Influence of time of 24-hour day on depth of processing in recall memory

In this paper we report hourly variations in recall of words in a short-term memory task. Tests were carried out at 18.00, 22.00, 02.00 and 06.00 hours. The words to be memorized were preceded by a question designed to shape the type of encoding adopted by the subjects. The question concerned semantic, typographic or structural characteristics of the word. After presentation of all the words, the subjects were tested in free recall. Time of day had no effect on overall recall scores. Nevertheless, we observed a phase difference in the recall of items processed at superficial or deep levels. Items encoded by typographical attributes were more readily recalled at 02.00 than at 18.00, whereas semantically encoded words were less well recalled at night than during the day. There was also an interaction between the serial position of the items and time of day. At 22.00 and 02.00, subjects recalled words from the end of the list better. The results are discussed in terms of resource allocation models.