Endurance athletes engaged in prolonged exercise and high training volumes are at increased risk of developing accumulated fatigue, which can result in a sustained decline in performance known as overtraining syndrome (OT) [1]. In addition to excessive training, factors such as inadequate sleep, psychological stress, and poor nutritional status are involved with increasing importance of daily monitoring. However, many assessments rely on single time-point measures, even though physiological variables (e.g., body temperature) present diurnal rhythms [2]; reliance on assessments at fixed time points may fail to detect abnormalities or disturbances in these rhythms. Thus, continuous monitoring of diurnal variations using non-invasive techniques, such as measurements of skin temperature, heart rate, and autonomic nervous system activity, may be more suitable for application in real-world training environments.

Core temperature follows a circadian rhythm; it is typically lower in the morning and higher in the evening [3]. These fluctuations are closely associated with sleep quality [4], mental health status [5,6], and nutritional condition [7]. Sleep plays an essential role in facilitating recovery [8]. However, high-intensity training has been shown to increase wake time after sleep onset, possibly due to elevated nocturnal core temperature and sympathetic nervous system activity [9]. Although core temperature rhythms can influence the secretion of sleep-related hormones, including cortisol and melatonin [10,11], the impact of daytime training on nocturnal core temperature and sleep quality in endurance athletes remains insufficiently explored.

In a study simulating trail-running exercise (48 min total, comprising a 3-min warm-up followed by five 9-min exercise blocks) conducted at night (9:00 P.M.), core temperature remained elevated during the initial 3 h after bedtime, resulting in disrupted sleep architecture in well-trained athletes [12]. Although core temperature assessment is considered the gold standard, it typically requires rectal thermometry, which limits its practicality. In contrast, recent advances in wearable sensor technology have enabled continuous, non-invasive monitoring of skin temperature, providing valuable data on circadian dynamics. A recent study demonstrated the potential for assessing circadian rhythms through 24-h monitoring of skin temperature fluctuations [13].

The present study investigated nocturnal skin temperature rhythms and sleep parameters after multiple sessions of endurance training in female long-distance runners.

The present study investigated the effects of multiple endurance training sessions (three sessions per day, totaling 130 min) on nocturnal skin temperature and sleep parameters in female long-distance runners. Multiple endurance training sessions did not affect the mean, maximum, minimum, or delta of nocturnal skin temperature during whole sleep. In contrast, the delta skin temperature during the first 3 h after bedtime was significantly lower on the Training Day than Rest Day. Also, a significant difference was observed in the temporal profile of skin temperature during the first 3 h after bedtime between the Training Day and Rest Day. Moreover, total sleep time was significantly longer after the Training Day. However, sleep efficiency, sleep latency, and WASO did not differ significantly between conditions; thus, the increased total sleep time may reflect greater homeostatic sleep pressure (increased sleep quantity) rather than improved sleep quality. These findings suggest that nocturnal skin temperature fluctuations reflect increased physiological stress induced by multiple endurance training sessions in female athletes. To our knowledge, this is the first study to report nocturnal proximal skin temperature fluctuations and sleep quality after multiple endurance training sessions in this population.

Continuous 24-h monitoring of core body temperature and melatonin levels is considered the gold standard for assessing circadian rhythms [15]. However, the invasiveness of rectal thermometry, high cost of ingestible sensors, and logistical challenges of collecting multiple blood samples limit the feasibility of such methods in field settings. Therefore, in the present study, we focused on proximal skin temperature during the initial 3 h of sleep, given the previously established inverse relationship between core temperature and proximal skin temperature at night [16-18]. As expected, skin temperature rapidly increased after bedtime in both conditions, consistent with previous studies [16,17]. This transient rise in skin temperature reflects enhanced peripheral heat dissipation, which facilitates a decline in core body temperature—a process considered essential for initiation of sleep [16,19,20].

