Map of Hallstadt on These characteristics also help distinguish normal sleep from a loss of consciousness due to injury, disease, or drugs. See also nrem sleep; rem sleep. sleep apnea the temporary cessation of breathing while asleep, which occurs when the upper airway briefly becomes blocked (obstructive sleep apnea) or when the respiratory centers in the brain fail to stimulate respiration (central sleep apnea). sleep cycle a recurring pattern of sleep stages in which a period of slow-wave sleep is followed by a period of rem sleep. In humans, a sleep cycle lasts approximately 90 min. sleep deprivation deliberate prevention of sleep, particularly for experimental purposes. Studies show that the loss of one night’s sleep has a substantial effect on physical or mental functioning; participants score significantly lower on tests of judgment and simple reaction time and show impairments in daytime alertness and memory. Map of Hallstadt 2016.