Dynamics and timekeeping in biological systems.

This article introduces three reviews on the theme of circadian rhythms.

The sleep-circadian interface: A window into mental disorders.

Sleep, circadian rhythms, and mental health are reciprocally interlinked. Disruption to the quality, continuity, and timing of sleep can precipitate or exacerbate psychiatric symptoms in susceptible individuals, while treatments that target sleep-circadian disturbances can alleviate psychopathology. Conversely, psychiatric symptoms can reciprocally exacerbate poor sleep and disrupt clock-controlled processes. Despite progress in elucidating underlying mechanisms, a cohesive approach that integrates the dynamic interactions between psychiatric disorder with both sleep and circadian processes is lacking. This review synthesizes recent evidence for sleep-circadian dysfunction as a transdiagnostic contributor to a range of psychiatric disorders, with an emphasis on biological mechanisms. We highlight observations from adolescent and young adults, who are at greatest risk of developing mental disorders, and for whom early detection and intervention promise the greatest benefit. In particular, we aim to a) integrate sleep and circadian factors implicated in the pathophysiology and treatment of mood, anxiety, and psychosis spectrum disorders, with a transdiagnostic perspective; b) highlight the need to reframe existing knowledge and adopt an integrated approach which recognizes the interaction between sleep and circadian factors; and c) identify important gaps and opportunities for further research.

Strengths and opportunities in research into extracellular matrix ageing: A consultation with the ECMage research community.

Ageing causes progressive decline in metabolic, behavioural, and physiological functions, leading to a reduced health span. The extracellular matrix (ECM) is the three-dimensional network of macromolecules that provides our tissues with structure and biomechanical resilience. Imbalance between damage and repair/regeneration causes the ECM to undergo structural deterioration with age, contributing to age-associated pathology. The ECM 'Ageing Across the Life Course' interdisciplinary research network (ECMage) was established to bring together researchers in the United Kingdom, and internationally, working on the emerging field of ECM ageing. Here we report on a consultation at a joint meeting of ECMage and the Medical Research Council / Versus Arthritis Centre for Integrated Research into Musculoskeletal Ageing, held in January 2023, in which delegates analysed the key questions and research opportunities in the field of ECM ageing. We examine fundamental biological questions, enabling technologies, systems of study and emerging in vitro and in silico models, alongside consideration of the broader challenges facing the field.

Two light sensors decode moonlight versus sunlight to adjust a plastic circadian/circalunidian clock to moon phase.

Many species synchronize their physiology and behavior to specific hours. It is commonly assumed that sunlight acts as the main entrainment signal for ∼24-h clocks. However, the moon provides similarly regular time information. Consistently, a growing number of studies have reported correlations between diel behavior and lunidian cycles. Yet, mechanistic insight into the possible influences of the moon on ∼24-h timers remains scarce. We have explored the marine bristleworm Platynereis dumerilii to investigate the role of moonlight in the timing of daily behavior. We uncover that moonlight, besides its role in monthly timing, also schedules the exact hour of nocturnal swarming onset to the nights' darkest times. Our work reveals that extended moonlight impacts on a plastic clock that exhibits <24 h (moonlit) or >24 h (no moon) periodicity. Abundance, light sensitivity, and genetic requirement indicate that the Platynereis light receptor molecule r-Opsin1 serves as a receptor that senses moonrise, whereas the cryptochrome protein L-Cry is required to discriminate the proper valence of nocturnal light as either moonlight or sunlight. Comparative experiments in Drosophila suggest that cryptochrome's principle requirement for light valence interpretation is conserved. Its exact biochemical properties differ, however, between species with dissimilar timing ecology. Our work advances the molecular understanding of lunar impact on fundamental rhythmic processes, including those of marine mass spawners endangered by anthropogenic change.

The molecular clockwork of mammalian cells.

