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Background Previous studies show contradictory results about the emergence of 24-h rhythms and the influence of external time cues on sleep–wake behavior in preterm compared to term infants. Aims To examine whether very preterm infants (< 32 weeks of gestational age) differ in their emergence of the 24-h sleep–wake rhythm at 5, 11 and 25 weeks corrected age compared to term infants and whether cycled light conditions during neonatal intermediate care affects postnatal 24-h sleep–wake rhythms in preterm infants. Study design: Prospective cohort study with nested interventional trial. Subjects: 34 preterm and 14 control term infants were studied. During neonatal hospitalization, preterm infants were randomly assigned to cycled light [7 am–7 pm lights on, 7 pm–7 am lights off, n = 17] or dim light condition [lights off whenever the child is asleep, n = 17]. Outcome measures: Sleep and activity behavior recorded by parental diary and actigraphy at 5, 11 and 25 weeks corrected age. Results Sleep at nighttime and the longest consolidated sleep period between 12 pm-6 am was longer (mixed model analysis, factor group: p = 0.02, resp. p = 0.01) and activity at nighttime was lower (p = 0.005) at all ages in preterm compared to term infants. Cycled light exposed preterm infants showed the longest nighttime sleep duration. Dim light exposed preterm infants were the least active. Conclusions Preterm infants show an earlier emergence of the 24-h sleep–wake rhythm compared to term infants. Thus, the length of exposure to external time cues such as light may be important for the maturation of infant sleep–wake rhythms. Trial registry number: This trial has been registered at www.clinicaltrials.gov (identifier NCT01513226). [ABSTRACT FROM AUTHOR]