In obstructive sleep apnea patients, who intermittently stop breathing at night for some seconds, functions of vigilance and attention seem to be impaired. The aim of our study was to investigate if nocturnal hypoxia as one possible detrimental factor is associated with the degree of modality shift effect expressing attention function at a very basic level of information processing. For the first time an experimental approach was applied to examine attention deficits in sleep apnea patients. Correlation analyses between pathophysiological parameters and attention function revealed a stronger association for the modality shift effect than for simple reaction times.