Early-night core body temperature decline is associated with sleep fragmentation and early-night slow-wave sleep duration in elite swimmers

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Sleep is a critical factor for health, performance and recovery, in elite athletes. Nocturnal thermoregulation, particularly the decline in core body temperature before and during sleep, is a key mechanism supporting sleep regulation. This study investigated the relationship between nocturnal core body temperature dynamics and sleep characteristics in elite swimmers. Data of twenty-six swimmers (42% women; 17 ± 1 years) were analyzed. Core body temperature was recorded using an ingestible thermometer, and sleep was assessed with a connected headband and wrist accelerometer during a single night. Retrospectively, participants were divided into high and low temperature delta groups based on the magnitude of nocturnal temperature decrease from one hour before light-out to four hours after. The high temperature delta group exhibited a greater nocturnal temperature decline, associated with longer slow-wave sleep duration during the first two cycles, earlier onset of the first sleep cycle, longer rapid-eye movement latency, and reduced sleep fragmentation, compared with low temperature delta group. No differences were observed for subjective sleep measures. Correlation analyses revealed that a higher temperature at the beginning of the night was associated with shorter slow-wave sleep duration during the first two cycles and greater sleep fragmentation. These findings support the relevance of early-night temperature dynamics, particularly within the −1 h to +4 h period around lights-out, in relation to objective sleep characteristics and slow-wave sleep duration. This period may represent a favorable thermoregulatory period for slow-wave sleep and could help refine cooling interventions aimed at supporting sleep quality and recovery in athletes.

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