Nicotine Acts Like Brain Fuel
Many smokers feel that the slight stimulant ability of nicotine acts like fuel for the human brain, but this is generally not true. While the first few times you smoke a cigarette may give a slight burst of energy and "fuel," this wears off very quickly. Tolerance then begins to creep up, and you become truly addicted to nicotine. Unfortunately, in addiction, the brain produces additional receptors to handle the chemicals sent through it. These receptors, in turn, become entirely dependent on the substance you are addicted to (in this case, nicotine). A cycle occurs where you begin to smoke just to bring your brain back to an even level, rather than to think more efficiently. This actually tells the brain to be lazy without a cigarette; smoking the cigarette just brings you back to the same level you were at before you began smoking.
Thoughts About Cigarettes
You must also consider that smoking leads to obsessive thoughts about smoking. This means that the quitting process can trigger intense concentration on smoking, cigarettes, and quenching those cravings. This takes away resources within the brain that would normally be dedicated to other thought processes. Think of your brain like a computer; it has limited ability to process "thoughts." If you are busy thinking about how to keep from smoking, it will likewise be difficult to concentrate in general. Food cravings also have this effect on the brain, especially when you are hungry. For some time, it can seem as if you can't think of anything else other than eating.
Your preoccupation is likely to be at its worst within the first 72 hours. This is the period of time in which nicotine fully flushes from the system. After this point, many people notice a significant reduction in symptoms.
Pavlov conditioning suggests that the brain makes links between different experiences. It states that smoking is often connected to everyday activities. One great example of this is the connection between smoking while drinking alcohol or coffee. While the cigarette really isn't any better during these times, your brain may connect the experiences you have while doing both with the other. For example, if you are out celebrating with friends, drinking martinis and smoking, when you think of smoking, you will think of having fun with friends. Much of this is subconscious, but the effects can be very strong. People often smoke before using the washroom, when they wake, before they sleep, when they sit down after work, and in any other number of situations. When you engage in these activities without smoking, the experience can seem lackluster. Much of this can be attributed to the lack of chemicals that the brain has grown used to. Instead of assuming that you need to smoke, choose to power through your cravings. You will find that within no more than 3 weeks, your brain has largely reset itself to the way it was before you smoked.
In short, while you will probably suffer slightly from concentration issues for at least part of your quitting process, this is a temporary experience. Focus on your coping skills to get you through the first few weeks, and things will lighten up dramatically for you. Your brain and body needs time to heal from smoking; you owe it to yourself to allow it to do so.