Excessive daytime sleepiness (EDS) is a prevalent topic in clinical care. Tiredness, fatigue, lack of energy, and sleepiness are complaints commonly encountered by psychiatrists and other physicians. Many surveys and epidemiological studies indicate that pathological daytime sleepiness, which is sleepiness that interferes with an individual's function most of the time, occurs in 7% to 13% of the population. Studies of patients within primary care practices suggest that this population suffers an even greater prevalence of sleepiness than the general public. Estimates of 20% to 25% or higher have been suggested by studies of large samples of primary care patient populations. Causal or contributing factors leading to a patient's experience of EDS range from the process of simply chronically allowing too little time for sleep on an ongoing basis, to idiopathic but profound central nervous system (CNS)-mediated debilitating hypersomnolence. Common among the causes of EDS are insufficient sleep, circadian rhythm disorders such as delayed sleep-phase syndrome, sleep disorders such as obstructive sleep apnea (OSA), CNS-related disorders such as post-infectious syndromes or CNS injury or lesions, and drugs—prescribed or otherwise.