I work overnight shifts (11pm-7am) and take caffeine pills to stay awake. When I get home, even if I take more caffeine I’m sometimes still too tired and fall asleep. Why does it eventually stop working? Is there some sort of chemical limit? Why can’t caffeine replace sleep for longer periods of time (like days on end)?
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Caffeine doesn’t make you not *need* sleep, it just makes you *feel* like you don’t need sleep. As you go through the day, your brain builds more and more *adenosine* which signals your body to feel fatigued. When you sleep, your body cleans up all that adenosine, so that in the morning you aren’t fatigued. The adenosine is really signaling to your brain and body that there’s a lot more going on – your brain needs to perform clean-up for a lot of different neurotransmitters and waste products.
The exact reason for why we need sleep is not well known. Probably we need to do that clean-up. Why can’t we do that while we’re awake? Scientists don’t know for sure. It probably interferes with your ability to form conscious, coherent thoughts. Sleep is also important for learning and building memories. Again, the exact mechanism isn’t known but during sleep your brain seems to build new connections better. So, what the adenosine is doing is telling your brain that it’s time for all that, *especially* the clean-up part.
Caffeine binds to the same receptors that the adenosine binds to, but caffeine doesn’t activate them. Instead, the caffeine just gets in the way and prevents the adenosine from working. It’s like the adenosine is a key and when it turns the “lock” in certain neurons, those neurons fire off “we’re tired” signals throughout your brain and body. The caffeine is like jamming the wrong key into that lock, so that it can’t unlock and the right adenosine “key” doesn’t fit in there anymore.
That works temporarily, but as your brain gets more and more “dirty” with waste products, and you keep putting off that clean-up cycle, the adenosine builds up more and more. You don’t build a “tolerance” for caffeine, so much as your tiredness just overwhelms the caffeine and you feel tired anyway. Eventually, your brain really just starts shutting down to preserve itself and do the clean-up tasks that it *must* do in order to function, regardless of how much caffeine you have.
Plus, caffeine stimulates the production of adrenaline, which is a hormone that signals your body to be alert and awake. For example, when you get scared, your brain dumps a ton of adrenaline to really ramp you up and get you ready to fight or run for your life. Your brain is always adding *some* adrenaline, though, to control how your body is functioning. Caffeine makes your brain increase the adrenaline that it produces, which gives you a bit more pep.
However, one organ significantly affected by adrenaline is your heart. Adrenaline makes your heart beat faster, spreading vital oxygen throughout your body when you might *really* need a lot (in order to fight or run). Your heart can’t keep that up forever, though. It never stops beating (until you die) but it *does* need to slow down and rest, just like your brain. Too much caffeine will cause an irregular heartbeat and can lead to palpitations (when you can feel your heart thumping in your chest). Normal caffeine consumption doesn’t really hurt you, but if you’re ingesting enough to keep you awake when it’s really unhealthy and you need to be sleeping – combined with your brain not really working the way it should be, the adrenaline can really mess with your heart start damaging your heart and fatigue your heart until it stops working correctly. Which is, you know, bad.
Point being, at some point your brain *must* sleep in order to function, and your heart *must* slow down and rest. Caffeine prevents those things from happening, which is ok when it’s in moderation, but pretty terrible if you try to postpone the inevitable.
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