The infant arousal response involves subcortical and cortical responses occurring as a sequence of stereotyped behaviour regardless of the eliciting stimulus. The spontaneous activity of these responses during sleep, however, is uncertain. We examined the spontaneous arousal pattern in normal infants to determine the sequence of responses, and to examine their periodicity and the effects of sleep state. We performed a nap polysomnographic study on 10 normal infants between 2 and 10 weeks of age. Electroencephalographic and electro-oculographic activity, and respiratory airflow and movements were measured, and video recordings were made throughout each study. Different levels of arousal behaviour were examined. We found that spontaneous arousal activity occurred frequently and the majority of responses occurred as a sequence involving an augmented breath followed by a startle and then cortical arousal. Subcortical arousals as reflected by augmented breaths and startles were more common than cortical arousals. Additionally, augmented breaths followed by apnoea were recorded and were not usually associated with other arousal responses. All of the responses occurred periodically either as bursts of activity or as isolated responses. Each of the responses occurred more frequently during rapid eye movement (REM) sleep than during non-rapid eye movement (NREM) sleep. We conclude that there is an endogenous rhythm of spontaneous activity in infants involving excitatory processes from the brainstem, which may or may not be closely followed by cortical excitation. The spontaneous arousal responses occur periodically but with a high level of irregularity and the level of activity is affected by sleep state.