We used to joke that smokers have great eyes because they take regular screen breaks to go outside and stare at the horizon.
Seriously though, one thing I did find was when I quit smoking my ability to solve complex problems at work went down for a while. I was no longer taking a forced 10 minute break every 60-90 minutes that let my brain reset. (Or explaining my problem out loud to another filthy smoker skulking around the back of the building :D )
Fortunately for me it was about the same time that the Pomodoro technique was featured on Lifehacker and I implemented a similar system to replace the regular nicotine craving induced breaks.
It might also be the lack of breaks too, but who knows.