Many years go I drank regular coffee all day long...even just before going to bed. I became relatively immune to the stimulating effect of the caffeine, and slept normally despite the high intake. I think this is an example of developing tolerance of a repeatedly ingested drug. It is akin to the drug addict who must take increasing doses to get the desired high.
A second, and more insidious result of the high caffeine intake was an adverse effect on my mental status. I found that I was becoming irritable and difficult, often finding myself snapping at subordinates or being inappropriately and publically critical and mean. I also felt emotionally uneasy.
Deciding that these negative effects were from too much caffeine intake, I switched to Decaf. and reduced the number of cups consumed.
I have been off of caffeine for many years now. And the undesirable mental issues indeed disappeared.
So, in my opinion, high caffeine intake results in reduction of the expected stimulation effect, and gradual appearance of undesirable mental effects.
As with almost everything in life, moderation seems to be the best policy.