The Impact of Dehydration on Cognitive Functions and Mood

Dehydration impairs concentration, short-term memory, and overall mood.

Since the brain is composed of 75% water, it would be interesting to study the relationship between dehydration and cognitive functions. This study examined the effect of dehydration on the ability to focus on tasks for extended periods and short-term memory.

During the study, participants were restricted from consuming food and water to achieve the required level of dehydration. Subjects were instructed to engage in routine daily activities while avoiding intense physical exertion. The experiment lasted 4 days, during which the participants underwent medical tests (urine and blood osmolality to assess dehydration levels; anthropometric measurements—weight, height, blood pressure), mood and thirst perception assessments, as well as cognitive tests:

• Forward and backward digit span — measures short-term auditory memory, working memory, and information manipulation skills.
• Symbol-digit coding test — assesses information processing speed, short-term visual memory, attention, and concentration.
• Stroop test — evaluates cognitive flexibility, information processing speed, and the ability to suppress interference.

The study involved 12 male volunteers who were medical college students from Cangzhou, China.

On the first day, all participants refrained from eating or drinking after 8:00 PM and were required to go to bed by 11:00 PM. During the night, they were not allowed to urinate to ensure the necessary tests could be conducted in the morning. The following morning at 8:00 AM, participants underwent medical tests, cognitive assessments, and psychological evaluations (baseline testing). After the tests, they were allowed to drink water and have breakfast.

On the second day, the procedure was repeated: from 8:00 PM to 8:00 AM the next day, participants did not eat or drink and went to bed no later than 11:00 PM.

From 8:00 AM on the third day to 8:00 AM on the fourth day (24 hours), participants refrained from drinking water. However, three meals (breakfast at 7:00 AM, lunch at 12:00 PM, and dinner at 5:30 PM) were arranged to achieve the required level of dehydration.

On the morning of the fourth day (8:00 AM), participants again underwent the previously described set of tests (dehydration testing). Starting at 8:30 AM, they drank 500 ml of water every 5 minutes over 15 minutes (by 8:45 AM, each participant consumed 1.5 liters of water). Over the next hour, participants rested while avoiding physical activity. At 9:30 AM, a follow-up round of testing was conducted (rehydration testing).

Throughout the study, participants were instructed to avoid intense physical activity. Those who violated the experimental requirements were excluded from further participation.

The study yielded the following statistically significant results:

• During dehydration, participants experienced decreased alertness, self-assessment scores, and digit span capacity, along with an increased error rate in paced task performance.
• Following rehydration, fatigue levels decreased, mood improved, and digit span capacity (forward, backward, and total) increased. Additionally, symbol-digit substitution accuracy, reading speed, and mental efficiency improved, while reaction times decreased.

The results of the study demonstrated that dehydration negatively affects alertness, self-assessment, short-term memory, and attention. Rehydration through water consumption reduces fatigue and improves overall mood, short-term memory, attention, and reaction time.