Definition and research guide
This page combines a MeSH definition, publication statistics and selected research abstracts. It is a starting point for reading the literature, not a medical textbook or a systematic evidence review.
Content available: 3 complete abstracts (488 words), 9 references. Full article text is not stored or reproduced here; follow the PMID, DOI or PMC links to read the original sources and their access terms.
Definition
The regular recurrence, in cycles of about 24 hours, of biological processes or activities, such as sensitivity to drugs or environmental and physiological stimuli.
Definition: NLM Medical Subject Headings (MeSH) scope note (Public domain (U.S. National Library of Medicine); attribution requested). View the MeSH record.
Also known as: Circadian Rhythms; Rhythm, Circadian; Rhythms, Circadian; Nycthemeral Rhythm; Nycthemeral Rhythms; Rhythm, Nycthemeral; Rhythms, Nycthemeral; Nyctohemeral Rhythm.
Where it fits in the MeSH classification
- Broader topics
- Periodicity
- Narrower topics
- Chronotype
- See also
- Biological Clocks; Photoperiod; Chronotherapy; Circadian Clocks
Research landscape
PubMed indexes 83,660 records tagged with the MeSH heading “Circadian Rhythm” (counts retrieved 2026-10-31). 12,652 of them (15.1%) were published from 2020 onward, and the busiest period so far is through 1999 with 35,742 records.
Reviews account for 10,309 records (12.3%) and meta-analyses for 274. Randomized controlled trials number 2,657 (3.2%) as a share of all records on the topic.
Show the data
| Period | Records |
|---|---|
| through 1999 | 35,742 |
| 2000-2009 | 17,511 |
| 2010-2019 | 17,755 |
| 2020 onward | 12,652 |
Publication-type chart data
| Type | Records | Share |
|---|---|---|
| Reviews | 10,309 | 12.3% |
| Meta-analyses | 274 | 0.3% |
| Randomized controlled trials | 2,657 | 3.2% |
Method: PubMed query "Circadian Rhythm"[MeSH Terms] with publication-type and date filters, retrieved 2026-10-31.
Reading the evidence in anatomy and physiology
Anatomy describes how body structures are built and physiology describes how they normally work, so most findings here are descriptive or mechanistic rather than evidence about treatment. Much of what is known comes from dissection, imaging, tissue studies and experiments in animals, and measurements in healthy volunteers can differ from those in people with disease or at different ages. Normal ranges vary between individuals and populations. Check whether a result was measured in people or in another species, the age and health of the participants, and the method used.
Evidence from open-access studies
These open-access studies come from PubMed’s best-match results for this heading (papers whose main topic it is), ranked here by citation count, with one recent addition. Each abstract is reproduced in full under its recorded open licence so that you can read the authors’ own words. They illustrate the literature; they are not a systematic summary of it. Citation counts measure attention, not reliability or clinical benefit.
| Study / authors | Year | Source label / design | Europe PMC citations | Licence / source |
|---|---|---|---|---|
| HIF-1α and BMAL1 in bone regeneration: crosstalk between hypoxia response and circadian rhythm | 2026 | Review article Design not recorded in structured metadata | 7 | cc by PMID 41690951 · DOI · PMC12906504 |
| A cellular basis for the mammalian nocturnal-diurnal switch | 2026 | Research article Design not recorded in structured metadata | 4 | cc by PMID 41747039 · DOI · PMC7618833 |
| Chronic intermittent hypoxia reshapes circadian metabolic architecture in a model of sleep apnea | 2026 | Research article Design not recorded in structured metadata | 3 | cc by PMID 41739942 · DOI · PMC12935056 |
HIF-1α and BMAL1 in bone regeneration: crosstalk between hypoxia response and circadian rhythm [1]
Abstract reading summary — source excerpts
- Opening excerpt
- Bone regeneration is initiated after a bone injury, such as a bone fracture or tooth extraction.
- Closing excerpt
- Additionally, we extend the discussion to their interactions in other bone-related diseases, and summarize the existing research directions and deficiencies, providing new insights for in-depth studies of the hypoxia response and circadian rhythm systems.
- Interpretation limits
- Verbatim excerpts from the authors’ abstract; the closing excerpt may describe a finding, limitation or recommendation. Design, population, effect size and uncertainty have not been independently extracted or appraised. Read the complete abstract and linked paper before interpreting the findings.
