Defective arousal mechanisms are viewed as contributory to sleep hypopnea disorders and sudden infant death syndrome. Sighs (i.e. augmented breaths) as well as startles have not traditionally been viewed as arousal-related phenomena in infants. We hypothesized that, if sighs and startles are the initial event in a sequential arousal process, then they might be associated with specific EEG activity changes, because activation of the arousal-related ascending reticular activating system can suppress thalamus-generated sigma spindle oscillations. We studied spontaneous sighs and startles and those elicited by briefly occluding the infants face mask airway in 12 normal infants (age 10-19 wk) during non-rapid eye movement sleep. We recorded EEG (C3-P3), ECG, O2 saturation, diaphragmatic electromyography, limb electromyography, and video of the infant. The startle intensity was scored on a scale of 0 to 3 based on video recorded movements. Sleep spindle periodicity was analyzed by using a threshold over a compressed spectral band array. Spontaneous sighs and sleep startles were immediately followed by an interspindle interval prolongation from (mean +/- SEM) 8.0 +/- 0.16 s (control period) to 17.9 +/- 1.45 s after spontaneous sighs, to 23.8 +/- 1.26 s after spontaneous sighs accompanied by startles and to 26.5 +/- 1.45 s after occlusion-related sighs and startles. Furthermore, the intensity of occlusion-evoked startles was positively correlated with the interspindle interval prolongation (p < 0.01). We conclude that spontaneous as well as evoked sighs and startles are immediately followed by a transient sleep spindle suppression. This phenomenon indicates a close linkage between sighs, startles, and reticular formation-related arousal mechanisms.