Implicit learning is better at subjectively defined non-optimal time of day

Individual preferences in morningness–eveningness rhythms modulate temporal fluctuations of cognitive performance over a normal day. Besides enhanced cognitive performance at individualʹs peak time as derived from morningness–eveningness questionnaires, a few studies have shown increased implicit memory abilities at a non-optimal (NOP) time of day. Various subjective factors might also determine the clock time for high or low cognitive efficiency. Using an artificial grammar learning (AGL) task, we show enhanced implicit learning of high-order information at NOP [vs optimal (OP)] time of day as subjectively defined by participants, irrespective of morningness–eveningness scores. Our results suggest that subjectively defined efficiency periods are a modulating factor in the testing of cognitive functions.