eli5: Since caffeine doesn’t actually give you energy and only blocks the chemical that makes you sleepy, what causes the “jittery” feeling when you drink too much strong coffee?


eli5: Since caffeine doesn’t actually give you energy and only blocks the chemical that makes you sleepy, what causes the “jittery” feeling when you drink too much strong coffee?

In: 5

That is not even close to all caffeine does, that’s just why caffeine helps you not feel sleepy. Caffeine is a pretty non-specific stimulant, meaning that it stimulates most of your nervous system across the board. It does so by basically the same mechanism as its anti-sleepiness effect: it blocks adenosine receptors all over the central nervous system. Adenosine acts to “calm down” nerves, so blocking it makes nerves more active. (Insofar as any neurotransmitter has simple effects, anyway – brain chemistry is complex.)

By stimulating the parts of the brain responsible for regulating other parts of the body – like heart rate, breathing, etc – caffeine stimulates activity throughout your body. It raises your heart rate, raises your breathing rate, ~~dilates~~ (EDIT: contracts, sorry) your blood vessels, and generally raises your level of alertness in the same way that natural stimulants like adrenaline do.

The specific heart jitters are due to an effect of an adenosine receptor on *pacemaker cells*, cells in the heart that actually trigger the electrical signal of a heartbeat. Adenosine acts to calm those cells down, slowing the pace of the heart and keeping its rhythm regulated. Caffeine stimulates those cells, causing a rapid pulse, and can cause slight irregularities in the rhythm. You can sometimes feel these irregularities as small “bumps” (palpitations – they feel like a sudden “extra strong beat”) in your heartbeat – anatomically, they’re [premature ventricular contractions](https://en.wikipedia.org/wiki/Premature_ventricular_contraction), where one part of the heart “jumps the gun” and beats too early.

(EDIT2: User /u/H097366e6 [objects to the claim that caffeine causes PVCs](https://www.reddit.com/r/explainlikeimfive/comments/1355nt5/eli5_since_caffeine_doesnt_actually_give_you/jij06d5/) and has brought reasonable citations to the party. I’m not editing this part out since a number of major medical establishments seem to claim that it does, but it’s a compelling enough post to be worth including here.)

In general, it’s quite rare for psychoactive substances – things that can get into your central nervous system and mess with the nerves there – to have only one effect, because the brain regulates tons of stuff (and because there are plenty of neurons elsewhere in the body for them to act on, too).

(EDIT: there’s a lot of discussion down below about ADHD – see [this comment](https://www.reddit.com/r/explainlikeimfive/comments/1355nt5/eli5_since_caffeine_doesnt_actually_give_you/jiiwg3r/) for more on why caffeine often calms people with ADHD.)

ChubbyEmu explains it better than I ever could. And probably better than anybody here ever will.


The next link is him explaining it in much greater detail, but also not eli5



My understanding is that caffeine blocks the receptors that would normally grab onto the chemicals that, when roaming free in your body cause you to release adrenaline, so your body releases a whole lot more adrenaline than normal, and *that is* what you feel.

Stimulating the hell out of you. It definitely does more than just preventing you from getting tired , every one who touched it can report that