Microbial Biomes, Biological Rhythms and Host Physiology
Findings on Microbial Biomes, Biological Rhythms and Host Physiology and how they affect each other nonlinear.
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1. The biological clock is more than a sleep timer
1. Short sleep can erase the daily rhythm of microbial metabolites in human blood
A controlled crossover study published in the Journal of Clinical Investigation in 2026 involved nine healthy adults. Participants completed three nights with 8.5 hours of sleep opportunity and three nights with 4.5 hours, with meals and calories controlled.
Researchers measured circulating metabolites every two hours over 24 hours. Short sleep altered the daily profiles of several metabolites, including butyrate and indole-3-propionic acid, which lost their usual rhythmicity.
The remarkable detail: a metabolite can remain measurable while its normal daily timing disappears. The study did not establish that the abundance of the bacteria producing these compounds had changed.
2. Bacterial fermentation products can shift the clock of intestinal cells
A 2022 Gastroenterology study investigated how short-chain fatty acids (SCFAs), produced during microbial fermentation, affect circadian rhythms in intestinal epithelial cells.
The researchers found that SCFAs could shift epithelial clock rhythms through mechanisms involving histone deacetylases. The work included experimental intestinal systems and organoids.
The remarkable detail: familiar fermentation products are not merely digestive end products; they can influence the timing of gene-expression programmes in host tissue.
Source: Gastroenterology, 2022 — SCFAs and intestinal epithelial circadian rhythms
3. Disrupting the intestinal clock changes microbial rhythms
A 2022 Nature Communications mouse study disrupted the circadian clock in intestinal epithelial cells. Researchers observed changes in microbial rhythmicity and the daily patterns of microbial products, including SCFAs and bile acids.
The remarkable detail: the timing system of the host tissue can influence the daily behaviour of the microbial community.
Source: Nature Communications, 2022 — Intestinal epithelial clock and microbial rhythmicity
4. A microbiota-associated bile acid can influence the clock of human colonic cells
A 2026 PNAS study investigated lithocholic acid, a bile acid associated with microbial transformation, and its effects on clock-gene timing in human colonic cells, alongside experiments in mice.
The study reported that lithocholic acid lengthened the circadian period of core clock genes in human colonic cells and altered transcription in mouse intestine.
The remarkable detail: a molecule associated with microbial bile-acid transformation is being investigated as a signal capable of changing the timing of host-cell activity.
Source: PNAS, 2026 — Lithocholic acid and colonic clock timing
5. The liver has a large daily programme of gene activity
Circadian-liver research has reported rhythmic oscillation in a substantial fraction of the liver transcriptome, with an often-cited estimate of roughly 10%.
Different metabolic pathways peak at different times. Liver function is therefore not simply switched on at night and off during the day.
Research note: the approximate 10% figure appeared in our earlier review material, but its precise original citation still needs to be recovered before publication.
2. Night-shift metabolism: different processes keep different schedules
6. Human bile-acid synthesis peaks twice a day, while cholesterol synthesis peaks overnight
A human study measured serum markers every 90 minutes in eight volunteers under standardised dietary conditions.
The bile-acid synthesis marker C4 showed two peaks, at approximately 1 pm and 9 pm. By contrast, the cholesterol synthesis marker lathosterol peaked between midnight and 4 am.
The researchers also noted that the human pattern differed from that observed in rodents.
The remarkable detail: two closely related aspects of lipid metabolism can follow distinctly different daily schedules, and those schedules can differ between species.
7. Sleep deprivation can disturb the timing relationship between bile acids and circulating lipids
A 2024 human study reported daily rhythms in circulating bile acids and found that sleep deprivation disturbed temporal relationships between bile acids and plasma lipids.
This is a different finding from simply measuring whether a substance is high or low. The relationship between two circulating substances can change across the day.
Research note: the exact article record for this finding needs to be recovered before the result is used in a published version.
8. Melatonin and cortisol follow different daily trajectories
Melatonin typically rises during the biological evening, while cortisol usually increases towards the latter part of the biological night and around waking.
Their timing varies with circadian phase, light exposure and sleep schedule.
The remarkable detail: clock time and internal biological time are not interchangeable. Two people going to bed at the same hour may be at different circadian phases.
9. Traditional Chinese medicine assigns organs two-hour periods
The traditional Chinese medicine organ clock assigns the liver to 1–3 am, the lungs to 3–5 am and the large intestine to 5–7 am, among other periods.
Modern chronobiology establishes daily rhythms in many organ functions, but has not validated those exact traditional windows as universal peak-function periods.
The research question: where do traditional timing observations overlap with measured physiological rhythms, and where do they diverge?
