While there has been much investigation, contradictions exist in the published studies. One chapter in that review was devoted to the effects on vision, including night vision. In it, comparable studies were summarized that supported each of three possible. Although it was reported that sufficient evidence existed to argue against selection of smokers for military night missions, the most compelling conclusion in the review was that more research was needed to adequately resolve this controversy. There are several factors that make investigation of the effects of smoking on night vision difficult.
Night vision is, in itself, a complex physiological process.
Visual function at night encompasses many parameters, e.g., scotopic dark adaptation, mesopic vision, contrast sensitivity, visual discrimination of moving targets, peripheral retinal sensitivity, glare recovery, and dark focus. Thus, it is difficult to limit night vision testing to single, easy tests and still obtain operationally meaningful information. Thus intersubject differences in dark adaptation of less than one order of magnitude are usually considered clinically insignificant. Accordingly, studies of night vision must have large sample sizes in order to detect small but significant effects. The effects of smoking are different for nonsmokers versus smokers versus deprived smokers.