Most organisms contain self-sustained circadian clocks. These clocks can be synchronized by environmental stimuli, but can also oscillate indefinitely in isolation. In mammals this is true at the molecular level for the majority of cell types that have been examined. A core set of "clock genes" form a transcriptional/translational feedback loop (TTFL) which repeats with a period of approximately 24 h. The exact mechanism of the TTFL differs slightly in various cell types, but all involve similar family members of the core cohort of clock genes. The clock has many outputs which are unique for different tissues. Cells in diverse tissues will convert the timing signals provided by the TTFL into uniquely orchestrated transcriptional oscillations of many clock-controlled genes and cellular processes.

Computational modeling of the synergistic role of GCN2 and the HPA axis in regulating the integrated stress response in the central circadian timing system.

The circadian timing system and integrated stress response (ISR) systems are fundamental regulatory mechanisms that maintain body homeostasis. The central circadian pacemaker in the suprachiasmatic nucleus (SCN) governs daily rhythms through interactions with peripheral oscillators via the hypothalamus-pituitary-adrenal (HPA) axis. On the other hand, ISR signaling is pivotal for preserving cellular homeostasis in response to physiological changes. Notably, disrupted circadian rhythms are observed in cases of impaired ISR signaling. In this work, we examine the potential interplay between the central circadian system and the ISR, mainly through the SCN and HPA axis. We introduce a semimechanistic mathematical model to delineate SCN's capacity for indirectly perceiving physiological stress through glucocorticoid-mediated feedback from the HPA axis and orchestrating a cellular response via the ISR mechanism. Key components of our investigation include evaluating general control nonderepressible 2 (GCN2) expression in the SCN, the effect of physiological stress stimuli on the HPA axis, and the interconnected feedback between the HPA and SCN. Simulation revealed a critical role for GCN2 in linking ISR with circadian rhythms. Experimental findings have demonstrated that a Gcn2 deletion in mice leads to rapid re-entrainment of the circadian clock following jetlag as well as to an elongation of the circadian period. These phenomena are well replicated by our model, which suggests that both the swift re-entrainment and prolonged period can be ascribed to a reduced robustness in neuronal oscillators. Our model also offers insights into phase shifts induced by acute physiological stress and the alignment/misalignment of physiological stress with external light-dark cues. Such understanding aids in strategizing responses to stressful events, such as nutritional status changes and jetlag.NEW & NOTEWORTHY This study is the first theoretical work to investigate the complex interaction between integrated stress response (ISR) sensing and central circadian rhythm regulation, encompassing the suprachiasmatic nucleus (SCN) and hypothalamus-pituitary-adrenal (HPA) axis. The findings carry implications for the development of dietary or pharmacological interventions aimed at facilitating recovery from stressful events, such as jetlag. Moreover, they provide promising prospects for potential therapeutic interventions that target circadian rhythm disruption and various stress-related disorders.

Morning exercise and pre-breakfast metformin interact to reduce glycaemia in people with type 2 diabetes: a randomized crossover trial.