This is a comparison to investigate, not a claim that the traditional clock and modern chronobiology are equivalent.
3. The nose and airways keep their own time
10. The two nostrils naturally alternate dominance
The nasal cycle is a normal physiological phenomenon in which congestion and airflow shift between the nostrils.
A study of 33 healthy, right-handed adults used 24-hour recordings to characterise this cycle. The average cycle was shorter during wakefulness, at approximately 2 hours, and longer during sleep, at approximately 4.5 hours. Body posture also influenced which nostril had greater airflow.
The remarkable detail: ordinary nasal airflow follows a changing autonomic pattern over the day and night.
Source: Measuring and Characterizing the Human Nasal Cycle
11. Nasal immune defence and mucus transport show opposing daily patterns
A 1990 human study examined secretory immunoglobulin A (IgA) in nasal secretions and mucociliary transport.
Secretory IgA was highest around 4 am and lowest around 10 am. Mucociliary transport also varied during the day, with an opposing pattern.
The authors proposed that the two functions could partly compensate for one another: when one defence mechanism was less active, the other was relatively more active.
The remarkable detail: two protective functions in the same tissue did not peak together.
Source: Passali, Bellussi and Lauriello, 1990 — Diurnal activity of the nasal mucosa
12. The upper-respiratory microbiome has measurable 24-hour oscillations
A 2026 Cell Reports study sampled 22 healthy adults every four hours over 48 hours, using oropharyngeal swabs.
Researchers found significant 24-hour cycles in microbial composition and biomass, with organisms following different patterns, including morning peaks and evening peaks. The oral cavity and oropharynx also showed different rhythmic patterns.
The researchers warned that temporal variation can produce false positives in microbiome comparisons: samples collected at different times may appear different partly because of the time of collection.
The remarkable detail: sampling time can be a major experimental variable in microbiome research.
13. Experimental inflammation disturbed airway microbial rhythms in mice
Preliminary research presented at the 2024 European Respiratory Society meeting reported daily oscillations in airway microbial diversity and lung clock-gene expression in mice. These rhythms were disturbed in animals with experimental inflammation.
This is preliminary animal evidence, not proof of the same mechanism in human lungs.
The remarkable detail: inflammation may be associated with changes not only in microbial composition, but also in the timing of microbial and host activity.
Research lead: European Respiratory Society
14. Nasal microbial communities can change during a single workday
A small human study of nine pig-truck drivers found changes in nasal bacterial diversity and microbial load during the workday. Exposure to animals and the working environment were potential confounders.
The remarkable detail: a respiratory microbial environment can change over hours, not just over months or years.
Research note: the exact study citation needs to be recovered before publication.
4. Skin is an active, rhythmic organ
15. Skin microbes participate in barrier maintenance and immune activity
A 2025 Journal of Clinical Investigation review examined evidence that skin microbes can influence immune function, microbial competition, colonisation resistance and epidermal barrier integrity.
Skin commensals interact with host cells, including keratinocytes and sebocytes. Local lipids, pH, desquamation and antimicrobial activity also shape the community.
The remarkable detail: skin microbes are not simply passengers living on a passive surface; particular microbial strains can affect the local tissue environment.
Source: Scharschmidt and Segre, JCI, 2025 — Skin microbiome and dermatologic disorders
16. Skin barrier function and repair-related processes vary across the day
Research on skin circadian physiology has described daily rhythms in properties such as blood flow, permeability, barrier recovery and repair-related gene expression.
The pattern depends on the particular function being measured and the experimental conditions.
The remarkable detail: the biological effect of a skin exposure may depend partly on when the exposure occurs, not only on its chemical composition.
Research lead: Skin microbiome and dermatologic disorders — JCI review
17. Skin immune-cell migration into lymphatics can follow a circadian pattern
A 2021 Nature Immunology study was discussed in our earlier research as evidence that dendritic-cell migration from skin into lymphatic vessels follows a daily rhythm in mice, peaking during the animals’ rest phase. Human skin showed corresponding rhythmic expression of signalling molecules, including CCL21.
The animal migration finding and the human molecular-expression finding are different kinds of evidence.
The remarkable detail: the movement of immune cells from a peripheral tissue into lymphatic pathways may itself be temporally regulated.
Research note: the exact 2021 paper citation needs to be recovered before publication.
18. Lymphoedema is associated with changes in the skin microbiome
A 2023 PLOS ONE study examined skin swabs from 30 patients with unilateral upper-extremity secondary lymphoedema, comparing affected and unaffected limbs.
The researchers identified 872 bacterial taxa. They did not find a significant difference in overall alpha diversity between affected and unaffected skin, but found substantial compositional heterogeneity. In patients without a history of infection, relative limb volume was associated with differences in microbial composition between paired limbs.
