Beyond circadian rhythms: emerging roles of ultradian rhythms in control of liver functions.

The mammalian liver must cope with various metabolic and physiological changes that normally recur every day and primarily stem from daily cycles of rest-activity and fasting-feeding. Although a large body of evidence supports the reciprocal regulation of circadian rhythms and liver function, the research on the hepatic ultradian rhythms have largely been lagging behind. However, with the advent of more cost-effective high-throughput omics technologies, high-resolution time-lapse imaging, and more robust and powerful mathematical tools, several recent studies have shed new light on the presence and functions of hepatic ultradian rhythms. In this review, we will first very briefly discuss the basic principles of circadian rhythms, and then cover in greater details the recent literature related to ultradian rhythms. Specifically, we will highlight the prevalence and mechanisms of hepatic 12-h rhythms, and 8-h rhythms, which cycle at the second and third harmonics of circadian frequency. Finally, we also refer to ultradian rhythms with other frequencies and examine the limitations of the current approaches as well as the challenges related to identifying ultradian rhythm and addressing their molecular underpinnings.

Adult-Juvenile interactions and temporal niche partitioning between life-stages in a tropical amphibian.

Divergence in ecological niche offers organisms the opportunity of exploiting different food and habitat resources, scaling down competition and predation both among species, and within different age or size-classes of the same species. In harsh environments, where abiotic factors determine a clustering of resources during short timespans, competition and predation between organisms is likely to be enhanced. This is the case in tropical dry forests, where amphibians have limited opportunities to feed, their activity being restricted to the short rainy season. One way to maximize resource exploitation while avoiding predation risk is by adopting different diel activity patterns. We tested this hypothesis by comparing activity patterns in adults and recently metamorphosed juveniles of Pacific horned frogs (Ceratophrys stolzmanni) during field surveys and in an experimental study. Field surveys showed that the adults are strictly nocturnal, whereas freshly metamorphosed juveniles can be found active above ground at all hours, with a peak activity during daytime. The average body condition index of juveniles found active during the night was higher than that of juveniles found active during the day, suggesting that the weaker individuals may be constrained to being active during the day. On the other hand, in a laboratory experiment, juveniles that were visually exposed to adults moved less than those in the absence of adults. Both field and experimental observations indicate a temporal niche divergence between life stages. The results of the experiment offer support to the hypothesis that the juveniles in this species display an inverse activity pattern compared to adults, which can reduce competitive interactions and predation pressure from the larger conspecifics.

Equation representing the dark-entrained transition from inaction to action in male and female mice.

The activation of behaviour in a daily rhythm governed by the light cycle is a universal phenomenon among humans, laboratory mammals and other vertebrates. For mice, the active period is during the dark. We have quantified the increase in activity when the lights shut off (Light to Dark, L to D) using a generalized CNS arousal assay with 20 ms resolution, rather than traditional running wheels. Data analysis yielded the rare demonstration of an equation which precisely tracks this behavioural transition and, surprisingly, its reverse during D to L. This behavioural dynamic survives in constant darkness (experiment 2) and is hormone-sensitive (experiment 3). Finally (experiment 4), mice on a light schedule analogous to one which proved troublesome for U.S. Navy sailors, had dysregulated activity bursts which did not conform to the transitions between D and L. These experiments show the lawfulness of a behavioural phase transition and the consequence of deviating from that dynamic pattern. And, in a new way, they bring mathematics to the realm of behavioural neuroscience.

Does 24-h Activity Cycle Influence Plasma PCSK9 Concentration? A Systematic Review and Meta-Analysis.

PURPOSE OF REVIEW: Higher plasma proprotein convertase subtilisin/kexin type 9 (PCSK9) concentration has been associated with a higher risk of atherosclerotic cardiovascular disease (ASCVD). Animal and human studies have examined the relationship between 24-h activity cycles (24-HAC) and PCSK9, but conflicting results exist. Therefore, this review aimed to examine the relationship between 24-HAC and plasma PCSK9 concentration in animals and humans.Three databases (PubMed, CINAHL, and Web of Science) were searched for eligible articles. Descriptive data were summarized using network meta-analysis. The effect size was estimated using pairwise meta-analysis. RECENT FINDINGS: The interventions designed to increase moderate to vigorous physical activities (MVPA) did not significantly change plasma PCSK9 concentration (Hedges' g = 0.137; p = 0.337). However, the effect was influenced by statin therapy and intervention delivery mode. Specifically, physical activity interventions in conjunction with statin therapy significantly increased plasma PCSK9 concentration (Hedges' g = 0.275; p = 0.007). Supervised exercise training significantly increased plasma PCSK9 concentration (Hedges' g = 0.630; p = 0.001), but physical activity counseling did not (p = 0.845). The effects of MVPA on plasma PCSK9 may be moderated by statin therapy, intervention delivery mode, or other potential unknown mechanistic factors. Thus, caution should be taken when using plasma PCSK9 as an outcome indicator for physical activity interventions aimed at decreasing the risk of ASCVD. Graphical abstract.

