Sleep is a condition of body and mind such as that which typically recurs for several hours every night, in which the nervous system is inactive, the eyes closed, the postural muscles relaxed, and consciousness practically suspended. When it is disrupted due to illness, injury or external conditions like stress, noise or travel, one becomes cranky; over time, this is considered a chronic condition called insomnia and may the be result of a sleeping disorder (such as apnea).