Circadian clocks govern daily physiological and behavioral processes and are crucial for health; disruptions can lead to various diseases. The circadian phase of entrainment—the phase of the internal circadian clock in relation to external environmental cycles—is influenced by both genetic and environmental factors, varies between individuals, and is reflected in daily behaviors such as sleep–wake patterns, cognitive performance, and physical activity. While circadian phase may also fluctuate within individuals, the dynamics and extent of such variation in daily life remain largely unexplored. The gold standard for circadian phase assessment, dim-light melatonin onset (DLMO), is impractical for large-scale studies, and blood-based molecular biomarkers, while promising, are limited in feasibility. To address these challenges, we developed HairTime, a noninvasive assay that estimates circadian phase from a single daytime hair sample. Developed and evaluated in two steps—a training and a validation study—HairTime demonstrated strong predictive power compared to DLMO. Suitable for large-scale studies, it was assessed using over 4,000 samples. Circadian phase estimations showed a normal distribution and were associated with age, sex, and notably, work schedules, with earlier timing on workdays, suggesting that societal factors can modulate internal rhythms. Together, these findings establish HairTime as a promising tool for assessing circadian phase in research and lay the foundation for future applications in personalized chronotherapy.

Circadian clocks orchestrate metabolic processes in the whole body and their response to food. Therefore, not only what and how much, but also when we eat has a significant impact on metabolism. In this context, daytime of carbohydrate and fat intake was recently shown to alter the metabolic state and potentially affect the disease risk. However, molecular mechanisms of this regulation in humans are poorly understood. In this crossover trial, we investigated the effects of two isocaloric 4-week dietary patterns – high carbohydrate in the morning and high fat in the afternoon (HC/HF) and the reverse (HF/HC) - on the subcutaneous adipose tissue (SAT) in overweight non-diabetic men. The SAT transcriptome was assessed using microarrays. Additionally, gene expression in peripheral blood monocytes (PBMCs) was analysed by qPCRs. Analysis of SAT samples collected across the day identified 1386 genes exhibiting diurnal oscillations. In SAT, both oscillatory and non-oscillatory genes related to lipid and glucose metabolism were modulated by the timing of macronutrient intake. Notably, expression of inflammatory response genes in SAT was elevated after HC/HF compared to HF/HC, suggesting that the HC/HF diet might promote an early proinflammatory state in SAT due to higher fat intake in the afternoon. Diet-induced remodelling of the SAT transcriptome was partly reflected in PBMCs. These findings demonstrate that diurnal macronutrient distribution significantly reshapes the SAT transcriptome, underscoring the relevance of eating timing-based (chrononutritional) strategies for prevention of metabolic dysfunction and systemic inflammation associated with obesity and type 2 diabetes.

Dysfunction of circadian and sleep rhythms is an early feature of many neurodegenerative diseases. Alzheimer's disease (AD) is a progressive neurodegenerative disorder resulting in cognitive and psychiatric disturbances. Although it is largely unclear whether dysfunctions in sleep and circadian rhythms contribute to the etiology of AD or are a consequence of the disease, there is evidence that these conditions are involved in a complex self-reinforcing bidirectional relationship. According to the recent studies, dysregulation of the circadian clock already occurs during the asymptomatic stage of the disease and could promote neurodegeneration. Thus, restoration of sleep and circadian rhythms in preclinical AD may represent an opportunity for early intervention to slow the disease course.

Keywords: Alzheimer's disease; circadian system; clock; neurodegeneration; sleep.

Background: Critical illness myopathy (CIM) increases mortality and causes long-term disabilities. CIM is characterized by reduced muscle excitability, muscle atrophy, weakness, and impaired glucose metabolism. Functional circadian rhythms are important for skeletal muscle homeostasis. Circadian rhythms are often disrupted during critical illness in the Intensive Care Unit (ICU). This analysis investigates whether diurnal temperature rhythms differ in critically ill CIM compared to no-CIM patients.

Methods: This is a secondary analysis of two prospective trials including critically ill patients with CIM (n = 32) or no-CIM (n = 30) based on electrophysiological tests. Diurnal body temperature rhythms were compared between CIM and no-CIM groups in reference to n = 16 participants included in a bed rest study. Cosinor analysis was performed to determine the rhythm parameters and classify into rhythm classes. Aggregated and longitudinal data were compared between groups using non-parametric tests. Rhythm parameters were correlated with muscle atrophy, weakness and insulin sensitivity.

