Smoking is a possible risk factor for obstructive sleep apnea syndrome (OSA), but its influence on the severity and importance of daytime sleepiness remains poorly documented. The scientific data evaluating the association of smoking and sleep apnea syndrome remains limited and controversial. Cigarette smoke can alter the mechanical and neural properties of the upper airway. Cigarette smoking is also associated with airway inflammation and edema, which leads to a decrease in airway caliber, which impacts sleep architecture with a greater latency to sleep and a shift to light sleep stages inducing excessive sleepiness. The aim of this monograph is to study the influence of smoking on the severity of OSA and its impact on the importance of daytime sleepiness during OSA. Smoking cessation would be the cornerstone in the management of this population.