Exercise is recommended in the treatment of type 2 diabetes and can improve insulin sensitivity. However, previous evidence suggests that exercise at different times of the day in people with type 2 diabetes may have opposing outcomes on glycaemia. Metformin is the most commonly prescribed initial pharmacological intervention in type 2 diabetes, and may alter adaptions to exercise. It is unknown if there is an interaction between metformin and diurnal exercise outcomes. We aimed to investigate glycaemic outcomes of moderate intensity morning vs. evening exercise in people with type 2 diabetes being prescribed metformin monotherapy. In this study, nine males and nine females with type 2 diabetes undergoing metformin monotherapy (age 61 ± 8.2 years, mean ± SD) completed a 16-week crossover trial including 2-week baseline recording, 6 weeks randomly assigned to a morning exercise (07.00-10.00 h) or evening exercise (16.00-19.00 h) and a 2-week wash-out period. Exercise arms consisted of 30 min of walking at 70% of estimated max heart rate every other day. Glucose levels were measured with continuous glucose monitors and activity measured by wrist-worn monitors. Food-intake was recorded by 4-day food diaries during baseline, first and last 2 weeks of each exercise arm. There was no difference in exercise intensity, total caloric intake or total physical activity between morning and evening arms. As primary outcomes, acute (24 h) glucose area under the curve (AUC), was lower (P = 0.02) after acute morning exercise (180.6 ± 68.4 mmol/l) compared to baseline (210.3 ± 76.7 mmol/l); and there were no differences identified for glucose (mmol/l) between baseline, morning and evening exercise at any specific time point when data were analysed with two-way ANOVA. As secondary outcomes, acute glucose AUC was significantly lower (P = 0.01) in participants taking metformin before breakfast (152.5 ± 29.95 mmol/l) compared with participants taking metformin after breakfast (227.2 ± 61.51 mmol/l) only during the morning exercise arm; and during weeks 5-6 of the exercise protocol, glucose AUC was significantly lower (P = 0.04) for participants taking metformin before breakfast (168.8 ± 15.8 mmol/l), rather than after breakfast (224.5 ± 52.0 mmol/l), only during morning exercise. Our data reveal morning moderate exercise acutely lowers glucose levels in people with type 2 diabetes being prescribed metformin. This difference appears to be driven by individuals that consumed metformin prior to breakfast rather than after breakfast. This beneficial effect upon glucose levels of combined morning exercise and pre-breakfast metformin persisted through the final 2 weeks of the trial. Our findings suggest that morning moderate intensity exercise combined with pre-breakfast metformin intake may benefit the management of glycaemia in people with type 2 diabetes. KEY POINTS: Morning moderate exercise acutely lowers glucose levels in people with type 2 diabetes being prescribed metformin. This difference appears to be driven by individuals that consumed metformin prior to breakfast rather than after breakfast. Morning exercise combined with pre-breakfast metformin persistently reduced glucose compared to morning exercise combined with post-breakfast metformin through the final week (week 6) of the intervention. Our study suggests it may be possible to make simple changes to the time that people with type 2 diabetes take metformin and perform exercise to improve their blood glucose.

Loss of temporal coherence in the circadian metabolome across multiple tissues during ageing in mice.

Circadian clock function declines with ageing, which can aggravate ageing-related diseases such as type 2 diabetes and neurodegenerative disorders. Understanding age-related changes in the circadian system at a systemic level can contribute to the development of strategies to promote healthy ageing. The goal of this study was to investigate the impact of ageing on 24-h rhythms in amine metabolites across four tissues in young (2 months of age) and old (22-25 months of age) mice using a targeted metabolomics approach. Liver, plasma, the suprachiasmatic nucleus (SCN; the location of the central circadian clock in the hypothalamus) and the paraventricular nucleus (PVN; a downstream target of the SCN) were collected from young and old mice every 4 h during a 24-h period (n = 6-7 mice per group). Differential rhythmicity analysis revealed that ageing impacts 24-h rhythms in the amine metabolome in a tissue-specific manner. Most profound changes were observed in the liver, in which rhythmicity was lost in 60% of the metabolites in aged mice. Furthermore, we found strong correlations in metabolite levels between the liver and plasma and between the SCN and the PVN in young mice. These correlations were almost completely abolished in old mice. These results indicate that ageing is accompanied by a severe loss of the circadian coordination between tissues and by disturbed rhythmicity of metabolic processes. The tissue-specific impact of ageing may help to differentiate mechanisms of ageing-related disorders in the brain versus peripheral tissues and thereby contribute to the development of potential therapies for these disorders.

Crickets in the spotlight: exploring the impact of light on circadian behavior.

