I can speak from my experience. I was for a long time drinking 2-3 cups of coffee in the morning. During the morning, generally things were good and I could crank out code. However, during early-mid afternoon I'd often get overwhelmed, tired, and unfocused, especially when multitasking, and often would lapse into a browse-reddit/agh-i'm-not-being-productive/browse-more-reddit cycle. Cutting down to one 1 cup in the morning has made a real difference - I no longer get the early afternoon slump, and can stay focused for longer in the afternoon without being overwhelmed by a long task list.
I'll throw in a personal experience with caffeine. I was feeling really bad (anxiety-attack-ish, according to Google) whenever I sat down to work on my thesis, a few years back. I'd use nice coffee to bribe myself to work on the thing. One day I went to work on the thesis after going to the gym and left my heart-rate monitor on. I noticed that this bad feeling coincided with a seated heart rate of 110 bpm, long enough after exercise that it should have been around 68. Investigation and experimentation ensued. Short version: caffeine + hard thinking about something scary launched me into a real physical anxiety attack; just switching to green tea allowed me to continue my work ritual without the panic.
I've found that when I drink coffee regularly over a long period, my intake invariably increases. Eventually, it reaches 2 cups of double-espresso-with-milk or equivalent a day, and goes past that too... after a few weeks or something months of that, I start getting strange episodes at random times that I describe as "light anxiety attacks".
The way they feel is that I get an impression that something, SOMETHING is wrong. I'm not sure quite what, but I don't realise that I'm not sure what until I realise I'm having one of these attacks. There's an unpleasant feeling of tension that starts in the chest and radiates out to the rest of the body - all the way to fingers and feet, through shoulders, knees, etc. That's the physiological aspects. Mentally, I feel like everything is going wrong. Nothing looks like it's going to work out. I tend to be more aggressive and curt with people, and extremely negative of course, since nothing seems like a good idea. In one of these episodes, I could easily find myself thinking that this business that I run, that employs 17 people and turns over over a million pounds, is doomed and that I should just leave it and run away from it and do something else. When I think something so ridiculous, usually some part of me wakes up and says "hey, that's not quite right", and then I realise I am having an anxiety attack.
If awake, once I realise that's what's going on I can deal with it fairly effectively. Get rid of any physical discomforts (e.g. go pee if I need to pee), have some water, take some deep breaths, and refocus on positive things. It used to take longer, but by now, once I realise it's happening, I can get out of it in a couple of minutes, even while I'm in the middle of conversation with others, so it's not noticeable to others.
If I'm asleep, it's more bothersome, because I'm unlikely to notice it in a half-dream state... so I'll lie in bed half-awake and half into some kind of mild nightmare where everything keeps going wrong and I feel stressed. Eventually, 30-60 minutes later, I'll actually realise what's going on, and go and open the window, take some deep breaths, go to the loo, have some water, etc... then once it's calmed away, I can go back to sleep.
I would not call this an "anxiety attack" in the full sense because usually people who suffer anxiety attacks can't get out of them so easily... but it definitely seems to overlap with the general understanding of what an anxiety attack is meant to be like. So I call this a "very mild anxiety attack".
I've noticed that when those are happening more regularly (e.g. once or twice a day), if I reduce my caffeine intake to zero, they go away almost entirely within a couple of days. So I'm pretty sure it's caffeine-induced.
