The average human body is around 70% water. The actual water content is controlled by the brain which keeps a certain percentage as a reserve in case it is suddenly required, most usually for sweating. Sweating is one of the ways in which our bodies maintain the correct temperature (sweating places moisture on the skin which removes heat from the skin as it evaporates), however, even when we are not actively sweating we still use water, for example in respiration.
The actual amount of the reserve we need is not a constant. In this, as with everything else, the body reacts to the world around it and the particular situation it is in. If the ambient temperature drops then in a fairly short space of time the brain will react to this. The reaction is that the brain concludes that due to the drop in temperature sweating is going to be less likely or less extreme so the excess water that the brain requires as a backup decreases. The body then rids itself of the unrequired excess through urination, which is why we often find we need to urinate when the temperature drops, even if we haven’t drunk any liquid for some time.
It is also the case that when this water reserve drops below a certain level, thirst is triggered. In this way the body / brain sends the message that more water is needed, we drink, and the reserve levels of water are topped up. The entire process works by way of the body gauging the level of reserve water required compared to what it currently holds.
The easiest way of picturing this is to think of the fuel gauge on a car. It shows how much fuel is in the tank, and if the fuel drops below a certain level usually a red light comes on as an additional warning. In the same way your body has a gauge that determines the level of reserve water and when this level drops below a certain level the thirst mechanism is triggered.
What alcohol does is to mess around with the operation of that gauge, specifically it recalibrates it such that your body thinks it has more water than it actually has. To go back to the car analogy, it is like recalibrating the fuel gauge so that the gauge shows full when the fuel tank is in fact half empty. In fact the analogy with the car isn’t ideal because a fuel gauge can only swing from empty to full, it cannot jettison additional fuel if the gauge showed that the tank had somehow been overfilled. However this is exactly what happens with the human body. The human body will jettison the reserve if it assesses that it has an unwanted excess of reserve water. Of course when the effect of the alcohol wears off the gauge is reset back to the correct level. The body then realises that it does not have enough reserve water and it looks to replenish this supply. It does this by triggering the thirst mechanism, we drink some liquid and the supply is topped up to the correct level. The important point to note is that the internal gauge will not return to normal until the effect of the alcohol has worn off.
Let’s now look at a practical example to highlight this by looking at a hypothetical person with a 100% correct water reserve, i.e., at that particular point in time they are perfectly hydrated in that they have all the water they need but no more (so they do not need to urinate). They then start drinking alcohol. Now, apart from the fact that alcohol is consumed as a liquid which will need to be urinated out, the alcohol also recalibrates their internal water gauge with the effect that a percentage of the reserve water is also expelled. This is why after a few drinks we start needing the toilet increasingly regularly and we do not just urinate out the exact amount of liquid that we consumed, but much more as our body also rids itself of (what it thinks is) an unwanted excess of reserve water. The next morning, or during the course of the next day (if we are not consuming more alcohol) the effect slowly wears off and the body gauges that it does not contain enough reserve water and we start to feel thirsty. If we take a (non-alcoholic) drink we replenish the water and start to return to normal.
As I have mentioned previously the important point to note is that the gauge is not recalibrated back to the correct position until the alcohol wears off. The important conclusion to note from this is that we will not feel thirsty as a result of the dehydration until the thing that has caused the recalibration of our internal water gauge has worn off. After all, why would our body be triggering the thirst mechanism when it is under the impression that we have the correct amount of water anyway? In this way we do not feel thirsty when we are actually drinking alcohol because the thirst mechanism is not triggered because, in turn, our body does not realise that it does not have enough water in it. You do not feel thirsty when you are dehydrated due to alcohol consumption until the effect of the alcohol has entirely worn off.
The further conclusion of this is that it is absolutely pointless trying to rehydrate yourself during a drinking session or immediately after it (such as on the way home). A friend of mine used to drink a couple of bottles of water on the way home after drinking in the hope that it would rehydrate him and ward off a hangover. In fact doing this will have done absolutely nothing at all other than disturb his already severely hampered sleep still further by causing him to need the toilet even more during the night. His body would not have kept hold of any of the water he drunk even though it was dehydrated at that point, because it was acting under the erroneous impression that it already had an adequate water content. It is only when the effect of the alcohol has worn off and the thirst mechanism is actually triggered that the water will be retained by the body and actually put to use, as it is at this point that the internal water gauge has returned to its correct position. You will always know when the gauge is returning to normal because you will begin to feel genuinely thirsty again.
This is why exercising after drinking, or exercising with a hangover, is so dangerous. Not only is it the case that the body simply doesn’t have the reserve of liquid to deal with strenuous exercise (which causes both sweating and increased respiration) but the internal water gauge is still malfunctioning so the body doesn’t even realise there is a problem (i.e., hydration levels have dropped to critical levels) until it is too late. When the body recognises that the water level is low it will send us messages telling us. We will feel thirsty, hot, exhausted, and we will be forced to stop what we are doing. However, if the body is not aware that the level is too low the messages telling us to stop, to cool down, and to drink water, will not be triggered.