The influence of feeding method on a mother's circadian rhythm and on the development of her infant's circadian rest-activity rhythm.

An infant's circadian sleep-wake rhythm is established during the first three months after birth. It is crucial to identify how entrainment factors, such as maternal behavioural influences, feeding conditions, and others, contribute to the infant's obtaining circadian rhythm. This study examined the influence of feeding method on the mother's rhythm and on the development of her infant's circadian rest-activity rhythm. The subjects were 24 healthy primiparas (mean age, 29.8) and their full-term infants. We retrospectively divided the subjects into two groups based on feeding method (breastfeeding, 17; mixed-feeding, 7). Actographic recordings for the infants and their mothers were made over 3-5 consecutive days during the 2nd-3rd week, the 6th week, and the 12th week. First, we calculated the mean values of the autocorrelograms from the autocorrelation coefficients and calculated their statistical significance to see their rhythmicity for all the mothers and their infants at each of the periods. Second, we evaluated the mean values of the autocorrelograms to see statistical differences between the feeding methods. For the breastfed infants, the mean values of the autocorrelograms at 24-h for the 6th week were significant. However, the mean values for the mixed-fed infants were unclear. At the 12th week, the mean values of the autocorrelograms at 24-h for both groups of infants were significant. The mean value for the breastfeeding mothers at 24-h was significantly larger than that for the mixed-feeding mothers. In conclusion, the breastfeeding mothers contributed more to their infants achieving circadian rhythm than did the mixed-feeding mothers.

Twilight activity patterns and angling vulnerability of yellowmouth barracuda (Sphyraena viridensis Cuvier, 1829), a range-expanding thermophilic fish.

Among the thermophilic fishes that have become established in the north-western Mediterranean as a consequence of sea warming, the yellowmouth barracuda (Sphyraena viridensis Cuvier, 1829) appears to be one of the most successful and abundant in the coastal rocky environment, having increasingly become the object of recreational and commercial exploitation in the study area. Lure-fishing sessions were carried out from May 2016 to November 2018 in the Catalan Sea (NE Spain) at dawn and dusk, with the aim of providing new insight into the behavioural, spatial and feeding ecology and vulnerability to angling of this poorly known species. Generalized mixed-effects linear models showed that S. viridensis is a crepuscular inshore dweller, whose vulnerability to angling is significantly influenced by solar and lunar light intensities, being highest in the pre-spawn and spawning periods. Asymmetries between dawn and dusk activity patterns were detected, evidently related to a drop in aggressiveness at dusk following the spawning period. The simple study design adopted may be applied to other contexts, aiming to the recognition of several levels of fish vulnerability to angling.

Disruption in daily eating-fasting and activity-rest cycles in Indian adolescents attending school.

A lifestyle with erratic eating patterns and habits predisposes youngsters to obesity. Through a two-phase feasibility study among Indian students living in the Delhi area, we longitudinally examined the following: (1) the daily eating-fasting cycles of students (N = 34) in school and college using smartphones as they transition from high school (aged 13-15 years; nIX = 13) to higher secondary school (HSSS; 16-18 years; nXII = 9) to their first year (FY) of college (18-19 years; nFC = 12); and (2) daily activity-rest cycles and light-dark exposure of 31 higher secondary school students (HSSS) using actigraphy. In phase 1, students' food data were analyzed for temporal details of eating events and observable differences in diet composition, such as an energy-dense diet (fast food (FF)), as confounding factors of circadian health. Overall, the mean eating duration in high school, higher secondary and FY college students ranged from 14.1 to 16.2h. HSSS exhibited the shortest night fasting. Although FY college students exhibited the highest fast food percentage (FF%), a positive correlation between body mass index (BMI) and FF% was observed only among HSSS. Furthermore, the body weight of HSSS was significantly higher, indicating that FF, untimely eating and reduced night fasting were important obesity-associated factors in adolescents. Reduced night fasting duration was also related to shorter sleep in HSSS. Therefore, food data were supplemented with wrist actigraphy, i.e., activity-rest data, in HSSS. Actigraphy externally validated the increased obesogenic consequences of deregulated eating rhythms in HSSS. CamNtech motion watches were used to assess the relationship between disturbed activity cycles of HSSS and other circadian clock-related rhythms, such as sleep. Less than 50% of Indian HSSS slept 6 hours or more per night. Seven of 31 students remained awake throughout the night, during which they had more than 20% of their daily light exposure. Three nonparametric circadian rhythm analysis (NPCRA) variables revealed circadian disruption of activity in HSSS. The present study suggests that inappropriate timing and quality of food and sleep disturbances are important determinants of circadian disruptions in adolescents attending school.