Read complete abstract (173 words)
Bone regeneration is initiated after a bone injury, such as a bone fracture or tooth extraction. It is a highly complex biological process involving multiple cell types, signaling molecules, and molecular pathways. The hypoxic microenvironment in the early stage of bone regeneration poses challenges to cell status and the final outcome of bone regeneration. During this phase, two key regulators-HIF-1α (the critical mediator of hypoxia response) and BMAL1 (the central component of the circadian rhythm)-orchestrate the activities of bone-regenerating cells, ensuring proper cellular function and orderly progression of bone repair. Existing studies have shown that there is a close crosstalk between HIF-1α and BMAL1, including regulation of gene expression, protein interaction, and regulation of downstream pathways. In this review, we discuss the respective regulatory roles of HIF-1α and BMAL1 in bone regeneration and further summarize their interactions within cells. Additionally, we extend the discussion to their interactions in other bone-related diseases, and summarize the existing research directions and deficiencies, providing new insights for in-depth studies of the hypoxia response and circadian rhythm systems.
Abstract reproduced from the open-access article under the recorded cc by licence; © the authors. PMID 41690951 · DOI · PMC12906504
A cellular basis for the mammalian nocturnal-diurnal switch [2]
Abstract reading summary — source excerpts
- Opening excerpt
- Early mammals were nocturnal while dinosaurs dominated the daytime.
- Closing excerpt
- These findings uncover a genetic and biochemical basis for nocturnal-diurnal switching, emphasizing how cellular signaling networks can encode complex phenotypes such as temporal niche selection.
- Interpretation limits
- Verbatim excerpts from the authors’ abstract; the closing excerpt may describe a finding, limitation or recommendation. Design, population, effect size and uncertainty have not been independently extracted or appraised. Read the complete abstract and linked paper before interpreting the findings.
Read complete abstract (126 words)
Early mammals were nocturnal while dinosaurs dominated the daytime. Mammalian transition to daytime activity accelerated after the Cretaceous-Paleogene extinction, but the underlying mechanisms remain unclear. We identified a conserved cell-intrinsic, thermodynamic mechanism that likely facilitated this shift. In cells from diurnal mammals, protein synthesis, phosphorylation, and circadian timing were less sensitive to temperature changes than were cells from nocturnal mammals. Comparative genomics revealed accelerated evolution within essential signaling pathways, including mechanistic target of rapamycin (mTOR), that increase the robustness of diurnal cellular clocks to thermal and osmotic perturbation. In nocturnal mice, mTOR inhibition shifted cells, tissues, and behavior toward diurnal activity. These findings uncover a genetic and biochemical basis for nocturnal-diurnal switching, emphasizing how cellular signaling networks can encode complex phenotypes such as temporal niche selection.
Abstract reproduced from the open-access article under the recorded cc by licence; © the authors. PMID 41747039 · DOI · PMC7618833
Chronic intermittent hypoxia reshapes circadian metabolic architecture in a model of sleep apnea [3]
Abstract reading summary — source excerpts
- Opening excerpt
- Obstructive sleep apnea (OSA), characterized by chronic intermittent hypoxia (IH) during sleep, is increasingly recognized as a driver of metabolic dysfunction.
- Closing excerpt
- Our findings establish chronic IH as a potent metabolic zeitgeber that rewires hepatic transcriptional and metabolic programs, revealing a circadian dimension to the metabolic consequences of sleep-disordered breathing.
- Interpretation limits
- Verbatim excerpts from the authors’ abstract; the closing excerpt may describe a finding, limitation or recommendation. Design, population, effect size and uncertainty have not been independently extracted or appraised. Read the complete abstract and linked paper before interpreting the findings.
Read complete abstract (189 words)
Obstructive sleep apnea (OSA), characterized by chronic intermittent hypoxia (IH) during sleep, is increasingly recognized as a driver of metabolic dysfunction. However, its impact on circadian metabolic regulation remains poorly understood. Here, we investigated how chronic IH reshapes 24-hour hepatic and systemic metabolic programs in a mouse model mimicking OSA-associated chronic hypoxia. Through integrated circadian transcriptomic, metabolomic, and physiological 24-hour profiling, we show that 4 weeks of rest phase-restricted IH reprograms hepatic and systemic metabolism in a time-specific manner. This reorganization involves the coordinated circadian regulation of glucose, lipid, and redox pathways, with a shift away from oxidative metabolism toward oxygen-sparing processes such as gluconeogenesis, glycogen turnover, and lipid mobilization. These changes align with the hypoxic phase exposure and coincide with reshaped hepatic metabolite oscillations and systemic autonomic rhythms, supporting a functional adaptation to intermittent oxygen availability. Mechanistically, we identify the cAMP-CREB1 pathway as a driver of circadian transcriptional remodeling in the liver and a central integrator of IH-dependent adrenergic stress. Our findings establish chronic IH as a potent metabolic zeitgeber that rewires hepatic transcriptional and metabolic programs, revealing a circadian dimension to the metabolic consequences of sleep-disordered breathing.
