Appliance of Science: Why do we need to hydrate so much?

This week Dr Naomi Lavelle answers your questions on the super important H2O.

I don’t think there is anyone in Ireland who hasn’t noticed the lovely weather we have had recently. It is a great excuse to down tools and head outdoors into the lovely sunshine and warmth. My son was delighted to spend the day playing outside recently, but after a while he came seeking me out for a little rest and to pose the question “why does the sun make us feel so sleepy?”

It is a good question as we have all experienced it; and as usual, there is likely more than one reason, so I’ll start with the most obvious one; we tend to be more active on sunny days as we like to get outside and partake in more physical activities. But what other factors are involved?

Staying cool

We all love the warm feeling of the sun on our skin, but this heat also changes the internal temperature of our bodies. The sun’s energy transfers heat to our bodies through a process called electromagnetic radiation. Normal body temperature is usually between 36C to 37.2C. The body works hard to maintain this temperature; a little too cold and we start to burn calories in order to bring our temperature up. A little too warm and we need to activate the body’s cooling mechanisms. These processes all require energy and can lead to fatigue.

One of the main ways we cool our bodies down is to start producing sweat. The body activates sweat glands (we have about 2m of them all over our bodies) to start secreting sweat out through the pores in our skin. This sweat contains fluids and salts. Under warm, dry conditions the sweat will evaporate off our skin but this process draws even more energy from our bodies, leaving us feeling tired. When our bodies heat up they experience some other physiological changes too, like increased heart rate and metabolic rate, which again can tire the body.


Sweating can easily lead to an imbalance in body fluid and can cause dehydration. One of the symptoms of dehydration is, of course… fatigue.

Chemical changes

Our bodies also undergo some chemical changes when exposed to the sun. Firstly certain cells in our bodies need to work harder to help protect our skin from the sun’s harmful UV rays. Cells called melanocytes produce a pigment called melanin, which is responsible for tanning (or colouring) our skin and acts to absorb some of these harmful UV rays, in order to prevent skin cell damage. All this extra work load on the body can deplete our energy levels. Even with this protection, our skin cells still experience a certain level of damage and the body again has to work harder to repair it. More fatigue.

Changing our rhythm

A neurotransmitter called melatonin plays a large part in regulating our sleep patterns and its production is controlled, in part, by light. During daylight hours our melatonin levels are low but these rise in darkness, making us feel drowsy. It is possibly our melatonin levels rise when we come in from the sun, adding to our tiredness.

Whatever the reasons I don’t think any of us would swap those sunny days for the sake of a little more energy. As for my son, a nice drink of water and a cooling ice cream were all he needed before heading back out into the beautiful day.

  • Naomi is a science communicator and mother to three inquisitive children. She can be found at
  • Feel free to email your questions to






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