Association of Time-Varying Rest-Activity Rhythm With Survival in Older Adults With Lung Cancer.

BACKGROUND: To the best of our knowledge, this is the first study to examine the relationship of rest-activity rhythm with survival in older adults with lung cancer and to consider variations in rest-activity rhythm over time. OBJECTIVE: The aim of this study was to explore the relationship between rest-activity rhythm variations and survival in 33 older adults with lung cancer by considering rest-activity rhythm as a time-dependent covariate over time. METHODS: In this prospective study with 5 repeated measurements, patients' rest-activity rhythm over 3 days was measured using actigraphy. The rest-activity rhythm was represented using the dichotomy index I (in-bed activity) < O (out-of-bed activity). The median I < O was used as the cutoff point, with an I < O of greater than or equal to 85.59% and less than 85.59% indicating robust and disrupted rest-activity rhythms, respectively. Data were analyzed using the Cox regression model with time-dependent repeated measurements of a covariate. RESULTS: In the time-dependent multivariate Cox model, a disrupted rest-activity rhythm was independently associated with a higher risk of death than was a robust rest-activity rhythm (hazard ratio, 16.05; P = .009). CONCLUSION: A time-varying rest-activity rhythm is incrementally associated with mortality in older adults with lung cancer and represents a rigorous and independent prognostic factor for their survival. IMPLICATIONS FOR PRACTICE: Clinicians may need to pay more attention to the rest-activity rhythms of older adults with lung cancer during disease progression. Future studies should account for the variation in rest-activity rhythm over time.

Gimme shelter: The effect of rocks and moonlight on occupancy and activity pattern of an endangered rodent, the garden dormouse Eliomys quercinus.

Information on spatial behaviour and temporal activity patterns is paramount for the conservation of animal species. This is particularly true for endangered taxa that are threatened by ongoing climatic and environmental changes. The garden dormouse Eliomys quercinus is a native European rodent (family Gliridae), whose populations are declining throughout the Continent. Notwithstanding this, neither International nor National laws explicitly require mandatory monitoring of populations. As a result, compelling information on the spatiotemporal behaviour of dormouse is lacking. We aimed to fill this gap by investigating occupancy patterns in relation to environmental features and activity rhythms in relation to moonlit nights in an Alpine population of dormouse within the Stelvio National Park, northern Italy. Data were collected between May and October 2015. Twenty camera-traps were deployed in a 500 ha coniferous forest, using a random tessellation approach; camera trap data were analyzed with occupancy models and kernel smoothers. Camera-traps provided a reliable assessment of the presence of the garden dormouse, with only 1 % of false absence and a high detection probability (68 %). The occurrence of the garden dormouse was positively influenced by the percentage of rock coverage on the ground. The species showed a strictly nocturnal behaviour, with an activity peak before midnight, negatively related to moonlit nights. The use of rocky areas for nesting, shelter site and thigmotactic movements and moonlight avoidance may represent adaptations of the garden dormouse to avoid predation risk. Our results shed some light on the habitat requirement of a poorly known, near-threatened species, and provide baseline information for future monitoring and conservation activities.

Nocturnal Activity Rhythm of Nyssomyia intermedia (Diptera: Psychodidae: Phlebotominae) in Campus Fiocruz Mata Atlântica, an American Cutaneous Leishmaniasis Transmission Area, Rio de Janeiro, Brazil.