Abstract reproduced from the open-access article under the recorded cc by licence; © the authors. PMID 41739942 · DOI · PMC12935056
Key reviews and meta-analyses
The most cited review-type papers among PubMed’s best-match results for this heading, listed with bibliographic details only. Follow the links for the full text and access terms.
- Research article Diurnal cortisol slopes and mental and physical health outcomes: A systematic review and meta-analysis [4]
- Research article Circadian variation in the timing of stroke onset: a meta-analysis [5]
- Research article Meta-analysis of the morning excess of acute myocardial infarction and sudden cardiac death [6]
- Research article The relationship between chronotype and depressive symptoms: A meta-analysis [7]
- Research article Beneficial Effects of Time-Restricted Eating on Metabolic Diseases: A Systemic Review and Meta-Analysis [8]
- Research article 24-Hour Blood Pressure Variability Assessed by Average Real Variability: A Systematic Review and Meta-Analysis [9]
References
- Weng Y, Xiong J, Zhao Q, Tan Z. HIF-1α and BMAL1 in bone regeneration: crosstalk between hypoxia response and circadian rhythm. . 2026. PMID 41690951 · DOI · PMC12906504
- Beale AD, Christmas MJ, Rzechorzek NM, Mihut A, Zeng A, Ellis C, et al.. A cellular basis for the mammalian nocturnal-diurnal switch. . 2026. PMID 41747039 · DOI · PMC7618833
- Montellier E, Vial G, Bouyon S, Ashtiani KC, Abdelkarim S, Lemarie E, et al.. Chronic intermittent hypoxia reshapes circadian metabolic architecture in a model of sleep apnea. . 2026. PMID 41739942 · DOI · PMC12935056
- Adam EK, Quinn ME, Tavernier R, McQuillan MT, Dahlke KA, Gilbert KE. Diurnal cortisol slopes and mental and physical health outcomes: A systematic review and meta-analysis. Psychoneuroendocrinology. 2017. PMID 28578301 · DOI · PMC5568897
- Elliott WJ. Circadian variation in the timing of stroke onset: a meta-analysis. Stroke. 1998. PMID 9596248 · DOI
- Cohen MC, Rohtla KM, Lavery CE, Muller JE, Mittleman MA. Meta-analysis of the morning excess of acute myocardial infarction and sudden cardiac death. Am J Cardiol. 1997. PMID 9185643 · DOI
- Au J, Reece J. The relationship between chronotype and depressive symptoms: A meta-analysis. J Affect Disord. 2017. PMID 28463712 · DOI
- Moon S, Kang J, Kim SH, Chung HS, Kim YJ, Yu JM, et al.. Beneficial Effects of Time-Restricted Eating on Metabolic Diseases: A Systemic Review and Meta-Analysis. Nutrients. 2020. PMID 32365676 · DOI · PMC7284632
- Mena LJ, Felix VG, Melgarejo JD, Maestre GE. 24-Hour Blood Pressure Variability Assessed by Average Real Variability: A Systematic Review and Meta-Analysis. J Am Heart Assoc. 2017. PMID 29051214 · DOI · PMC5721878
About this article
- Sources
- NLM Medical Subject Headings (definition, synonyms); NCBI PubMed (counts, identifiers); Europe PMC (open-access abstracts and citation counts).
- Verification
- Every reference was resolved against PubMed on 2026-10-31; title, year and DOI matched.
- Review state
- automated source verified; no human expert review has been recorded yet.
- Version
- 1, last updated 2026-10-31.
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Evidence strength
EvidenceHelix's computed view of the study designs behind this topic: how common meta-analyses, randomized trials and reviews are, how recent the literature is, and a plain-language reading. It describes the design mix of the literature. It is not a quality appraisal of any study, and it is not medical advice.