Crickets serve as a well-established model organism in biological research spanning various fields, such as behavior, physiology, neurobiology, and ecology. Cricket circadian behavior was first reported over a century ago and prompted a wealth of studies delving into their chronobiology. Circadian rhythms have been described in relation to fundamental cricket behaviors, encompassing stridulation and locomotion, but also in hormonal secretion and gene expression. Here we review how changes in illumination patterns and light intensity differentially impact the different cricket behaviors as well as circadian gene expression. We further describe the cricket's circadian pacemaker. Ample anatomical manipulations support the location of a major circadian pacemaker in the cricket optic lobes and another in the central brain, possibly interconnected via signaling of the neuropeptide PDF. The cricket circadian machinery comprises a molecular cascade based on two major transcriptional/translational negative feedback loops, deviating somewhat from the canonical model of Drosophila and emphasizing the significance of exploring alternative models. Finally, the nocturnal nature of crickets has provided a unique avenue for investigating the repercussions of artificial light at night on cricket behavior and ecology, underscoring the critical role played by natural light cycles in synchronizing cricket behaviors and populations, further supporting the use of the cricket model in the study of the effects of light on insects. Some gaps in our knowledge and challenges for future studies are discussed.

On hidden factors and design-associated errors that may lead to data misinterpretation: An example from preclinical research on the potential seasonality of neonatal seizures.

Unintentional misinterpretation of research in published biomedical reports that is not based on statistical flaws is often underrecognized, despite its possible impact on science, clinical practice, and public health. Important causes of such misinterpretation of scientific data, resulting in either false positive or false negative conclusions, include design-associated errors and hidden (or latent) variables that are not easily recognized during data analysis. Furthermore, cognitive biases, such as the inclination to seek patterns in data whether they exist or not, may lead to misinterpretation of data. Here, we give an example of these problems from hypothesis-driven research on the potential seasonality of neonatal seizures in a rat model of birth asphyxia. This commentary aims to raise awareness among the general scientific audience about the issues related to the presence of unintentional misinterpretation in published reports.

CLOCK gene circannual expression in cluster headache.

BACKGROUND: Cluster headache is a primary headache disorder characterized by bouts with circadian and circannual patterns. The CLOCK gene has a central role in regulating circadian rhythms. Here, we investigate the circannual CLOCK expression in a population of cluster headache patients in comparison to matched controls. METHODS: Patients with cluster headache were sampled two to four times over at least one year, both in or outside bouts, one week after each solstice and equinox. The expression of CLOCK was measured by quantitative real-time polymerase chain reaction (RT-PCR) in the peripheral blood. RESULTS: This study included 50 patients and 58 matched controls. Among the patient population, composed of 42/50 males (84%) with an average age of 44.6 years, 45/50 (90%) suffered from episodic cluster headache. Two to four samples were collected from each patient adding up to 161 samples, 36 (22.3%) of which were collected within a bout. CLOCK expression for cluster headache patients was considerably different from that of the control population in winter (p-value mean = 0.006283), spring (p-value mean = 0.000006) and summer (p-value mean = 0.000064), but not in autumn (p-value mean = 0.262272). For each season transition, the variations in CLOCK expression were more pronounced in the control group than in the cluster headache population. No statistically significant differences were found between bout and non-bout samples. No individual factors (age, sex, circadian chronotype, smoking and coffee habits or history of migraine) were related to CLOCK expression. CONCLUSIONS: We observed that CLOCK expression in cluster headache patients fluctuates less throughout the year than in the control population. Bout activity and lifestyle factors do not seem to influence CLOCK expression.

The Ultra-Long-Term Sleep study: Design, rationale, data stability and user perspective.