INTRODUCTION: Nyssomyia intermedia is an important vector of American cutaneous leishmaniasis. METHODS: Sand flies were captured in a Shannon trap and after landing on human collectors and retrieved monthly. Nocturnal activity was estimated using a log-linear model. RESULTS: Peak sand fly collection in the Shannon trap occurred in summer and winter, and peak fly collection on humans occurred in spring and summer. CONCLUSIONS: Ny. intermedia was captured in both the hottest and coldest months, indicating its adaptability to different seasons and the possibility of disease transmission at any time of the year in Campus Fiocruz Mata Atlântica, Brazil.

Endogenous cycles, activity patterns and energy expenditure of an intertidal fish is modified by artificial light pollution at night (ALAN).

The increase of global light emissions in recent years has highlighted the need for urgent evaluation of their impacts on the behaviour, ecology and physiology of organisms. Numerous species exhibit daily cycles or strong scototaxic behaviours that could potentially be influenced if natural lighting conditions or cycles are disrupted. Artificial Light Pollution at Night (ALAN) stands for situations where artificial light alters natural light-dark cycles, as well as light intensities and wavelengths. ALAN is increasingly recognized as a potential threat to biodiversity, mainly because a growing number of studies are demonstrating its influence on animal behaviour, migration, reproduction and biological interactions. Most of these studies have focused on terrestrial organisms and ecosystems with studies on the effects of ALAN on marine ecosystems being more occasional. However, with the increasing human use and development of the coastal zone, organisms that inhabit shallow coastal or intertidal systems could be at increasing risk from ALAN. In this study we measured the levels of artificial light intensity in the field and used these levels to conduct experimental trials to determine the impact of ALAN on an intertidal fish. Specifically, we measured ALAN effects on physiological performance (oxygen consumption) and behaviour (activity patterns) of "Baunco" the rockfish Girella laevifrons, one of the most abundant and ecologically important intertidal fish in the Southeastern Pacific littoral. Our results indicated that individuals exposed to ALAN exhibited increased oxygen consumption and activity when compared with control animals. Moreover, those fish exposed to ALAN stopped displaying the natural (circatidal and circadian) activity cycles that were observed in control fish throughout the experiment. These changes in physiological function and behaviour could have serious implications for the long-term sustainability of fish populations and indirect impacts on intertidal communities in areas affected by ALAN.

Nocturnal activity rhythm of nyssomyia intermedia (diptera: psychodidae: phlebotominae) in campus fiocruz mata atlântica, an american cutaneous leishmaniasis transmission area, Rio de Janeiro, Brazil

Abstract INTRODUCTION: Nyssomyia intermedia is an important vector of American cutaneous leishmaniasis. METHODS: Sand flies were captured in a Shannon trap and after landing on human collectors and retrieved monthly. Nocturnal activity was estimated using a log-linear model. RESULTS: Peak sand fly collection in the Shannon trap occurred in summer and winter, and peak fly collection on humans occurred in spring and summer. CONCLUSIONS: Ny. intermedia was captured in both the hottest and coldest months, indicating its adaptability to different seasons and the possibility of disease transmission at any time of the year in Campus Fiocruz Mata Atlântica, Brazil.

Thermoregulation and the ultradian basic rest-activity cycle.

Daily life involves interactions with the external environment. In rats these apparently spontaneous interactions, often associated with the search for food, alternate with periods of rest in both the dark and light periods of the 24-hour day. Kleitman, in whose laboratory rapid eye movement sleep was discovered, referred to the temporal pattern as "the basic rest-activity cycle." The active periods of the basic rest-activity cycle occur approximately every 1-2 hours in an irregular stochastic pattern that has been described (perhaps unhelpfully) as ultradian rhythmicity. Both the spontaneous interactions and those evoked by salient, potentially threatening environmental events are accompanied by increases in brown adipose tissue (BAT) temperature of approximately 1°C. The heat produced in BAT contributes to associated increases in the temperature of the brain (approximately 0.8°C) and the body (approximately 0.6°C). These temperature changes require extension of the conventional "homeostasis" framework of temperature regulation. They may function to facilitate the cognitive processing that underlies the vital decision making necessary for safe and effective interaction with the external environment.

Toward a theory of coactivation patterns in excitable neural networks.

