Cluster headaches (CH) frequently recur at the same point in the circadian cycle, often during sleep. They may, in some cases, represent a susceptible individual's response to hypoxemia or other physiological changes induced by obstructive sleep apnea (OSA). If and when this mechanism exists, timing of CH close to the onset of sleep-and therefore OSA-might be expected. We questioned 36 subjects with CH about the times at which their CH usually occurred and about several symptoms known to be predictive of OSA, including habitual snoring, loud snoring, observed apneas and excessive daytime sleepiness. We then used logistic regression to determine whether occurrence of CH in each of six time periods was associated with OSA symptoms. The 23 subjects (64%) who reported CH in the first half of a typical night's sleep also tended to report headaches during the midday/afternoon period. Symptoms of OSA, and in particular habitual snoring, were predictive of both first-half-of-the-night and midday/afternoon CH (p<.05). Thirty-one subjects (86%) reported that their CH were sleep-related, usually occurring during any part of the night or on awakening, but symptoms of OSA were not predictive of this timing pattern. In short, several OSA symptoms showed an association with CH occurrence in the first half of the night but not with sleep-related CH in general. These findings suggest that in some patients, physiological consequences of OSA may trigger CH during the first few hours of sleep and thereby influence the timing of subsequent daytime headaches.