Effect of maternal separation on the responsiveness to melatonin in the anticipation for light onset. Anticipation of the predicted light onset during stage 3 was recorded by measuring the spontaneous locomotor activity prior to the predicted light onset time according to the original 11:11 hr light:dark cycle in both peer- and mother-reared animals. Spontaneous locomotor activity profile for the last 70 min prior to expected light onset in peer-reared (C, D) and mother-reared (A, B) animals continuously treated with vehicle (ethanol; A, C) (stages 1–3) or treated with melatonin in stage 2 (B, D). *P < 0.05, One-way ANOVA and Newman–Keuls post hoc analysis. Error bars represent s.e.m. Identical letters are assigned to identify time intervals compared by Newman–Keuls post hoc analysis. Data presented are normalized to baseline activity.