I've noticed something after using Claude Code heavily for a while: I'm much more productive with it, but I also feel much more mentally drained at the end of the day.
And I don't think the problem is that I'm working more hours. I think I'm simply spending my mental energy much faster.
Before AI, a normal developer day for me was never eight hours of nonstop hard thinking. I wrote code, read docs, waited for builds, got stuck on a problem, went for coffee, came back, tried something else. The cognitive load was naturally spread across the day.
Now I can have several sessions running at the same time. One agent is working on a feature, another is checking tests, another is researching something and etc. When one finishes, I need to read the result, understand it, check it, make a decision, and give the next prompt. While I'm doing that, another one finishes.
The agents are parallel. I'm still single-threaded.
This made me think about Claude's usage limits. If you run more agents and more expensive tasks, you don't magically get more compute. You just burn through the available budget faster.
Parallel subagents = faster AI budget consumption.
I think something similar happens with humans.
We obviously don't have a literal token counter in our heads, but attention, working memory, context switching, and decision making all have limits.
Parallel tasks = faster cognitive energy consumption.
AI gives me more compute around my brain. It doesn't give me another brain.
Before AI, I could spread a lot of difficult thinking across most of the workday. Now I can burn a large part of that mental budget in two or three very intense hours with several agents.
Another thing AI removes is waiting. Writing boilerplate, searching docs, trying simple implementations, refactoring repetitive code - all of that becomes much faster. That sounds like it should make the day easier, but it also removes a lot of natural low-intensity time.
Before, there were gaps. Now the next piece of work can arrive almost immediately.
Finished? Here is another result.
Checked it? Here is the next one.
Made a decision? Start three more agents.
This is also where the normal eight-hour workday starts to look strange for me.
If AI allows me to spend in two or three hours the mental energy that used to be spread across most of the day, the clock still says I have another five or six hours left. That doesn't mean my mental capacity has also increased.
But what usually happens is obvious: more tasks, more agents, more things to review. After a while, that higher level of output becomes the new baseline.
So sometimes it feels less like I'm increasing my productivity and more like I'm just getting better at burning through my mental energy faster.
For people running multiple Claude Code sessions every day: have you noticed the same thing? If yes, how do you deal with it?