Why staying hydrated helps your brain health in a heatwave

This summer, the UK has already recorded more days above 30-degrees than it did in the whole of 1976 or the whole of 2020, two summers widely remembered for historic heat. The Met Office has issued multiple heat warnings with forecasters expecting temperatures could reach 36-degrees. While the majority of people enjoy some heat, as a nation, temperatures approaching 30-degrees are not something we’re equipped to handle. The North Atlantic Drift and jet stream result in the UK having a predominantly maritime climate – with mild, wet winters and relatively warm, but changeable summers. British infrastructure is not built to deal with continuous heat – made more apparent by the fact that houses are fitted with central heating, not air conditioning units and are often built to retain the heat. This can quickly make everyday life feel uncomfortable and challenging.

Within the human body, water makes up approximately 73-75% of the brain – it is essential to how neurons in the brain ‘fire’, how nutrients move through brain tissue, and how waste products are cleared by the glymphatic system when we sleep. When the body loses water faster than it replaces it – with even a small amount of fluid loss (up to 1-2% of body weight) – research suggests the brain may immediately be impacted, even before you become thirsty. You may begin to experience effects such as brain fog, a lower attention span, or slower memory and processing speed.

On top of natural water loss, exercise, humidity, and heat can greatly increase water output. When the core body temperature rises, the body prioritises cooling itself by redirecting blood towards the skin to dissipate heat. This may result in less blood, oxygen and glucose (the brain’s main source of energy) being available for the brain’s everyday workload. In the 2026 paper Nutrition and Hydration Strategies for Heatwave Adaptation it was found in one study that even mild dehydration – where the body loses a small amount of fluid, and lacks enough water to perform at its best – is enough to affect cognitive output. Dehydration has also been linked to short-term memory loss, slower reaction times, impaired concentration, and a drop in executive function.

As well as affecting the body during the day, the heat can also disrupt sleep. Overnight temperatures that stay uncomfortably high may interfere with the deep stages of sleep. The brain needs this time to clear the toxins accumulated throughout the day, but also to consolidate the memories formed. When the heat impacts brain health at night, we’re not able to fully rest and reset, which is needed to help us carry out everyday executive function and perform at our best in everyday life. 

Older adults are also vulnerable during a heatwave, for as we age, the body’s natural thirst response blunts, and so many older people don’t feel thirsty even though they are. In addition, impaired temperature sensing can be experienced – this means that the regions of the brain responsible for thermoregulation find it harder to sense and respond to extreme heat. The resulting combination of dehydration and heat stress can have a significant negative impact on cognitive wellbeing, and can worsen confusion, agitation and brain fog. This becomes a dangerous loop if an individual has a neurodegenerative disease such as Alzheimer’s or other forms of dementia, or they are unable to help themselves. This is why the UK Health Security Agency (UKHSA) specifically flags risk for older populations with heat alerts, and where the cognitive and physical effects of heat and dehydration arrive faster and hit much harder later in life.

Five ways to stay hydrated in a heatwave

Unlike many risk factors for cognitive decline, dehydration is one that is within our power to correct within hours – rather than over months or years. Here are five practices that will make a big difference to brain health during hot weather:

1. Drink before you’re thirsty. By the time you’re thirsty, mild dehydration has already kicked in. A great way to stay hydrated is to sip water steadily throughout the day, rather than gulping large amounts when you’re thirsty. This will help keep fluid levels stable and can help keep headaches, low mood, and irritability at bay. 
2. Balance water with electrolytes. When the body is hot, it sweats to control our body temperature. Heavy sweating depletes electrolytes including sodium and potassium (alongside water), which are essential for basic functioning. Therefore replacing those fluids is key. Good sources can be found in your daily diet, such as sodium in salt and potassium can be found in fruits and vegetables like bananas and spinach.  
3. Eat fresh and hydrating foods to increase water intake. Good examples are watermelon and cucumber, which contain up to 92% and 96% water respectively. Eating them helps to replenish your body with vital fluids and electrolytes needed on the hotter days. 
4. Try to time cognitively demanding work around the heat. If possible, try to tackle work that requires high levels of focus and concentration during the cooler parts of the day, and treat the hottest hours as a time to slow down. 
5. Check in on older relatives and neighbours. Because the thirst response depletes as we get older, take the time to check in on others, especially those who are unable to help themselves easily. This has the power to catch a hydration problem before it becomes a medical one.

Heatwaves are becoming more frequent and more intense across the UK and it’s unlikely that this summer will be the last series of heatwaves we experience. Beyond the immediate advice to drink more water and understand the brain and body’s tell-tale signs of dehydration, it’s also worth putting this advice into practice beyond the heat. Hydration is key for good health – hot weather or not – so treating these practices as foundational rather than as a summer special will help to protect brain health longevity in later life. 

For more information on how to look after your brain health in later life visit www.brain.health.