Sleep deprivation and poor sleep quality are significant societal challenges that negatively impact individuals' health. The interaction between subjective sleep quality, objective sleep measures, physical and cognitive performance, and their day-to-day variations remains poorly understood. Our year-long study of 20 healthy individuals, using subcutaneous electroencephalography, aimed to elucidate these interactions, assessing data stability and participant satisfaction, usability, well-being and adherence. In the study, 25 participants were fitted with a minimally invasive subcutaneous electroencephalography lead, with 20 completing the year of subcutaneous electroencephalography recording. Signal stability was measured using covariance of variation. Participant satisfaction, usability and well-being were measured with questionnaires: Perceived Ease of Use questionnaire, System Usability Scale, Headache questionnaire, Major Depression Inventory, World Health Organization 5-item Well-Being Index, and interviews. The subcutaneous electroencephalography signals remained stable for the entire year, with an average participant adherence rate of 91%. Participants rated their satisfaction with the subcutaneous electroencephalography device as easy to use with minimal or no discomfort. The System Usability Scale score was high at 86.3 ± 10.1, and interviews highlighted that participants understood how to use the subcutaneous electroencephalography device and described a period of acclimatization to sleeping with the device. This study provides compelling evidence for the feasibility of longitudinal sleep monitoring during everyday life utilizing subcutaneous electroencephalography in healthy subjects, showcasing excellent signal stability, adherence and user experience. The amassed subcutaneous electroencephalography data constitutes the largest dataset of its kind, and is poised to significantly advance our understanding of day-to-day variations in normal sleep and provide key insights into subjective and objective sleep quality.

Is the association between social jetlag and BMI mediated by lifestyle? A cross-sectional survey study in the Dutch general population.

OBJECTIVE: Social jetlag is a discordance between the social and biological rhythm and is associated with higher HbA1c, higher BMI, and higher odds of obesity. The pathways that could explain these associations are still debated. This study aims to assess the mediating role of several lifestyle factors in the cross-sectional association between social jetlag and BMI. METHODS: We used cross-sectional data from 1784 adults from urban areas in the Netherlands, collected in 2019. Social jetlag (difference in midpoint of sleep between week and weekend nights) was categorized as low(<1 h), moderate(1-2h), and high(>2 h). BMI(kg/m2) was calculated from self-reported height and weight. The association between social jetlag and BMI was assessed using linear regression, adjusted for sex, age, education, and sleep duration and stratified for the effect modifier stress (high vs. low). Mediation analysis was performed for self-reported smoking, physical activity, alcohol consumption, and adherence to a healthy diet. RESULTS: High social jetlag was associated with higher BMI (0.69 kg/m2,95%CI 0.05;1.33). This association was stronger in people with high stress (0.93 kg/m2,95%CI 0.09;1.76). Social jetlag was also associated with higher odds of smoking, lower physical activity, higher alcohol consumption, and lower healthy diet adherence. In people with high stress, these factors mediated 10-15% of the association between social jetlag and BMI. CONCLUSIONS: Social jetlag is associated with higher BMI and this association is stronger in people with high stress. In people with high stress, healthy diet adherence mediated 12% of this association. Other pathways involved in this association should be further investigated.

Personalized laboratory medicine in the digital health era: recent developments and future challenges.

Interpretation of laboratory data is a comparative procedure and requires reliable reference data, which are mostly derived from population data but used for individuals in conventional laboratory medicine. Using population data as a "reference" for individuals has generated several problems related to diagnosing, monitoring, and treating single individuals. This issue can be resolved by using data from individuals' repeated samples, as their personal reference, thus needing that laboratory data be personalized. The modern laboratory information system (LIS) can store the results of repeated measurements from millions of individuals. These data can then be analyzed to generate a variety of personalized reference data sets for numerous comparisons. In this manuscript, we redefine the term "personalized laboratory medicine" as the practices based on individual-specific samples and data. These reflect their unique biological characteristics, encompassing omics data, clinical chemistry, endocrinology, hematology, coagulation, and within-person biological variation of all laboratory data. It also includes information about individuals' health behavior, chronotypes, and all statistical algorithms used to make precise decisions. This approach facilitates more accurate diagnosis, monitoring, and treatment of diseases for each individual. Furthermore, we explore recent advancements and future challenges of personalized laboratory medicine in the context of the digital health era.

