Why am I dehydrated in the morning?
When sleeping, we stop taking in water (obviously - who drinks in their sleep!?). We still lose water though: through the skin as sweat, through moisture when we breathe, in our faeces, and through the making of urine. This means that we net lose water while we sleep, so we get dehydrated at night.
Why do I wake up more dehydrated when I have a cold?
When we breathe in, the air is heated and humidified as it moves into the lungs. When we breath back out, some of this heat and water is lost. The conditions that you’re sleeping in affect this, for example colder nights lead to drier air and more water lost when you breathe out.
Breathing through your nose is your body’s natural mechanism to reduce water loss through breathing in your sleep, because you’re able to hold onto more of that water than you can through mouth breathing (take note mouth breathers!). With a blocked nose, like when you have a cold, you’re forced to breathe through your mouth, so you lose more water and wake up more dehydrated.
Why do I wake up more dehydrated when it is hot?
During the night your body doesn’t need to do very much, so it can quite literally chill out, and be at a slightly cooler temperature than in the daytime, to save energy.The body cools as we fall asleep, through a few mechanisms:
- an increase in blood flowing to the skin where the heat it carries can be lost to thermal radiation
- Sweating: at night we sweat more at any given body temperature than during the day in order to cool off. For every gram of water you lose through sweat, your body cools accordingly. So sleeping in a warmer environment means we need to sweat more to get to that chilled out sleeping temperature. This means we lose more water, and so we wake up more dehydrated!
Why do I wake up thirsty?
When your brain senses that you’re low on water, it releases a hormone called ADH (antidiuretic hormone). This tells your kidneys to absorb more water back into the body, rather than making urine. It also makes you feel thirsty to get you to drink more water.
At night, we release more ADH from the brain as a natural way to stop us getting dehydrated during the night. As we get older though, the daily rhythm of the part of the brain that releases the ADH is thought to decrease, which may be why as we age we tend to need to get up to visit the bathroom more often. This was highlighted by a recent study that showed that peeing more at night and disturbed sleep happens to people who lack that increased release of ADH at night.
How can I be more hydrated in the morning?
Scientists have shown in some animals that they release more ADH before going to sleep, which makes them thirsty so they drink before bed. Although we haven’t shown this in humans yet, drinking more water before bed can similarly improve morning hydration. A hydrating beverage or just plain water is better than a fruit juice or soda, which may not actually hydrate us very well.
Maintaining your hydration levels throughout the day is also important. Fruit and vegetables are a good source of water, and those of us who eat more of them have shown better hydration. We get 20 - 30% of our water intake from foods, so diet is important for hydration too. Drinking lots as soon as you wake up won’t necessarily hydrate you, but instead produce lots of dilute urine as your body tries to deal with the sudden increase in fluid. It’s better to drink steadily through the morning and rest of the day, or to drink a product containing electrolytes that will help your body to retain that water. Why not try out Hydrant?
 Iglesia, I et al, Fluid consumption, total water intake and first morning urine osmolality in Spanish adolescents from Zaragoza: Data from the HELENA study, European Journal of Clinical Nutrition, 70.5, 541-547, (2016) (This paper looks at hydration in teenagers and how different foods and drinks influence this. Generally, those who ate more fruit and vegetables were better hydrated.)
 Jéquier, E, et al, Water as an essential nutrient: The physiological basis of hyhdration, European Journal of clinical nutrition, 64.2, 115-123 (2010) (This is a good overview of the importance of water and its different roles in the body!)
 Van Tilburg, C, Attitudes toward medical aid to developing countries, Wilderness and environmental medicine, 6.3, 264-268 (1995) (If we get dehydrated when we sleep normally, imagine how bad it must be when sleeping up a mountain! This paper looks at how in this extreme environment people can try to reduce the water loss caused by this dry, cold air.)
 Thornton, S, Dehydration during sleep affects cognitive performance, Sleep medicine, 13.1, 118 (2012) (This paper argues that dehydration overnight can negatively impact cognition in the morning.)
 Geschickter, E et al, Nocturnal body temperature regulation in man: a rationale for sweating in sleep, Journal of Applied Physiology, 21.2, 623-630, (1996) (Why do we sweat overnight? A paper explaining the cooler temperature that our body has to get to when we sleep and how we cool down.)
 Timbal, J, Circadian variations mechanism, Journal of Applied Physiology, 39.2, 226-230, (1975) (More information on how and why our body temperature is slightly lower when we sleep.)
 Gizowski, C, Central and peripheral roles of vasopressin in circadian defense of body hydration, Best practice and research: clinical endocrinology and metabolism, 31.6, 535-546 (2017) (An in depth look at the hormone ADH (vasopressin). It also discusses how ADH regulates overnight hydration to make sure we aren’t too dehydrated when we wake up.)
 Colwell, C, Preventing dehydration during, Nature Neuroscience, 13.4, 403-404 (2010) (A more accessible paper on how ADH (vasopressin) release late in the night prevents dehydration as we sleep.)
 Miller, E et al, Effect of nighttime eating on next morning hydration status and running performance in female endurance athletes, ProQuest Dissertations publishing (2015) (Although not the main focus of this paper (they were looking at whether or not drinking chocolate milk before bed made us more hydrated in the mornings than other drinks… sadly not!), the experiments done show that drinking fluid before bed makes us more adequately hydrated when we wake up.)
 Thornton, S, Thirst and hydration: Physiology and consequences of dysfunction, Physiology and behaviour, 100.1, 15-21 (2010) (Look at this paper for specifics on the importance of hydration, how it is detected and how our body responds. Interestingly, we don’t respond that well to thirst so may remain in a slightly dehydrated state, contributing in the long term to poor health.)
 Cheuvront, S et al, The void in using urine concentration to assess population fluid intake adequacy or hydration status, American Journal of clinical Nutrition, (2016) (This paper looks at how reliable measuring urine concentration is as a method for determining your hydration status. There are lots of confounding factors which makes it difficult.)