The relationship between the structural connectivity (SC) and functional connectivity (FC) of neural systems is of central importance in brain network science. It is an open question, however, how the SC-FC relationship depends on specific topological features of brain networks or the models used for describing neural dynamics. Using a basic but general model of discrete excitable units that follow a susceptible-excited-refractory activity cycle (SER model), we here analyze how the network activity patterns underlying functional connectivity are shaped by the characteristic topological features of the network. We develop an analytical framework for describing the contribution of essential topological elements, such as common inputs and pacemakers, to the coactivation of nodes, and demonstrate the validity of the approach by comparison of the analytical predictions with numerical simulations of various exemplar networks. The present analytic framework may serve as an initial step for the mechanistic understanding of the contributions of brain network topology to brain dynamics.

Artificial light at night shifts daily activity patterns but not the internal clock in the great tit (Parus major).

Artificial light at night has shown a dramatic increase over the last decades and continues to increase. Light at night can have strong effects on the behaviour and physiology of species, which includes changes in the daily timing of activity; a clear example is the advance in dawn song onset in songbirds by low levels of light at night. Although such effects are often referred to as changes in circadian timing, i.e. changes to the internal clock, two alternative mechanisms are possible. First, light at night can change the timing of clock controlled activity, without any change to the clock itself; e.g. by a change in the phase relation between the circadian clock and expression of activity. Second, changes in daily activity can be a direct response to light ('masking'), without any involvement of the circadian system. Here, we studied whether the advance in onset of activity by dim light at night in great tits (Parus major) is indeed attributable to a phase shift of the internal clock. We entrained birds to a normal light/dark (LD) cycle with bright light during daytime and darkness at night, and to a comparable (LDim) schedule with dim light at night. The dim light at night strongly advanced the onset of activity of the birds. After at least six days in LD or LDim, we kept birds in constant darkness (DD) by leaving off all lights so birds would revert to their endogenous, circadian system controlled timing of activity. We found that the timing of onset in DD was not dependent on whether the birds were kept at LD or LDim before the measurement. Thus, the advance of activity under light at night is caused by a direct effect of light rather than a phase shift of the internal clock. This demonstrates that birds are capable of changing their daily activity to low levels of light at night directly, without the need to alter their internal clock.

A Model-Based Approach to Optimizing Ultradian Forced Desynchrony Protocols for Human Circadian Research.

The human circadian system regulates internal 24-h rhythmicity and plays an important role in many aspects of human health and behavior. To investigate properties of the human circadian pacemaker such as intrinsic period and light sensitivity, experimental researchers have developed forced desynchrony (FD) protocols in which manipulations of the light-dark (LD) cycle are used to desynchronize the intrinsic circadian rhythm from the rest-activity cycle. FD protocols have typically been based on exposure to long LD cycles, but recently, ultradian FD protocols with short LD cycles have been proposed as a new methodology for assessing intrinsic circadian period. However, the effects of ultradian FD protocol design, including light intensity or study duration, on estimates of intrinsic circadian period have not, to our knowledge, been systematically studied. To address this gap, we applied a light-sensitive, dynamic mathematical model of the human circadian pacemaker to simulate ultradian FD protocols and analyze the effects of protocol design on estimates of intrinsic circadian period. We found that optimal estimates were obtained using protocols with low light intensities, at least 10 d of exposure to ultradian cycling, and a 7-h LD cycle duration that facilitated uniform light exposure across all circadian phases. Our results establish a theoretical framework for ultradian FD protocols that can be used to provide insights into data obtained under existing protocols and to optimize protocols for future experiments.

Diurnal behavior and activity budget of the golden-crowned flying fox (Acerodon jubatus) in the Subic bay forest reserve area, the Philippines.

Acerodon jubatus (the Golden-Crowned flying fox) is an endemic species in the Philippines, which was suspected to be a host of the Reston strain of the Ebola virus. As nocturnal animals, the flying foxes spend daytime at the roosting site, which they use for self-maintenance and reproduction. To understand the variation in diurnal behavior and time allocation for various activities in the Golden-Crowned flying fox, we investigated their daytime behavior and activity budget using instantaneous scan sampling and all occurrence focal sampling. Data collection was performed from 07:00 to 18:00 hr during January 8-17, 2017. The most frequent activity was sleeping (76.3%). The remaining activities were wing flapping (5.0%), self-grooming (4.2%), hanging relaxation (3.4%), wing spread (2.9%), movement (2.4%), mating/courtship (2.4%), aggression (1.9%), hanging alert (1.2%), excretion (0.1%) and scent marks (0.05%). The frequency of sleeping, wing flapping, self-grooming, hanging relaxation, aggression, mating/courtship and movement behaviors changed with the time of the day. Females allocated more time for resting than males, while males spent more time on the activities that helped enhance their mating opportunities, for example, movement, sexual activity and territorial behavior.

