1 GIGA-CRC Human Imaging Unit, Sleep and Chronobiology Laboratory, University of Liège, Liège, Belgium.
2 Psychology and Neuroscience of Cognition Research Unit, Faculty of Psychology and Educational Sciences, University of Liège, Liège, Belgium.
3 UR2NF, Neuropsychology and Functional Neuroimaging Research Unit, Center for Research in Cognition and Neurosciences, ULB Neurosciences Institute, Universite Libre de Bruxelles, Brussels, Belgium.
4 Centre de Recherche de l'Institut Universitaire de Gériatrie de Montréal, Concordia University, Montreal, Canada.
5 Athinoula A. Martinos Center for Biomedical Imaging, Department of Radiology, Massachusetts General Hospital and Harvard Medical School, Boston, MA, USA.
6 Centre for Chronobiology, Psychiatric Hospital of the University of Basel, Basel, Switzerland.

Thermoregulatory processes are closely linked to sleep initiation and maintenance throughout the circadian cycle, and may contribute to the increased tendency to nap in older adults. This cross-sectional study examined whether habitual napping in healthy older individuals is associated with altered skin temperature-derived heat-loss dynamics and their relationship with sleep onset. Thirty self-reported habitual nappers and 28 non-nappers (59-82 y) completed a 40-hour multiple-nap protocol under controlled laboratory conditions, with continuous polysomnography and distal-proximal skin temperature gradients (DPG) recordings. DPG was analyzed across scheduled wake episodes and at lights-off preceding each nap opportunity. Habitual nappers exhibited distinct changes in thermoregulatory dynamics compared to non-nappers. Overall, they had a lower DPG during scheduled wakefulness, particularly during the afternoon nap window (14:45-17:30). Their circadian organization of the DPG also differed markedly: they had a higher 24-hour DPG amplitude, a more pronounced 12-hour component, and an earlier DPG phase than the non-nappers. During nap opportunities, shorter sleep onset latency (SOL) was associated with a faster increase in DPG after lights-off in both groups. However, in habitual nappers, sleep onset occurred more rapidly despite a reduced dependence on pre-sleep DPG increase. Together, these findings indicate that habitual napping in older adults is accompanied by altered heat-loss rhythms across the circadian cycle and a reduced coupling between pre-sleep thermoregulatory dynamics and sleep initiation. Circadian-driven thermoregulatory changes may underlie the greater propensity to nap in older adults and differentiate habitual nappers from non-nappers. However, the causal direction of this relationship requires further investigation.