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Sleep apnea syndromes are more common in men than in women. The ventilatory response to arousal from sleep may be an important determinant of respiratory stability/instability and could contribute to this sex difference. We therefore compared changes in ventilation, end-tidal carbon dioxide (CO₂), upper airway resistance, heart rate, and finger photoplethysmogram pulse wave amplitude after both spontaneous and tone-induced arousal from non–rapid eye movement sleep in 13 men and 13 women. At sleep onset, ventilation fell and both upper airway resistance and end-tidal CO₂ rose, but these changes were not different between sexes. Spontaneous arousal (duration, 6.6 ± 0.2 seconds) resulted in a biphasic ventilatory response consisting of brief hyperventilation (5 seconds) followed by prolonged hypoventilation (30–40 seconds) on resumption of sleep. The biphasic ventilatory response was greater in …