Calculating Aging: Analysis of Survival Curves in the Norm and Pathology, Fluctuations in Mortality Dynamics, Characteristics of Lifespan Distribution, and Indicators of Lifespan Variation.

The article describes the history of studies of survival data carried out at the Research Institute of Physico-Chemical Biology under the leadership of Academician V. P. Skulachev from 1970s until present, with special emphasis on the last decade. The use of accelerated failure time (AFT) model and analysis of coefficient of variation of lifespan (CVLS) in addition to the Gompertz methods of analysis, allows to assess survival curves for the presence of temporal scaling (i.e., manifestation of accelerated aging), without changing the shape of survival curve with the same coefficient of variation. A modification of the AFT model that uses temporal scaling as the null hypothesis made it possible to distinguish between the quantitative and qualitative differences in the dynamics of aging. It was also shown that it is possible to compare the data on the survival of species characterized by the survival curves of the original shape (i.e., "flat" curves without a pronounced increase in the probability of death with age typical of slowly aging species), when considering the distribution of lifespan as a statistical random variable and comparing parameters of such distribution. Thus, it was demonstrated that the higher impact of mortality caused by external factors (background mortality) in addition to the age-dependent mortality, the higher the disorder of mortality values and the greater its difference from the calculated value characteristic of developed countries (15-20%). For comparison, CVLS for the Paraguayan Ache Indians is 100% (57% if we exclude prepuberty individuals as suggested by Jones et al.). According to Skulachev, the next step is considering mortality fluctuations as a measure for the disorder of survival data. Visual evaluation of survival curves can already provide important data for subsequent analysis. Thus, Sokolov and Severin [1] found that mutations have different effects on the shape of survival curves. Type I survival curves generally retains their standard convex rectangular shape, while type II curves demonstrate a sharp increase in the mortality which makes them similar to a concave exponential curve with a stably high mortality rate. It is noteworthy that despite these differences, mutations in groups I and II are of a similar nature. They are associated (i) with "DNA metabolism" (DNA repair, transcription, and replication); (ii) protection against oxidative stress, associated with the activity of the transcription factor Nrf2, and (iii) regulation of proliferation, and (or these categories may overlap). However, these different mutations appear to produce the same result at the organismal level, namely, accelerated aging according to the Gompertz's law. This might be explained by the fact that all these mutations, each in its own unique way, either reduce the lifespan of cells or accelerate their transition to the senescent state, which supports the concept of Skulachev on the existence of multiple pathways of aging (chronic phenoptosis).

Circadian Regulation of Cardiac Physiology: Rhythms That Keep the Heart Beating.

On Earth, all life is exposed to dramatic changes in the environment over the course of the day; consequently, organisms have evolved strategies to both adapt to and anticipate these 24-h oscillations. As a result, time of day is a major regulator of mammalian physiology and processes, including transcription, signaling, metabolism, and muscle contraction, all of which oscillate over the course of the day. In particular, the heart is subject to wide fluctuations in energetic demand throughout the day as a result of waking, physical activity, and food intake patterns. Daily rhythms in cardiovascular function ensure that increased delivery of oxygen, nutrients, and endocrine factors to organs during the active period and the removal of metabolic by-products are in balance. Failure to maintain these physiologic rhythms invariably has pathologic consequences. This review highlights rhythms that underpin cardiac physiology. More specifically, we summarize the key aspects of cardiac physiology that oscillate over the course of the day and discuss potential mechanisms that regulate these 24-h rhythms.

Time matters: an insight into the relationship between chrononutrition and diabetes.

