Adequate alertness is necessary for proper daytime functioning. Impairment of alertness or increase in sleepiness results in suboptimal performance and adversely affects the quality of life. While some causes of somnolence are intrinsic to the brain circuitry and neurochemical architecture, others are due to maladaptive behaviors and disorders affecting the normal sleep homeostasis. Identification of the problem and understanding the underlying etiology is the key to timely treatment and better outcomes.