The remarkable detail: a lymphatic disorder can be associated with changes in the composition of the local skin microbial community without necessarily producing a measurable reduction in overall diversity.
Source: Campbell et al., 2023 — Skin microbiome alterations in upper extremity secondary lymphedema
5. Antibacterial soap: composition, diversity and microbial relationships
19. Antibacterial soap changed skin community composition for at least two weeks in one study
A 2018 study in rural Madagascar investigated antibacterial soap use among 20 adult men. Ten participants used the supplied antibacterial soap for one week, while ten served as controls.
The researchers found that the amount of soap used predicted changes in skin microbial community composition. Those changes persisted for at least two weeks after soap use stopped.
Overall microbial richness and alpha diversity did not change significantly.
The remarkable detail: community membership can shift even when the total number of detected types remains broadly stable.
Source: Antibacterial soap use impacts skin microbial communities in rural Madagascar
20. Short-term cleanser studies do not all show major microbiome disruption
A 2016 study found that common cleansers did not significantly alter the bacterial community on forearm skin over the short term. However, antimicrobial compounds including benzalkonium chloride and triclocarban affected the survival of an applied test bacterium.
The effects depend on the product, compound, exposure and outcome being measured.
The remarkable detail: the presence of an antimicrobial ingredient does not tell us, by itself, how an entire microbial community will respond.
Source: Acute exposure to common cleansers and skin bacteria
21. Mild cleansers can change microbial network measurements without reducing diversity
A 2025 British Journal of Dermatology study reported stable skin microbiome diversity after use of mild soap and bodywash. Some products increased microbial co-occurrence network properties.
The authors interpreted the changes as potentially consistent with improved microbial network resilience.
The remarkable detail: microbial diversity and the relationships between community members are separate measurements. A community can retain similar diversity while its network of co-occurrence changes.
6. Candida: a normal resident with a flexible role
22. Candida is a fungus, not a bacterium, and can be a normal commensal
Candida species can inhabit the mouth, gut and other body sites without causing disease. Reviews commonly report Candida colonisation in roughly half of healthy adults, depending on the body site and population.
Colonisation is not the same as infection. The outcome depends on the fungal species, its location, the surrounding microbial community, host immunity and barrier condition.
The remarkable detail: an organism capable of causing disease can also exist as a normal member of the human microbial ecosystem.
Source: Ost and Round, 2023 — Commensal fungi in intestinal health and disease
23. Candida is a prominent member of the oral fungal community
A review of the oral mycobiome noted that Candida is the fungal genus most clearly demonstrated to attain substantial biomass and a distinct ecological presence in the mouth.
Candida can interact with plaque bacteria. In some conditions, these interactions contribute to acid production and dental caries.
The remarkable detail: the effect of an oral fungus depends partly on which other organisms are present and the environment in which they interact.
Source: Oral mycobiome review
24. Commensal fungi can stimulate immune responses
Research reviewed in Nature Reviews Gastroenterology & Hepatology describes interactions between commensal fungi and immune pathways, including Th17 responses. These responses can contribute to immune protection against fungal infection.
The same review describes how bacteria can constrain fungal growth through competition, metabolites and immune mechanisms. Antibiotics can disrupt these relationships and permit fungal expansion in some settings.
The remarkable detail: a commensal fungus can contribute to immune education, while the surrounding bacterial community helps determine whether that fungus remains controlled.
Source: Ost and Round, 2023 — Commensal fungi in intestinal health and disease
25. Bacterial metabolites can influence Candida physiology
A 2024 review examined how bacteria-derived short-chain fatty acids can influence Candida gene expression, metabolism, signalling and post-translational protein modifications through acylation.
These mechanisms provide a molecular basis for interactions between bacteria and fungi within the same ecosystem.
The remarkable detail: microbial communication is not restricted to microbes of the same kingdom. Bacterial metabolites can alter fungal physiology.
26. Researchers were measuring daily variation in salivary Candida counts in 1972
J. J. Williamson published Diurnal variation of Candida albicans counts in saliva in the Australian Dental Journal in February 1972.
A second paper, published in April 1972, examined the extent of daily variation in salivary Candida counts.
The numerical details of the Candida peak times still need to be extracted from the original papers.
Sources:
- Williamson, February 1972 — Diurnal variation of Candida albicans counts in saliva
- Williamson, April 1972 — A study of extent of variation in daily counts of Candida albicans in saliva
27. Saliva itself has measurable circadian rhythms
C. Dawes’s 1972 Journal of Physiology study collected saliva from eight subjects over periods ranging from four to 26 days.