Chrononutrition is a branch of chronobiology that evaluates nutrients and the pathways implicated in their regulation in accordance with circadian rhythms. Sleep deprivation and disturbances have been strongly associated with the progression of different metabolic alterations, and the time of food intake plays a fundamental role in maintaining metabolic homeostasis. It has been demonstrated that not only the components of food are important, but quantity and quality are also crucial elements of a healthy eating pattern. Chrononutrition is an emerging tool that could help improve dietary interventions beyond those derived from consuming an adequate amount of each nutrient. Diabetes is a complex endocrine pathology characterized by sustained hyperglycemia. Dietary changes are a key component in obtaining adequate control and preventing long-term complications. Recent studies emphasize the use of chrononutrition and its components as a novel dietary intervention that could improve metabolic control. The use of chrononutrition as a dietary intervention is faced with challenges such as the presence of gaps in the literature that limit its implementation. This emphasizes the imperative need for additional research that can lead to an evidence-based use of this intervention.

Monkeypox virus replication underlying circadian rhythm networks.

The mammalian brain has an endogenous central circadian clock that regulates central and peripheral cellular activities. At the molecular level, this day-night cycle induces the expression of upstream and downstream transcription factors that influence the immune system and the severity of viral infections over time. In addition, there are also circadian effects on host tolerance pathways. This stimulates adaptation to normal changes in environmental conditions and requirements (including light and food). These rhythms influence the pharmacokinetics and efficacy of therapeutic drugs and vaccines. The importance of circadian systems in regulating viral infections and the host response to viruses is currently of great importance for clinical management. With the knowledge gained from the COVID-19 pandemic, it is important to address any outbreak of viral infection that could become endemic and to quickly focus research on any knowledge gaps. For example, responses to booster vaccination COVID-19 may have different time-dependent patterns during circadian cycles. There may be a link between reactivation of latently infected viruses and regulation of circadian rhythms. In addition, mammals may show different seasonal antiviral responses in winter and summer. This article discusses the importance of the host circadian clock during monkeypox infection and immune system interactions.

Sex differences, chronobiology and general anaesthesia in activities of the autonomic nervous system in rats.

NEW FINDINGS: What is the topic of this review? Changes in heart rate variability in rats with sex differences and the use of different anaesthesia during light-dark cycles. What advances does it highlight? The review highlights and discusses synthesized current results in order to advance knowledge and understanding of sex differences with an emphasis on changes in the autonomic nervous system determined by heart rate variability. ABSTRACT: Heart rate variability (HRV) is commonly used in experimental studies to assess sympathetic and parasympathetic activities. The belief that HRV in rodents reflects similar cardiovascular regulations in humans is supported by evidence, and HRV in rats appears to be at least analogous to that in humans, although the degree of influence of the parasympathetic division of the autonomic nervous system (ANS) may be greater in rats than in humans. Experimental studies are based on control or baseline values, on the basis of which the change in ANS activity after a given experimental intervention is assessed, but it is known that the ANS in rats is very sensitive to various stress interventions, such as the manipulation itself, and ANS activity can also differ depending on sex, the time of measurement, and whether the animals are under general anaesthesia. Thus, for correct assessment, changes in ANS activity and their relationship to the observed parameter should be based on whether ANS activity does or does not change but also to what extent the activity is already changed at the start of the experiment. Since rats are considered to be the most suitable model animal for basic cardiovascular research, in this review we point out existing differences in individual HRV frequency parameters at the start of experiments (control, baseline values), taking into account sex in relation to time of measurement and anaesthesia.

David S. Saunders: man of insects and photoperiodism (1935-2023).

David S. Saunders was an outstanding scientist, who devoted his life to his family and to insects. He has made many fundamental contributions to our understanding of how insects reproduce and adapt their reproduction and development to the seasonal changes on our planet. Most importantly, he was a pioneer in demonstrating the role of the circadian clock in insect photoperiodic time measurement, first in the jewel wasp Nasonia vitripennis, and later in varies species of flies. His books on biological rhythms and insect clocks are important undergraduate, graduate and research reference literature. David was also a brilliant teacher and mentor and played a major role in establishing and teaching a series of successful Erasmus-funded Chronobiology Summer Schools in Europe. He leaves behind a legacy, both professionally and personally.