Results: CIM and no-CIM patients had severe multiorgan failure (median SOFA score 12 in both groups, p = 0.39). The temperature rhythm nadir timepoint was shifted in CIM patients (10:43 [09:21, 12:22]) and no-CIM (11:12 [09:43, 13:30]) compared to the healthy bed rest group (5:03 [3:22, 6:36]) p < 0.001. CIM patients showed lower temperature rhythm mesors than no-CIM patients (p = 0.041). The temperature rhythm amplitude was lower in both CIM and no-CIM patients compared to the healthy bed rest group (CIM: 0.3 °C [0.2, 0.4]; no-CIM: 0.2 °C [0.2, 0.3]; healthy bed rest: 0.5 °C [0.2, 0.6]; p < 0.01). Compared to no-CIM patients, CIM patients had higher temperature rhythm amplitudes (p = 0.021) and showed a less pronounced reduction in temperature rhythm amplitudes during ICU stay (p = 0.017). A higher temperature rhythm amplitude correlated negatively with M. vastus lateralis myocyte cross-sectional area.

Conclusions: Heterogeneous phase shifts of diurnal temperature rhythms in CIM and no-CIM groups compared to healthy bed rest volunteers may indicate ICU-related circadian disruption. Suppression of temperature rhythm amplitude during ICU stay could represent an adaptive response to this disruption. Blunted amplitude suppression observed in CIM compared to no-CIM patients might reflect reduced adaptation, potentially contributing to muscle catabolism. This hypothesis-generating analysis underlines the need for mechanistic studies exploring circadian regulation in skeletal muscle during critical illness.

Keywords: Body temperature; Circadian rhythm disruption; Circadian rhythms; Critical illness; Critical illness myopathy; Diurnal rhythms; Muscle weakness; Skeletal muscle atrophy; Temperature rhythm.

Background: Circadian rhythms are often severely disrupted in critically ill patients in the ICU. The ICU environment, characterized by irregular light-dark signals, continuous nutrition, and round-the-clock interventions, contributes to this disruption by providing weak and conflicting timing cues to the circadian system. Extensive scientific research has demonstrated that circadian rhythms play a vital role in regulating physiology and maintaining overall health. Therefore, integrating circadian principles into critical care may represent a promising strategy to improve patient outcomes in the ICU.

Research question: What are the key challenges of integrating circadian medicine into critical care, what steps can address these challenges, and which recommendations can guide future study designs and clinical implementation?

Study design and methods: We convened a 5-day workshop in September 2024 that brought together 24 international experts with backgrounds in circadian biology, critical care, and implementation science. Each day was organized around a predefined theme, with morning presentations and plenary discussions, and afternoons dedicated to drafting a list of Propositions and Recommendations in breakout groups. Propositions and Recommendations were finalized via a post-workshop survey requiring ≥ 75% agreement.

Results: This roadmap summarizes the discussions and outcomes of the workshop, structured around a set of Propositions and Recommendations, and provides a framework for building a robust evidence base for integrating circadian principles into ICU practice. Key recommendations include the development of circadian outcome measures tailored for use in the ICU and using standardized frameworks for evaluating the effect of circadian interventions in clinical trials.

Interpretation: Altogether, this roadmap provides an interdisciplinary framework resulting from a collaborative effort of ICU clinicians, circadian biologists, and implementation specialists, for building a robust evidence base for integrating circadian principles into ICU research and practice.

Keywords: ICU; circadian rhythms; critical care; critical illness; meeting report.

Time-restricted eating (TRE) is a promising strategy to improve metabolic outcomes. However, it remains unclear whether TRE has cardiometabolic benefits in an isocaloric setting and whether its effects depend on the eating timing. We conducted a randomized crossover trial in 31 women with overweight or obesity to directly compare the effects of a 2-week early TRE (eTRE; eating from 8:00 to 16:00) and a 2-week late TRE (lTRE; eating from 13:00 to 21:00) on insulin sensitivity, cardiometabolic risk factors, and the internal circadian phase. During the restricted 8-hour eating period, participants were asked to consume their habitual food quality and quantity. Insulin sensitivity did not differ between (-0.07; 95% CI, -0.77 to 0.62; P = 0.60) or within (eTRE: 0.31; 95% CI, -0.14 to 0.76; P = 0.11; lTRE: 0.19; 95% CI, -0.22 to 0.60; P = 0.25) interventions. Twenty-four-hour glucose, lipid, inflammatory, and oxidative stress markers showed no clinically meaningful between- or within-intervention differences. Participants demonstrated high timely adherence (eTRE, 96.5%; lTRE, 97.7%), unchanged dietary composition and physical activity, minor daily calorie deficit (eTRE, -167 kilocalories/day), and weight loss (eTRE, -1.08 kilograms; lTRE, -0.44 kilograms). In lTRE, the circadian phase in blood monocytes (24 minutes; 95% CI, -5 to 54 minutes; P = 0.10) and sleep midpoint (15 minutes; 95% CI, 7 to 23 minutes; P< 0.001) occurred later compared with eTRE. Overall, in an intended isocaloric setting, neither eTRE nor lTRE improves insulin sensitivity or other cardiometabolic traits, despite a shift of internal circadian clocks.