The present study was conducted under free-living conditions without strict regulation of sleep timing, to reflect the natural lifestyle of endurance athletes. Given that the study took place during winter (average ambient temperature approximately 7°C), skin temperature data collected during the day were likely influenced by environmental temperature. Therefore, our analyses focused on the 3-h period after bedtime, during which a consistent increase in skin temperature was observed within the first 20 min, indicating the promotion of heat loss at sleep onset. Previous studies have suggested that such thermoregulatory changes at bedtime contribute to reduced sleep latency [16]; however, no significant difference in sleep latency was observed between conditions in the present study. Further research involving continuous monitoring of core temperature and simultaneous measurement of both proximal and distal skin temperatures (e.g., distal-proximal skin temperature gradient) under controlled environmental conditions would help clarify the thermoregulatory mechanisms that influence sleep after intensive training.

Currently, evidence regarding the effects of multiple daily endurance training sessions on nocturnal thermoregulation and sleep parameters in athletes remains limited. Aloulou et al. reported that a 48-min simulated trail run performed at night (9:00 P.M.) increased core temperature during the first 3 h of sleep and reduced total sleep time in well-trained athletes [12]. In contrast, our findings demonstrated an increase in total sleep time after the Training Day. This discrepancy may be explained by environmental factors, such as the lower ambient temperature during outdoor winter training in the present study, which could have mitigated the post-exercise elevation in body temperature. Additionally, the timing of the final exercise session (starting from 3:30 P.M.) in the present study was earlier than that of the nighttime session utilized by Aloulou et al. [12], possibly reducing the impact on nocturnal physiological responses.

Thomas et al. found that total sleep time was significantly increased after both high-intensity interval running and low-intensity steady-state running performed in the evening (6:00-7:00 P.M.), relative to a no-exercise condition [21]. They attributed this effect to elevated energy expenditure and/or enhanced peripheral blood flow. A similar mechanism may explain the prolonged sleep duration observed in our participants after endurance training, although the present study did not assess energy expenditure or skin blood flow, precluding definitive conclusions regarding their contributions.

The present study had several limitations. First, we were unable to control individual factors that may influence nocturnal skin temperature and sleep parameters, including the timing of bathing [22,23], menstrual cycle phase [17,24,25], seasonal environmental variations [26,27], and external factors during sleep (e.g., clothing, bedding). In particular, menstrual cycle phase and the presence or absence of menstruation may have affected body temperature fluctuations with increased inter-individual variabilities. Although we were not able to clarify the impact of menstruation for nocturnal skin temperature change due to limited numbers of submits, further determination would be valuable. Also, external factors (e.g., sleep environment) were not strictly controlled among participants, but these conditions did not differ within participants across the two conditions. Therefore, the impact of different sleep environment would be minor. Second, core body temperature a critical component of thermoregulatory assessment was not measured. Recent studies have demonstrated the potential of continuous monitoring of distal skin temperature (e.g., at the wrist) for health-related applications, including predicting changes in heat dissipation and circadian rhythmicity [7,28-30]. Therefore, future studies should incorporate simultaneous monitoring of proximal and distal skin temperatures to better characterize thermoregulatory responses. In addition, skin temperature was measured at a single proximal site (groin), which may not fully represent whole body thermoregulatory responses. Fourth, the comparison between “Training Day” and “Rest Day” may be excessive. Further determination of the impact of different training volume would be required. Finally, although skin temperature was continuously recorded over a 24-h period, only nighttime data were analyzed. Given that data collection occurred in December (winter) in Japan, outdoor training sessions during the day likely influenced daytime skin temperatures due to ambient conditions. Thus, further studies are warranted to evaluate circadian rhythms using continuous skin temperature monitoring under both controlled and free-living conditions in highly trained athletes.

In conclusion, multiple endurance training sessions (three sessions per day) altered the time course of nocturnal proximal skin temperature during the initial 3 h after bedtime and were associated with increased total sleep time in female long-distance runners. These findings provide novel insights into the thermophysiological and sleep-related responses to endurance exercise in elite female athletes.