Circadian rhythms in the wild: Skin temperature versus actimetry for short-term field assessment in varying sleep conditions.

Circadian phenotype is associated with diurnal variation in human exercise performance.

It is demonstrated that endurance exercise performance is enhanced in the evening compared with the morning and suggested that behavioural markers of circadian phenotype are associated with the magnitude of diurnal variation in performance.

Circadian rhythm of heart rate variability in tropical horses: age-associated autonomic alterations and environmental air pollution effects in an urban field setting

ABSTRACT Background and Aim: Heart rate variability (HRV) reflects autonomic nervous system activity and is influenced by age, environmental conditions, and circadian rhythms. Information regarding these interactions in horses maintained under tropical conditions remains limited. This study aimed to characterize 24-h heart rate (HR) and HRV patterns in horses raised under tropical conditions and to investigate the effects of aging and environmental factors, including air pollution, on autonomic regulation. Materials and Methods: Fifteen clinically healthy horses aged 4-20 years were allocated to three age groups: 4-7 years, 8-14 years, and 15-20 years (n = 5 per group). Continuous 24-h recordings of HR and HRV were obtained using a Polar H10 sensor. Time-domain and frequency-domain HRV indices were analyzed using Kubios Scientific software. Simultaneously, environmental parameters, including temperature, humidity, feels-like temperature, light intensity, air quality index (AQI), and particulate matter ≤2.5 μm (PM2.5), were continuously monitored under field conditions in Bangkok, Thailand. Circadian variations, age-associated differences, and correlations between HRV and environmental variables were evaluated. Results: Only HR and normal-to-normal intervals differed significantly between daytime and nighttime, whereas other HRV indices exhibited limited circadian variation, indicating attenuated nocturnal parasympathetic predominance. Older horses (15–20 years) demonstrated significantly lower parasympathetic-related indices, including root mean square of successive differences (59.44 ± 7.25 ms) and high-frequency (HF) power (805.60 ± 213.37 ms²), compared with younger groups (p < 0.05). Total power was reduced, whereas the low-frequency (LF)/HF ratio increased with age, suggesting diminished autonomic flexibility and sympathetic predominance. Air pollution variables showed significant associations with autonomic imbalance. Both AQI and PM2.5 were negatively correlated with HRV indices, including root mean square of successive differences, percentage of normal-to-normal intervals differing by >100 ms, and HF power, while showing positive correlations with the LF/HF ratio (p < 0.001). Conclusion: Tropical horses exhibited blunted circadian autonomic modulation and age-related reductions in parasympathetic activity. Furthermore, elevated PM2.5 and AQI levels were associated with impaired autonomic balance, providing the first evidence linking ambient air pollution with resting HRV alterations in horses. These findings emphasize the importance of age-specific management and environmental monitoring to promote equine welfare in tropical urban environments.

Skin Temperature Dynamics during Sleep Onset Latency under Different Ambient Temperatures

The bedroom thermal environment is a key determinant of sleep onset, yet most studies manipulate ambient temperature and evaluate sleep onset latency (SOL) between conditions. Continuous skin-temperature dynamics during the SOL, particularly across bedroom temperatures, remain poorly characterised. This study examined skin-temperature dynamics before objectively defined sleep onset across multiple bedroom temperatures. Twelve healthy adults (6 males, 6 females; 23–42 years; BMI 17.7–32.9 kg/m²) completed four overnight sessions at 22°C, 24°C, 26°C, and 28°C. From 23:00 lights-out to 07:00 lights-on, skin temperatures at nine sites were recorded every 30 seconds, while polysomnography was continuously monitored. Sleep stages were scored according to the American Academy of Sleep Medicine manual, and the first non-wake epoch defined sleep onset. Temperature series were aligned to this point, and distal (DST), proximal (PST), and mean skin temperatures (MST) were derived. The distal–proximal gradient (DPG) was calculated as the difference between distal and proximal temperatures. Linear mixed-effects models with ambient temperature and SOL segment (early: −30 to −15 min; late: −15 to 0 min) as fixed factors showed that ambient temperature strongly affected DST, MST, and DPG in the early period of SOL, but effects were markedly attenuated in the late period. Across all four bedroom temperatures, DST, MST, and DPG converged toward similar levels as sleep onset approached. These findings suggest that the body is not passively constrained by ambient temperature but actively adjusts skin temperature and heat dissipation through distal thermoregulation to reach a relatively stable sleep-conducive state.

