It also decreases oxygen flow to your lungs and brain. That alone can make you feel light headed. I get the feeling from your post that you are an occasional smoker? If so, give it up now while you can...it's not worth it. I've just spent 10 horrible days fighting this addiction.
First off, smoking dehydrates you pretty badly. You need to drink twice the amount of water than the average person if you are a consistent smoker. All of the chemicals in cigs also produce many toxins in your body. The smoke you breathe in circulates in your brain, just as the oxygen you inhale travels to your brain. There are a bunch of reasons you could be feeling this way.
Have you thought about quitting? How long have you smoked?
The additives in cigarettes create a stimulant effect in your body like an energy drink or cocaine. But, some people can become very relaxed and even dizzy. If it's been awhile since your last cigarette then that in itself could make you very dizzy which, in turn, can create a drowsy feeling. Also, I knew a guy who was always tired and drained and it turned out that the cigarettes were harming him so much that the cigarettes were creating a sludge in his lungs and liver, leading to excessive lack of energy. I smoke and I know it's hard to quit but quitting would probably be helpful for you. Warning though, if you get very dizzy, cold sweats, shakiness, and sometimes nausea all together can be a sign you may faint or lose consciousness. Be careful and good luck to you!