Coffee — specifically the caffeine — is a stimulant. Stimulants are a class of chemical/drug that give you energy (“uppers”). You may be familiar with some of the other things they do like make your pupils larger, make your heart beat faster, raise your blood pressure, etc.
It turns out your body does not like your blood pressure elevated significantly above normal for a prolonged period of time. It can do bad things over time like cause an aneurysm (a balloon-like bulging in your blood vessels can rupture, which can essentially be fatal even with rapid medical attention), stroke, kidney damage, dementia, etc. Your body has three mechanisms to lower your blood pressure. One is to widen your blood vessels (vasodilation), one is to slow your heart rate, and another is to get rid of the fluid in them. Your body uses the latter to counteract caffeine.
The body uses sodium ions to retain water/fluid within its blood vessels. When you purge the salt, water/fluid follows. Caffeine stops the kidneys from holding on to salt causing it to be expelled into urine with water following soon thereafter.
Ask someone to sing “We are the Champions” by Queen and they’ll close with the oft-sung “We are the champions…..of the world!” Except, Queen never sang that in the song. Other examples of things you’ll swear you remember that just are not true include the famous Forrest Gump line never uttered “Life is like a box of chocolates.” (Life was like a box of chocolates) and Darth Vader uttering “Luke, I am your father.” ( “No, I am your father.” )
How can this be?! The human memory is known to be less than perfect. In fact, it is known that how you remember things depends on your biases and prejudices, the word choice someone uses to ask you to recall an event, and how others witnesses recall the event. Further, science shows every time you recall an event, the memory (and the neuronal circuitry used to store/retrieve it) changes slightly. It is believed that false memories arise and are spread such that they become taken as truth.
This phenomenon in which false memories propagate is known as the Mandela Effect. It was coined by paranormal researcher Fiona Bloome when a popular false memory circulated regarding Nelson Mandela’s death broadcast in the 80s (who would not pass until 2013).
Interestingly, some have gone on to claim these inconsistencies are proof of parallel universes or that the timeline we live on is being altered.
Exercise and your body’s normal metabolism uses energy. ATP (Adenosine triphosphate) is energy currency — if you will — used by the body to carry out those functions. Adenosine is a byproduct that signals to the body that energy is being depleted and it would probably be a good idea to rest and wind down. It signals this to the brain via adenosine receptors A1 — more directly associated with fatigue and A2a — more associated with the mood related to fatigue). Caffeine stops adenosine from telling the brain you’re tired when it blocks A1, and A2a blockade causes a release of dopamine which makes you feel good.
These effects are really pronounced in someone who drinks coffee rarely or who has just begun to drink coffee. They are diminished over time when someone drinks coffee regularly. This is because to combat the effect of caffeine blocking the A1 and A2a receptors, the brain produces more of both. This is called upregulation. Because the same amount of caffeine can only block the same amount of receptors, adenosine can signal tiredness more effectively. This is why coffee drinkers find they may need to drink more to get the same effect. This is called tolerance.
**Fun fact, caffeine causes vasoconstriction — the tightening of blood vessels. When one stops drinking coffee, their blood vessels vaso-dilate or widen. It is believed vasodilation of the blood vessels in the brain are responsible for headaches.