Short-term activity cycles impede information transmission in ant colonies.

Rhythmical activity patterns are ubiquitous in nature. We study an oscillatory biological system: collective activity cycles in ant colonies. Ant colonies have become model systems for research on biological networks because the interactions between the component parts are visible to the naked eye, and because the time-ordered contact network formed by these interactions serves as the substrate for the distribution of information and other resources throughout the colony. To understand how the collective activity cycles influence the contact network transport properties, we used an automated tracking system to record the movement of all the individuals within nine different ant colonies. From these trajectories we extracted over two million ant-to-ant interactions. Time-series analysis of the temporal fluctuations of the overall colony interaction and movement rates revealed that both the period and amplitude of the activity cycles exhibit a diurnal cycle, in which daytime cycles are faster and of greater amplitude than night cycles. Using epidemiology-derived models of transmission over networks, we compared the transmission properties of the observed periodic contact networks with those of synthetic aperiodic networks. These simulations revealed that contrary to some predictions, regularly-oscillating contact networks should impede information transmission. Further, we provide a mechanistic explanation for this effect, and present evidence in support of it.

Morningness-eveningness and amplitude - development and validation of an improved composite scale to measure circadian preference and stability (MESSi).

Measuring morningness-eveningness is an important aspect of individual differences because it is associated with many aspects of personality and health. The present study outlines recent advancements in the field of measurement and proposes an improved assessment of morningness-eveningness, such as the measurement of circadian amplitude, updating and reflecting new item developments, addressing the clock time based measures, the morning-biased items and the aspect of uni versus multidimensionality. Four studies have been carried out in Germany to present a novel development (with a total sample of N = 1181). In study I, the exploratory factor analysis (EFA) revealed three dimensions, one of morningness, one of eveningness and one of amplitude/stability. Then, items were reduced to present a clearer factor structure by removing ambiguous items. In the second study, a shortened questionnaire was applied, with 15 items (5 per construct), but Confirmatory Factor Analysis (CFA) did not provide acceptable fit indices. Refining items were made in study III, which again showed a clearer factor structure in EFA, and subsequently, in study IV, the refined set of 15 items provided a good fit of a CFA. The final questionnaire was tested for validity by applying clock times, personality questions and alertness ratings. Thus, this newly developed questionnaire contains three distinct dimensions. To reflect the new content, the scale is labelled morningness-eveningness-stability-scale improved (MESSi).

Male prairie voles display cardiovascular dipping associated with an ultradian activity cycle.

Mammals typically display alternating active and resting phases and, in most species, these rhythms follow a circadian pattern. The active and resting phases often are accompanied by corresponding physiological changes. In humans, blood pressure decreases during the resting phase of the activity cycle, and the magnitude of that "nocturnal dipping" has been used to stratify patients according to the risk for cardiovascular disease. However, in contrast to most mammals, prairie voles (Microtus ochrogaster) have periods of activity and rest that follow an ultradian rhythm with period lengths significantly <24h. While rhythmic changes in blood pressure across a circadian activity cycle have been well-documented, blood pressure patterns in species that display ultradian rhythms in activity are less well-studied. In the current study, we implanted pressure-sensitive radiotelemetry devices in male prairie voles and recorded activity, mean arterial pressure (MAP), and heart rate (HR) continuously for 3days. Visualization of the ultradian rhythms was enhanced using a 1h running average to filter the dataset. Positive correlations were found between activity and MAP and between activity and HR. During the inactive period of the ultradian cycle, blood pressure decreased by about 15%, which parallels the nocturnal dipping pattern seen in healthy humans. Further, the duration of inactivity did not affect any of the cardiovascular measures, so the differences in blood pressure values between the active and inactive periods are likely driven by ultradian oscillations in hormones and autonomic function. Finally, specific behavioral patterns also were examined. Both the instrumented animal and his non-instrumented cagemate appeared to show synchronized activity patterns, with both animals displaying sleep-like behavior for more than 90% of the inactive period. We propose that the prairie vole ultradian rhythm in blood pressure is an analogue for circadian blood pressure variability and can be used to study the long-term effects of commonly prescribed drugs on blood pressure dipping.