It shaves off 10 to 20 IQ points in me when I'm on a bad streak. I have failed FAANG interviews because of bad sleep. I still probably would've failed them, but it'd have been much closer. I literally remember one coding interview where I was thinking "if I just had enough sleep, I'd have a solid shot" (the other coding interview was so easy I could even do it in my half-ragged sleep-deprived state).
No matter what I tried I could not get the code to compile.
I still laugh about it now but it definitely highlights the importance of sleep.
The next day, well rested, I realized I could do the same thing with a for loop because I already had all the data I needed.
1. I periodically go off coffee, for various reasons, but frankly never notice much difference to my sleep quality.
2. I have 2 kids that deprive me of sleep much more effectively. Or, put otherwise, no matter how much I drank during the day, I fall asleep like a log. And then it's not coffee disturbing my sleep, but crying children.
I can see though that coffee abuse _could_ lead to sleep deprivation and related issues. But it's like alcohol could lead to alcoholism, but doesn't have to. Though, incidentally, apparently deliberate sleep deprivation is one method of tackling depression - so it's not all bad.
https://www.scientificamerican.com/article/why-sleep-depriva...
But I agree that abuse is what is causing majority of the problems (and it was in my personal case)
A lot of it doesn't require any domain knowledge to verify.
But different rates of caffeine tolerance and digestion certainly conform with my personal observations, Walker or not.
Briefly and temporarily, but efficacy quickly drops off with near-term repetition. As (apparently) with psychedelics, it's good for a temporary change of perspective that can be leveraged with the right techniques into lasting improvement, but much more of an adjuvant to therapy than a therapy in its own right.
There seems to be a wide variety in different people's caffeine sensitivity, but for me, a coffee habit leads to a downwards spiral that ends with permanent tension and migraine episodes. I have tried this cycle enough times by now to know the cause and effect. I have to wonder what other effects the continued stimulation of caffeine has.
I drink espresso in the morning, black coffee through out the day and stop before dinner. It's around 4-8 mugs every day. On holidays I often go a days without any coffee for various reasons with no ill effects. I have gone an entire month substituting coffee with water to test the hypothesis that it doesn't really affect me and I felt no different. HR, BP, sleep quality, tiredness, mental performance, digestion all remained stable.
My neighbour gets a headache if he skips coffee for a day.
I for one have coffee in the mornings and stare at my phone until I fall asleep, and I sleep like the dead at night. But, that's purely anecdotal.
I don't think people will listen to others saying 'stop drinking coffee!' or 'stop looking at screens!' if they don't actually have problems. On that note, one shouldn't start self-medicating with sleep hormones (currently available over the counter) to try and optimize their sleep. If you have to force yourself to sleep so you can min/max your day, you're heading to a crash and burnout.
I abstain from coffee (normally eight cups daily) and alcohol (normally three bottles of beer weekly) for a two-week period as a prescription against chemical dependency.
Typically this results in one day in which I am more irritable, nothing more.
Any consumptive habit can be evil in the absence of moderation and discipline.
My gut feeling is telling me that coffee and digital devices are responsible for big parts of the population suffering from mental illnesses, caused via chronic sleep deprivation. Of course majority of those mental illnesses are under reported - being sleep deprived for months does not make you bipolar maybe, but depression can and does happen (as well as myriad of other issues).
There are too many studies who show relationship between sleep and all kinds of illnesses (majority - mental), and nobody can argue that coffee is NOT responsible for that at least partially.
When I recently quit on coffee, I had such strong withdrawal symptoms, I was unable to function for 4 days. I had strong headaches and was almost vomiting.
No study will convince me this is a good/healthy thing for a human body. It is drug which is so omni-present, if you look around it is scary.