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effects of dehydration on the brain Why drinking water is important hydration and brain function how long does it take to rehydrate your brain

Why drinking water is important: Cognitive health benefits of staying hydrated.

Published Mar 14, 2023 | Updated Feb 8, 2024

“Stay hydrated.” is a universal health message that is encouraged across all life stages. Getting adequate fluids is imperative for normal functioning of your body. 60-70% of your body is water, while your brain is 75-80% water [1]. Hydration and brain function is strongly linked because keeping your brain hydrated will keep it functioning well. Even mild dehydration (losing 1-2% of body water) is known to affect the ability to focus and temporarily impact memory functions [2]. Losing over 2%, can seriously impact cognitive functions. The good news is rehydrating is quick, simple, and effective and in most cases, water does the job. 

Effects of dehydration on the brain: Reasons to stay hydrated.

Imagine, a hot summer day with temperatures around 33°C and you are working out in the heat for 50 mins [6]. Lack of adequate hydration in this scenario can easily dehydrate you, and you can lose up to 2% body weight in water.  This can lead to physical and cognitive changes in the brain, which are reversible when you re-hydrate yourself. Your hydration levels can make a big difference to your brain health: 

  • Better attention and memory

    Upon mild dehydration, sustained attention was affected. Those who abstained from water for over 24 hrs (which equates to being dehydrated to the same level as exercising or being exposed to heat for up to 2hrs), reported difficulties in completing cognitive tasks. They were 20% more tired and 18% less alert and reported needing additional mental effort to complete the memory and attention tasks [9]. If you have a test coming up, having a glass of water just before you begin can improve performance. In a study where participants consumed 120-330ml water just before tests reported improvement in sustained attention, and those who drank more, performed better [10]. In some school students, adequate hydration improved attention tests scores by 50% [11]. During dehydration, the levels of stress hormone, cortisol spikes because your body assumes this to be a stressful situation. High levels of cortisol reduce your processing speed and short-term memory [12]. 
  • Improved mood and energy

    If you are feeling low, it might be worth upping your fluids. Keeping yourself hydrated can help improve mood and energy levels. How long does it take to rehydrate your brain? It takes under 2 hours to rejuvenate your brain. Those who drank 500ml water following an overnight fast, reported 55% improvement in energy levels and 5% improvement in mood, after 90 minutes [13]. People who drank less water (<1L/day) reported improvement in sleep, mood, and energy levels when they increased their intake to 2.5L/day for a week [14].
  • Shrinking of brain structures: Since the brain is almost 80% fluid, lack of adequate hydration impacts the volume of the structures. 16 hours of dehydration reduced the brain volume by 0.55%, and when rehydrated by 1.5L water intake, the volume increased by 0.72% [7]. Reduced brain volume has a negative impact on cognition and is a reason for lower cognition in old age. Those who were dehydrated due to low fluid intake (<150ml/day) were found to have lower volume of brain cells and the connections between these cells [8]. This can help explain why you experience lower focus and brain fog when you are less hydrated because your brain is running on sub-optimal capacity

    How much fluid should you drink?

    The European Food Safety Authority (EFSA) recommends consumption of 2 litres of water for women and 2.5 litres for men, for maintenance of normal physical and cognitive functions [3]. The NHS Eatwell Guide recommends 6-8 glasses of fluid per day, including water, low-fat milk, and lower-sugar beverages including tea and coffee [4]. It is essential to top off any water losses and maintain a balance, especially in cases where you lose water due to being in hot and humid conditions or after a workout. The colour of your urine is a good indication of your hydration status [5]. Aim for a light straw-coloured urine. A darker colour signals dehydration since urine is concentrated due to insufficient water available in the body. 


    1. Oros-Peusquens et al. (2019). A Single-Scan, Rapid Whole-Brain Protocol for Quantitative Water Content Mapping With Neurobiological Implications. Frontiers in Neurology.
    2. Kostelnik, S. B. et al. (2021). The Validity of Urine Color as a Hydration Biomarker within the General Adult Population and Athletes: A Systematic Review. Journal of the American College of Nutrition40(2), 172–179.
    3. EFSA (2010). Scientific Opinion on Dietary Reference Values for water. EFSA Journal, [online] 8(3).
    4. National Health Service (2019). The eatwell guide. [online] 
    5. Belasco, R. (2020). The Effect of Hydration on Urine Color Objectively Evaluated in CIE L*a*b* Color Space. Frontiers in nutrition7, 576974.
    6. MacLeod, H. et al. (2018). Effects of heat stress and dehydration on cognitive function in elite female field hockey players. BMC Sports Science, Medicine and Rehabilitation, 10(1)
    7. Zhang, N. et al. (2022). Dehydration and rehydration affect brain regional density and homogeneity among young male adults, determined via magnetic resonance imaging: A pilot self-control trial. Frontiers in nutrition9, 906088.
    8. Streitbürger, D.-P. et al (2012). Investigating Structural Brain Changes of Dehydration Using Voxel-Based Morphometry. PLoS ONE, 7(8), p.e44195
    9. Szinnai, G. et al. (2005). Effect of water deprivation on cognitive-motor performance in healthy men and women. American journal of physiology. Regulatory, integrative and comparative physiology289(1), R275–R280.
    10. Liska, D. et al. (2019). Narrative Review of Hydration and Selected Health Outcomes in the General Population. Nutrients, [online] 11(1), p.70.
    11. Samy, H. (2021). Effect of Hydration Status of School Children on Cognitive Performance and Impact of Health Education on Their Drinking Behavior. The Egyptian Journal of Community Medicine, [online] 39(2)
    12. Masento, N.A. et al. (2014). Effects of hydration status on cognitive performance and mood. British Journal of Nutrition, 111(10), pp.1841–1852.
    13. Zhang, J. et al. (2020). Different Amounts of Water Supplementation Improved Cognitive Performance and Mood among Young Adults after 12 h Water Restriction in Baoding, China: A Randomized Controlled Trial (RCT). International journal of environmental research and public health17(21), 7792.
    14. Pross, N. et al. (2014). Effects of changes in water intake on mood of high and low drinkers. PloS one, [online] 9(4), p.e94754.

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