Time-of-day effects on eating behaviors: Examining circadian timing and caloric intake patterns.

Chronotype describes differences in the timing of daily rhythms, regulated by the circadian system. These patterns are classified as morning or evening chronotypes. Studies show increased impulsivity, poorer health behaviors, and greater cardiometabolic risk among evening chronotypes. However, few studies evaluate these effects relative to circadian biomarkers. Additionally, time of day may impact how chronotype affects behaviors. Participants completed an eating in the absence of hunger task during morning and evening sessions. Participants were served oatmeal, then given access to snack foods. Calories consumed from each snack type was the primary outcome. Participants fasted for ≥8 h before a 9 AM morning task. For the evening task, participants fasted for 2 h and completed an 8 PM task. Dim light melatonin onset (DLMO) was measured the day following the morning task. Covariate-adjusted models evaluated differences in caloric intake across food types and sessions, and tested associations between caloric intake and DLMO. 113 participants completed the protocol (57 female; age: M = 35.6, SD = 9.87). Significant time-of-day effects were observed, with lower intake of bland (B = -5.67, p < 0.001) and salty foods (B = -15.53, p = 0.015) in the evening. Order effects showed greater intake during the second administration for total calories and all categories (ps < 0.01). Later DLMO was linked to greater appetite ratings. These finding suggest that those with later circadian timing may have greater hedonic interest in food. Future research should explore interactions between chronotype, circadian timing, and environmental factors shaping eating behaviors.

Variable Morning Melatonin Profiles Limit Determination of Dim Light Melatonin Offset (DLMOff) in Preschool-Aged Children

Circadian rhythms have been shown to regulate sleep-wake timing across the lifespan, yet many questions remain about early childhood circadian physiology. Understanding dim light melatonin onset (DLMO) and offset (DLMOff), established markers of circadian phase, is essential for characterizing circadian rhythms in early childhood. We examined the distribution of salivary DLMO and DLMOff and their relationship with actigraphic sleep timing across 20 healthy preschoolers (M = 4.31 ± 0.34 years, 45% female). After maintaining a consistent sleep schedule for seven days, children completed an in-home circadian assessment. Children were awoken 1.5 h before habitual wake time and saliva samples were collected in 20–30 min intervals throughout the morning to determine DLMOff, then in the evening until 50 min past habitual bedtime to assess DLMO. A 4 pg/ml threshold was used to calculate each phase marker. DLMO ranged from 17:22 to 20:40 (M = 18:55 ± 0:54) and was positively associated with bedtime, sleep onset, and midsleep. In contrast, morning melatonin levels were highly variable, allowing DLMOff calculation in only eight participants. Within this small subsample, later DLMOff was associated with later chronotype (r = 0.81), sleep offset (r = 0.84), and midsleep (r = 0.80). Across the full sample, interpolated melatonin levels at habitual wake remained ≥ 4 pg/ml for 45% of children, a pattern broadly consistent with findings in adults, in which a majority of participants exhibit DLMOff after habitual wake time. These findings indicate that although evening melatonin profiles were consistently well-defined, permitting reliable calculations of DLMO across all participants, morning melatonin patterns were often irregular in young children. When able to be calculated, DLMOff showed strong associations with sleep timing, suggesting it could be a reliable marker of circadian phase. However, high variability and fluctuating morning melatonin patterns make DLMOff difficult to determine in many preschool-aged children.

Overnight Temperature Regulation Improves Circadian Rhythm and Cardiovascular Recovery in Postmenopausal Women and Age-Matched Men