Unstimulated whole saliva showed significant circadian rhythms in flow rate and in sodium and chloride concentrations. Stimulated parotid saliva showed rhythms in protein and several electrolytes, including sodium, potassium and chloride.
The remarkable detail: different components of the same secretion can have different rhythmic patterns. Saliva is not a chemically static fluid.
Source: Dawes, 1972 — Circadian rhythms in human salivary flow rate and composition
7. Light, sleep and brain-fluid clearance
28. Ordinary room light before bedtime can shorten the melatonin signal by about 90 minutes
A study of 116 healthy young adults compared melatonin profiles under room-light conditions of less than 200 lux with profiles under dim-light conditions of less than 3 lux.
Compared with dim light, room light before bedtime produced a later melatonin onset in 99% of participants and shortened melatonin duration by approximately 90 minutes.
The remarkable detail: light exposure itself can change the timing and duration of the biological night, not merely keep someone awake.
29. Evening blue-enriched light can reduce early-night slow-wave activity
A study of 30 healthy young adults compared two hours of evening exposure to polychromatic light at different colour temperatures.
Sleep architecture across the first three sleep cycles did not differ significantly between conditions. However, slow-wave activity during the first NREM sleep cycle was significantly lower following exposure to blue-enriched 6500 K light than after exposure to warmer 2500 K and 3000 K light.
The remarkable detail: the reported effect concerned slow-wave activity during a particular part of the night, rather than a general disappearance of deep sleep.
Source: Acute exposure to evening blue-enriched light impacts on human sleep
30. Norepinephrine oscillations are closely linked to brain-fluid clearance during sleep
A 2025 Cell study investigated what drives glymphatic clearance during NREM sleep.
Researchers identified tightly synchronised oscillations in norepinephrine, cerebral blood volume and cerebrospinal fluid as strong predictors of glymphatic clearance. Experimental stimulation of arterial oscillations enhanced cerebrospinal-fluid inflow.
The study also reported that zolpidem suppressed norepinephrine oscillations and glymphatic flow in the experimental setting.
The remarkable detail: brain-fluid movement was linked to coordinated neural and vascular dynamics, not merely to the fact that the subject was asleep.
31. Mouse glymphatic and lymphatic fluid flow follows daily rhythms
A 2020 Nature Communications study investigated circadian control of brain glymphatic and lymphatic fluid flow in mice.
The researchers found that glymphatic influx and clearance peaked during the animals’ mid-rest phase. Cerebrospinal-fluid drainage to lymph nodes also varied over the day, with a pattern opposite to glymphatic influx. The study reported a daily pattern in perivascular AQP4 polarisation.
A later mouse study, published in 2025, directly tested whether AQP4 expression and perivascular distribution differed between rest and active phases. It did not find significant day–night differences in the measured AQP4 parameters.
The remarkable detail: the rhythmicity of fluid movement is an active research question, and the specific mechanisms underlying it remain under investigation.
Sources:
- Hablitz et al., 2020 — Circadian control of brain glymphatic and lymphatic fluid flow
- 2025 study — Are diurnal variations in glymphatic clearance driven by circadian regulation of AQP4 expression?
32. A systematic review found links between sleep and the gut microbiota across 41 human studies
A 2026 systematic review examined human research on sleep, circadian health and gut microbiota.
The review found associations between sleep restriction or disturbance, circadian misalignment and differences in microbial composition or function. However, the studies were heterogeneous, and diversity results were not consistent across the literature.
The remarkable detail: the evidence does not point to one universal microbial signature of poor sleep. Different studies measure different organisms, metabolites, sleep conditions and outcomes.
Source: 2026 systematic review — Gut microbiota, sleep quality, and cognitive function in adults
8. What these individual findings establish
The observations above cover several distinct research areas:
- microbial metabolites that lose daily rhythmicity after sleep restriction;
- fermentation products that influence the timing of intestinal cells;
- host clocks that alter microbial rhythms;
- bile-acid metabolism and cholesterol synthesis following different schedules;
- nasal immune defence and mucus transport following opposing rhythms;
- daily oscillations in upper-respiratory microbial communities;
- skin barrier physiology and immune-cell movement changing over time;
- changes in skin microbial composition associated with lymphoedema;
- antibacterial products affecting community composition without necessarily reducing diversity;
- Candida interacting with bacteria, metabolites and immune pathways;
- light exposure altering melatonin timing;
- sleep-related neural and vascular oscillations accompanying brain-fluid clearance.
These are individual findings, produced by different research groups using different methods. They should be evaluated on their own experimental evidence before being combined into a broader explanation.
The purpose of this collection is to retain the detail: the organism, tissue, molecule, time of day, experimental conditions, result and original study. Any wider interpretation should come after this record is complete, not replace it.
