Polysomnograms were obtained from two groups of normal subjects (20 medical students and 20 elderly persons). The recordings included the usual sleep parameters (electroencephalograph, electrooculograph, and electromyograph of chin muscles) and respiratory data obtained by means of a pneumotachygraph (tidal volumes and respiratory frequency) and thoracic and abdominal strain gauges. In the older group, sleep was more disturbed and respiratory events (hypopneas and apneas) were more frequent both in non-rapid eye movement (NREM) and in REM sleep. The apneas were predominantly of the obstructive type. A correspondence analysis carried out on the 40 subjects showed that a high frequency of hypopneas and obstructive apneas was linked to old age and age-related parameters (disturbed sleep and obesity) both during NREM and REM sleep. A high frequency of central apneas during light slow-wave sleep was linked to the male sex; a high frequency of central apneas during REM sleep was linked to the amount of REM sleep. No relationship could be demonstrated between the decrease of minute ventilation from wakefulness to steady sleep and the incidence of sleep respiratory events.