Inadequate sleep is a burning issue in today's modern world. Sleep and cognition are closely interrelated. Sleep deprivation negatively affects various aspects of cognition including working, short-term and long-term memories, attention, reaction time, visuomotor performance, reasoning ability, and judgment. Disorders with cognitive abnormalities also have negative impact on sleep. This review studies the clinical aspects of interplay between sleep and cognition. We have also focused on the assessment of sleep and cognition. Researchers are required in the field of sleep and cognition. Sleep has potential to become future diagnostic and therapeutic marker in cognitive disorders.