Rationale: Mechanical ventilation (MV) is life-saving but may evoke ventilator-induced lung injury (VILI). Objectives: To explore how the circadian clock modulates severity of murine VILI via the core clock component BMAL1 (basic helix-loop-helix ARNT like 1) in myeloid cells. Methods: Myeloid cell BMAL1-deficient (LysM (lysozyme 2 promoter/enhancer driving cre recombinase expression)^Bmal1-/-) or wild-type control (LysM^Bmal1+/+) mice were subjected to 4 hours MV (34 ml/kg body weight) to induce lung injury. Ventilation was initiated at dawn or dusk or in complete darkness (circadian time [CT] 0 or CT12) to determine diurnal and circadian effects. Lung injury was quantified by lung function, pulmonary permeability, blood gas analysis, neutrophil recruitment, inflammatory markers, and histology. Neutrophil activation and oxidative burst were analyzed ex vivo. Measurements and Main Results: In diurnal experiments, mice ventilated at dawn exhibited higher permeability and neutrophil recruitment compared with dusk. Experiments at CT showed deterioration of pulmonary function, worsening of oxygenation, and increased mortality at CT0 compared with CT12. Wild-type neutrophils isolated at dawn showed higher activation and reactive oxygen species production compared with dusk, whereas these day-night differences were dampened in LysM^Bmal1-/- neutrophils. In LysM^Bmal1-/- mice, circadian variations in VILI severity were dampened and VILI-induced mortality at CT0 was reduced compared with LysM^Bmal1+/+ mice. Conclusions: Inflammatory response and lung barrier dysfunction upon MV exhibit diurnal variations, regulated by the circadian clock. LysM^Bmal1-/- mice are less susceptible to ventilation-induced pathology and lack circadian variation of severity compared with LysM^Bmal1+/+ mice. Our data suggest that the internal clock in myeloid cells is an important modulator of VILI.

Patients admitted to the intensive care unit (ICU) are in need of continuous organ replacement strategies and specialized care, for example because of neurological dysfunction, cardio-pulmonary instability, liver or kidney failure, trauma, hemorrhagic or septic shock or even preterm birth. The 24-h nursing and care interventions provided to critically ill patients significantly limit resting and/or recovery phases. Consecutively, the patient's endogenous circadian rhythms are misaligned and disrupted, which in turn may interfere with their critical condition. A more thorough understanding of the complex interactions of circadian effectors and tissue-specific molecular clocks could therefore serve as potential means for enhancing personalized treatment in critically ill patients, conceivably restoring their circadian network and thus accelerating their physical and neurocognitive recovery. This review addresses the overarching issue of how circadian rhythms are affected and disturbed in critically ill newborns and adults in the ICU, and whether the conflicting external or environmental cues in the ICU environment further promote disruption and thus severity of illness. We direct special attention to the influence of cell-type specific molecular clocks on with severity of organ dysfunctions such as severity of brain dysfunction, pneumonia- or ventilator-associated lung inflammation, cardiovascular instability, liver and kidney failure, trauma, and septic shock. Finally, we address the potential of circadian rhythm stabilization to enhance and accelerate clinical recovery.

Time-restricted eating (TRE) is a promising strategy against metabolic disorders, but its effects on lipid metabolism remain controversial. The present research assesses and compares the impact of early (eTRE) versus late (lTRE) TRE on the plasma lipidomic profile. This is an exploratory outcome of the previously published randomized crossover trial, which examines 31 women with overweight or obesity who follow a two-week eTRE and a two-week lTRE in an intended isocaloric setting. Blood plasma and subcutaneous adipose tissue biopsies are analyzed using shotgun lipidomics and transcriptomics, respectively. Between interventions and within the lTRE, lipid species and classes, as well as enzyme activity indices, are not substantially changed. Within the eTRE, changes are observed for 103 lipid species, including a reduction of ceramide and phosphatidylcholine classes, and for the desaturation indices D5D, D6D, and D9D, as well as the elongation index ELOVL6. Combined analysis of plasma lipidome and adipose tissue reveals alterations in the glycerophospholipid pathway and in the expression of phospholipase enzymes PLB1, PLA2G6, and PLAG4B, dependent on TRE timing. These results suggest that eating timing during TRE may be crucial for remodeling the plasma lipidome and adipose tissue transcriptome and highlight the need of future lipidomic research in TRE.