Cough and sleep are vital functions. The effects of cough on sleep and vice versa are important for a number of reasons. Sleep disruption is common in patients with cough and is often the reason why they seek medical attention. Sleep suppresses cough and the biological mechanisms for this action are poorly understood. Cough has recently been reported as a presenting symptom of obstructive sleep apnea. It is uncommon for healthy people to cough at night; however, approximately 50% of patients with chronic cough report sleep disruption due to cough. Cough frequency is much lower at night than during the day. There is reduced exposure to tussive stimuli at night and decreased cough reflex sensitivity. Cough is more difficult to induce in REM sleep compared to slow-wave sleep. Studies of anesthetized humans have shown that the cough reflex is suppressed; however, the expiratory reflex is less affected. The sleep-cough interaction has implications for the physician. The measurement of cough frequency with 24-h ambulatory cough monitors in patients with chronic cough suggests that the presence or absence of nocturnal cough is not helpful in establishing the etiology. Nocturnal cough may be a useful outcome parameter for clinical trials of antitussive drugs since it is under less voluntary control than daytime cough. Most antitussive drugs are sedatives. This suggests that part or all of their action may be through an effect on cortical neural pathways. Unexplained chronic cough has recently been reported as a presenting feature of obstructive sleep apnea. Patients are likely to be female and report gastroesophageal reflux and rhinitis. Continuous positive airway pressure therapy is effective in alleviating cough. Greater awareness of this condition is needed.