A few cites from Cecil:
http://www.straightdope.com/classics/a2_223a.html
Dear Cecil:
*
What’s the Straight Dope on nicotine? I know for a fact that nicotine is a stimulant, yet smoking seems to calm people. Can you explain this paradox? --A reader, Chicago
Cecil replies:
Give me a running start and enough Old Style, pal, and I can explain anything. It’s pretty generally agreed that smoking is mildly addictive, at least for some people, and that the anxiety smokers feel when they’re deprived of weed is actually drug withdrawal. Naturally, as soon as you light up, the symptoms disappear. A study by Columbia University researchers some years ago indicated that the craving for nicotine is related to acid levels in the urine–the more acid, the more you want to smoke. Moreover, stress seemed to make the urine even more acidic, causing you to want to smoke all the more. It was therefore suggested that smokers who are trying to cut down stoke up on acid-killers like bicarbonate of soda or Alka-Seltzer, particularly when they anticipate stressful situations, and at the same time avoid acidic substances like orange juice. I guarantee nothing, you understand, but it’s worth a shot.*
http://www.straightdope.com/classics/a2_372.html
*Dear Cecil:
What is it about smoking a cigarette that will sometimes send me to the bathroom for a number two? --C.B.M., Washington
Dear C.:
During my foolish youth this is the kind of question I would kiss off with some totally uncalled-for if hilarious remark. Now that I am mature, however, I know that behind many seemingly inane queries there lie deep scientific truths. It is even so in the present case. The reason smoking makes you feel like heading for the john is that your body has been specially programmed to respond to the products of the R.J. Reynolds company. No fooling. Your guts are crammed with “nicotinic receptors,” which are part of the parasympathetic nervous system, which controls your digestion, among other things. When you use tobacco, these receptors are stimulated and your innards commence to churning. If you were pretty full already, you may well feel the need to visit the john.
Some people become dependent on this odd phenomenon. Cecil has heard of a case involving an old coot who used to spend every waking moment with a chaw of tobacco between cheek and gum. One time he had to go into the hospital for prostate surgery. While he was stretched out in the recovery room, a nurse noticed that his abdomen was swelling up. The attending physicians feared that they had pierced his intestine during the operation and infected him. But then another doctor strolled in and remarked, “Hey, that’s the first time I’ve ever seen old Charlie without his wad.” Remembering the nicotinic receptors, the physicians immediately guessed that old Charlie had become so accustomed to tobacco that his intestines wouldn’t work without it. His abdomen was swelling because his bowels were filling up with air like a balloon, an event known as an ileus. They promptly sent out for a family-size can of Red Man and administered a double dose. (I presume they woke Charlie up first.) The swelling soon subsided*
This cite says that it takes 2-5 hours for nicotine levels to drop
http://tinyurl.com/grbrl
This cite:
http://whyquit.com/whyquit/A_Symptoms.html
“Like Pavlov’s dogs, who he conditioned to salivate upon the ringing of a bell, the nicotine addict’s mind has been conditioned to expect nicotine as soon as it begins to feel any discomfort associated with the onset of physical withdrawal. In response to falling nicotine levels, the habitual mind has been conditioned to intensify “desire” in order to cause us to ingest new nicotine and thereby avoid any discomfort. When we smoked, most of us received a gentle “desire” reminder every 20 to 30 minutes. If not satisfied, the desire would build and escalate in intensity to the point of becoming an influential urge or extremely demanding crave. Our mind has stored the means and manner by which it motivates us to ingest another hit of nicotine” and "*Our Habit Triggers - Although our basic time trigger served as our mind’s foundation for conditioning it to generate desire attacks, we have also conditioned it to expect new nicotine during certain events, at specific times each day, upon experiencing certain emotions or when we engage in specific activities. The mind is a “steal trap.” You many not consciously remember what you were doing, feeling or where you were when you fed yourself nicotine in the past, but those memories have been locked away deep within your subconscious.
Each of these emotions, events, specific times or locations will trigger our mind’s crave generator to begin pulsating desire when next encountered. Reconditioning each trigger is a very necessary part of recovery and should not be feared but anticipated. "*
Hope that helps. Good luck!