Hypoxaemia during the rapid eye movement phase of sleep is common in older healthy normal subjects over 55 years of age; the sleep apnoea syndromes--such as obstructive sleep apnoea, where oro-nasal airflow ceases for more than 10 seconds on many separate occasions throughout the night, due to failure of contraction of the genio-glossus muscle; "blue and bloated" patients with chronic bronchitis and emphysema, where profound nocturnal hypoxaemia is common in REM sleep, and is associated with further elevation of pulmonary arterial pressure; the overlap syndrome--where "blue and bloated" chronic bronchitis is associated with an obstructive sleep apnoea syndrome; and bronchial asthma, where hypoxaemia is associated with irregular breathing and possibly nocturnal bronchoconstriction. Although absolute recognition depends upon all night sleep studies, monitoring of ear oxygen saturation, breathing patterns, and EEG, the clinical features when awake can lead to suspicion of sleep hypoxaemia--as, for example, obesity and obstructive sleep apnoea with loud snoring and restlessness in sleep, hypoxaemia during wakefulness in the overlap syndrome, and nocturnal awakening with wheeze in bronchial asthma. Treatment depends on the cause, and may vary from weight loss and nasal continuous positive airway pressure in obstructive sleep apnoea, to nocturnal oxygen in "blue bloaters", a combination of these two in the overlap syndrome, and long acting bronchodilators such as slow release theophyllines in nocturnal asthma. Recognition and appropriate treatment of nocturnal hypoxaemia is an important new development in respiratory medicine.