Part I of the thesis contains a brief review of the clinical importance of nocturnal asthma, and an account of the methods used to study breathing during sleep. The original work of the thesis is divided into three further parts: In Part II are described studies of breathing and oxygenation during sleep in normal subjects. These studies were performed to establish a normal range of apnoea, hypopnoea and oxygenation during sleep and to determine the effects of age and sex on these variables. Part III contains the results of similar studies in patients with asthma. Breathing patterns and oxygenation during sleep in asthmatic patients were compared with those of normal subjects and those of patients with chronic bronchitis and emphysema. The studies described in Part IV were designed to explore the relationship between sleep and bronchoconstriction. I wished to establish whether sleep was essential for nocturnal bronchoconstriction and to determine whether bronchoconstriction was associated with one particular stage of sleep. The results are summarised at the end of each section, and the clinical implications of the results are discussed in Part V.