Researchers found those who habitually drink less release greater levels of the stress hormone cortisol, despite not feeling any thirstier than people who drink more.

Keeping a water bottle nearby during stressful periods could be beneficial for long-term health, researchers suggest.

The study, led by experts at Liverpool John Moores University (LJMU), included 16 people who drank under 1.5 litres of water a day, along with 16 people who regularly met daily recommended guidelines for fluid intake.

Researchers used the European Food Safety Authority (EFSA) guidelines, which recommend men should drink 2.5 litres of water a day while women should drink two litres.

In the UK, the Eatwell Guide suggests adults should drink between six and eight cups of fluid a day, which is roughly 1.5 to two litres.

Researchers monitored hydration in both groups over seven days using urine and blood samples

However, people may need to drink more when it is hot, if they are very active, if they are recovering from an illness, or if they are pregnant or breastfeeding.

Researchers monitored hydration in both groups over seven days using urine and blood samples, after which they were invited into the lab for a stress test.

The test involved an impromptu job interview, for which each individual was given 10 minutes to prepare for.

After that time, they were invited into another room where a fake camera was set up and asked to do the interview to a panel of three people dressed in white coats.

Following the fake interview, individuals were then asked to do a mental arithmetic challenge which involved subtracting numbers as fast as they can.

People who have a low daily fluid intake are likely to be poorly hydrated

Professor Neil Walsh, of the LJMU School of Sport and Exercise Sciences, described the test as “really flustering”.

He told the PA news agency: “We know that people who have a low daily fluid intake, who don’t meet the recommendations, are likely to be poorly hydrated.

“But what we didn’t know was whether, when you then stress those people under controlled conditions, they would have a greater stress hormone response.”

Researchers collected saliva samples from the group before and after the stress test to measure cortisol levels.

Cortisol, known as the stress hormone, is produced by the adrenal glands in response to stress.