink water.

3. The Water Is Actually Sweet

If you are entirely healthy and your water always tastes sweet, it may be that you have sweet water.

Sweet water usually comes from wells or natural springs, which are more common in rural areas. Water from wells and springs contains high levels of calcium and iron, which create a pleasant taste.

Is It Ok To Drink Water That’s Been Sitting Out?

It is safe to drink water that has been sitting out overnight, despite the odd taste.

The chemical changes that take place in water, such as the carbon dioxide and chlorine reactions, are not hazardous to human health.

Drinking stale water is only a cause for concern when:

  • the water has been sitting out for more than 12 hours
  • the container is dirty
  • the glass or container is uncovered
  • the water has been in direct sunlight
  • you shared the water, possibly transferring saliva, sweat, or skin cells
  • you have a compromised immune system.

Under these conditions, bacteria can develop in the water and have potentially dangerous effects: think of how algae develop in standing pools of water.

If you want water next to your bed, cover it before leaving it overnight. Discard any leftover water in the morning and wash the glass.

How Long Can Water Sit Out Before It Goes Bad?

Water left sitting out will remain safe to drink overnight but should be thrown out the following day, especially if exposed to the air.

Water does not contain nutrients like protein or sugar, so it cannot go bad like other food. However, it can contain bacteria, which can contaminate the water. 

Most developed countries’ tap water contains enough chlorine to keep bacteria at bay, but leaving water sitting out allows the chlorine to dissipate. Chlorine will evaporate within a day or two, leaving the bacteria free to flourish. 

A warm environment encourages bacterial growth, so avoid water that has been sitting in the sun for a few hours. Sharing a water container increases the possibility of bacteria; instead, have individual containers.

How To Keep Your Water Tasting Good

These handy tips will help you avoid waking up to a nasty glass of water:

  • Replace your water frequently. Ideally, finish each serving of water before pouring fresh water.
  • Always use a clean glass. Reusing the same container without washing it can lead to contamination.
  • Cover the glass or carafe, or use a closed water bottle.
  • Don’t leave water out for days, even if it is covered and in a clean container.
  • Don’t share an unwashed glass or water bottle with others.
  • Never leave water in direct sunlight.


Water tastes bad in the morning after sitting out all night because it absorbs carbon dioxide, creating sour-tasting carbonic acid. The chlorine in the water will evaporate, also altering the taste. Your oral environment may influence how you taste water, sometimes making it seem sweeter.

To avoid water contamination, keep fresh water in a clean, covered container. In the morning, discard leftover water and wash the container.

Tom Hambly

Tom Hambly is the founder of Totally Drinks. He loves to cook and drink nice things - his favorite drinks are wine, beers and whiskey. About Tom Hambly.

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