The relationship between morningness±eveningness, time-of- day, speed of information processing, and intelligence John Song, Con Stough* Brain Sciences Institute, Swinburne University of Technology, PO Box 218, Hawthorn, Victoria 3122, Australia Received 22 June 1998; received in revised form 24 November 1999; accepted 12 January 2000 Abstract Morningness±Eveningness refers to individual dierences in circadian phase position of spontaneous sleep±wake rhythms and to subject alertness. There is some evidence indicating that performance on cognitive tasks may be in¯uenced by Morningness±Eveningness and time-of-day. Given the potential importance of such a ®nding for the assessment of cognitive ability we conducted a study assessing the relationship between Morningness±Eveningness, time-of-day, and performance on the Multidimensional Aptitude Battery IQ (MAB-IQ) and Inspection Time (IT) task. Twenty male and 50 female participants classi®ed according to their scores on the Morningness±Eveningness dimension (Horne & O È stberg, 1976) were administered the MAB and IT tasks in the morning (0900 h) and in the late afternoon (1500 h). No signi®cant eect of time of testing, and Morningness±Eveningness was observed except for the Spatial subtest of the MAB. Morning Type-participants performed signi®cantly worse in the morning session in Spatial subtest and better in the late afternoon session and Evening Type-participants performed signi®cantly better in the morning than in the late afternoon session. These results do not support the hypothesis that there is a reliable relationship between Morningness±Eveningness, time-of- day and cognitive ability. 7 2000 Elsevier Science Ltd. All rights reserved. Keywords: Circadian rhythms; Morningness±Eveningness; Intelligence; Time-of-day; Processing speed Personality and Individual Dierences 29 (2000) 1179±1190 0191-8869/00/$ - see front matter 7 2000 Elsevier Science Ltd. All rights reserved. PII: S0191-8869(00)00002-7 www.elsevier.com/locate/paid * Corresponding author. Tel.: +61-9214-8822; fax: +61-9214-5525.