The aim of this study was to evaluate the organisation of EEG patterns in 24-h recordings of preterm and near-term neonates. In particular, the distribution of the different EEG codes at different postmenstrual ages (PMA) and the variations of sleep-related EEG pattern organisation was studied, during day (8.00 a.m.-8.00 p.m.) and night (8.00 p.m.-8.00 a.m.) time. The age of appearance of different neonatal EEG patterns, previously described in literature for short lasting records, was confirmed in this 24-h study. The medium-voltage continuous EEG pattern (pattern "3") was less represented approaching term age, in coincidence with the appearance of the two low-voltage continuous patterns ("1" and "2"), which are also related to active sleep and wakefulness. Discontinuous pattern ("7") was also less represented with age, but in day-time only. The percentage of time occupied by this pattern, related to quiet sleep, was significantly higher during day-time hours, than at night.