Dehydration results in reducing the efficiency of the basic cellular function and disruption of normal functioning of organs
Dehydration occurs when a body loses more water and fluids than it takes in. Nearly all of the major systems in your body depend on water. On a biological level, water is a universal solvent that transport nutrients and oxygen; helps with absorbing and assimilating minerals, vitamins, amino acids, and other substances; carries various molecules into and out of the cells; creates a medium for chemical reactions; flushes out toxins and waste, and more.
According to Professor Neumyvakin, dehydration starts first with losing inner cell liquid volume (by 66%) resulting in aging symptoms: dry skin, loose elasticity, wrinkles, etc. Then liquid volume decreases between cells (by 26%), and after that a body will take water from a blood stream by 8%. All of that is done to keep water in our brains. To prevent water loss in brain, a human body tries get water out of blood vessels that results in decreasing their diameter, thickening of blood and clogging the blood vessels. Dehydration results in reducing the efficiency of even the basic cellular function, disruption of normal functioning of organs and immune system.
But the most dangerous is decreasing water in brains cells that generally contains 85-92% of water. Losing more that 1% of water in a brain could cause irreversible effect on a brain activity. According to National Library of Medicine, a body water loss of 1–2% can impair cognitive performance.
Signs of dehydration could be: headache, light-headedness, vertigo; fatigue, depression, insomnia, dry skin, etc.
That is why it important not to wait until you feel thirsty but intake water regularly throughout a day.