Rhythmic Characterisation of Extreme Circadian Preferences in Everyday Life

The aim of this study was to compare extreme circadian preferences by analysing motor activity rhythms in a naturalistic environment. Overall, 84 participants (64 females, 20 males) (42 evening types and 42 morning types), with a mean age of 23.3 years (standard deviation = 4.2), were selected for this study. Each participant wore the Actigraph Micro Motionlogger Watch (Ambulatory Monitoring Inc., Ardlsey, NY, USA) around the non-dominant wrist for 1 week. We analysed quantitative and qualitative sleep features. We extracted motor activity counts, minute-by-minute, over 24 h and used the functional linear modelling (FLM) framework to examine circadian differences across chronotypes. Moreover, we used the Locomotor Inactivity During Sleep (LIDS) to compare, for the first time, nocturnal ultradian activity differences between evening types and morning types. The results did not reveal significant differences in sleep features nor in several circadian or ultradian rhythmicity parameters. We found a significant difference in all the indicators of the circadian and ultradian phases. The rate of decline of LIDS during the night was more pronounced in the morning type than in the evening type. Overall, the results support the idea that circadian preference reflects a different process of synchronisation with the environment.