Biological rhythms are repetitive natural phenomena depending on the time of day or year. Most human life processes have a circadian rhythm lasting about 24 hours, such as cortisol secretion. Shorter rhythms are called ultradian rhythms, while infradian rhythms are endogenous rhythms with longer than 28-hour periods; the seasonal cycle is the best-known example [1]. The light/dark rotation is the strongest zeitgeber responsible for the synchronisation of rhythms [2]. The light impulses from the retina reach the suprachiasmatic nucleus (SCN) in the hypothalamus region, which is found to be the primary regulator of endogenous rhythms (“body clock”) [3].

Therefore, the human body clock is susceptible to seasonal variations in the natural light-dark cycle, leading to the expanded biological night in winter compared with summer [4]. Seasonal rhythms influenced by environmental changes are commonly described in mammals [5]. In humans, seasonal variations in diurnal rhythms may be affected by various exogenous factors, such as dietary patterns, physical activity, ambient temperature, and natural light exposure [6]. Honma et al. [7] indicated that the circadian rhythms of the individual biological parameters (such as the timing of sleep or the rectal temperature) might also depend on seasons.

In addition, seasonal variations in mood and behaviour refer to depressive symptoms caused by seasonal changes [8]. Seasonal affective disorder (SAD) is a recurrent seasonal condition with depressive episodes commonly in autumn or winter and remissions in spring or summer. The prevalence is estimated to be from 1.5% to 9% worldwide [9]. The potential risk factors include family history, female sex, living in more northern latitudes, and young adult age (30 years) [10]. In addition to depressed mood, prolonged sleep duration, increased appetite, and weight gain can be experienced [11]. These symptoms directly correlate with overproduction of melatonin [12]. Moreover, SAD patients demonstrate a circadian rhythm delay relative to the sleep/wake cycle [13]. Earlier studies also found associations between seasonality and metabolic syndrome, diabetes, obesity, and cardiovascular diseases (CVDs) [14, 15, 16, 17]. Furthermore, some reports confirmed higher morbidity and mortality of CVDs during winter than in summer [18, 19].

Myeloperoxidase (MPO) is a heme peroxidase expressed primarily in neutrophils and monocytes. It is one of the most important enzymes responsible for local immunity and host defence [20, 21]. MPO catalyses the formation of reactive species (including hypochlorous acid) involved in microbial killing [22, 23]. Due to the potentially defensive role, MPO is increased in conditions associated with oxidative stress and inflammation [24, 25]. For instance, we found that salivary MPO could predict differential diagnosis and treatment effectiveness in inflammatory bowel disease patients [21, 26], but it may be affected by daily secretion variability. In addition, circadian variations in MPO concentrations might accompany some other time-dependent events, such as atherosclerotic plaque destabilisation, which plays a role in the occurrence of CVDs [27, 28]. MPO is elevated in patients with acute coronary syndrome [29], which is found to be associated with seasonality [30].

However, no study has assessed the direct association of MPO with SAD. Galijasevic et al. [31] presented that MPO could be inhibited by melatonin under physiological-like conditions as well as during inflammation. Interestingly, the melatonin-MPO interactions may be bidirectional, and MPO may act as a chelator for melatonin, limiting its bioavailability. Therefore, these interactions could have biological implications for wake-sleep rhythmicity regulation. The authors speculated that increased inflammation and decreased immunity caused by chronic sleep loss could result in the insufficient inhibitory protective effects of melatonin on MPO during prolonged waking hours.

Moreover, many antioxidative enzymes manifested diurnal rhythmicity in several organisms and tissues, but the detailed rhythm parameters, such as amplitudes or acrophases, were significantly divergent [32]. Understanding the interaction between oxidative status and diurnal, seasonal rhythms might be essential to understand better the pathogenesis of some inflammatory diseases (including coronary artery diseases) and undertake a more effective therapy considering both components [33].

To our knowledge, the daily and seasonal variations of MPO secretion have not been evaluated. In Poland, the seasonal daylight fluctuations are intense, and thus, the Polish population is a good subject for studying seasonality. The salivary cortisol secretion was assessed as a marker of diurnal and seasonal rhythm. Cortisol secretion in the saliva is characterised by a circadian rhythm with higher values in the morning than in the evening [34] and with seasonal changes [35, 36].

Our study aimed to determine if the salivary secretion of MPO exhibits diurnal variations in healthy men, if the daily variability differs seasonally between summer and winter, and whether SAD could affect MPO rhythms.

The study protocol followed the guidelines for biological rhythm research on human beings by Portaluppi et al. [37], was approved by the Bioethics Committee of the Poznan University of Medical Sciences (223/19), and followed the principles of the Declaration of Helsinki [38]. All participants gave written informed consent.

In the present study, we wished to determine whether the secretion of MPO in saliva demonstrated diurnal and seasonal variations using cosinor analysis. The findings suggest that salivary MPO may have shorter 12-hour ultradian rhythms, although the power of these models was relatively moderate. The MPO acrophases were observed in the early morning and the early evening, with no noticeable variability depending on the seasons. Also, the presence of self-assessed SAD showed no significant variances. MPO daily variations do not perfectly follow a sinusoidal waveform, so we consider alternatives to the cosinor method, such as non-parametric approaches to investigate MPO patterns. We found that increased self-assessed SAD (higher GSS values) was associated with stronger relative amplitude in winter season only.

Moreover, the presence of the circadian rhythm of cortisol secretion in saliva confirmed the rhythmicity of the volunteers. They demonstrated the cortisol peaks in the morning hours, with the observed delay in acrophase in summer compared to winter. The participants with self-assessed SAD demonstrated a stronger shift of peak to earlier morning hours, especially in winter.

To our best knowledge, this is the first study to address the potential rhythmicity of MPO secretion in healthy volunteers, using saliva as a diagnostic medium helpful in chronobiology. Saliva samples are relatively easy to collect in a non-invasive way, unlike blood [46]. Due to its dual nature, MPO exhibits prooxidative and antioxidant properties, especially in saliva [47, 48]. Thus, salivary MPO levels reflect the oxidative status of the human organism - both systemic elements associated, for example, with increased metabolic states, as well as local ones caused even by microinflammation in the oral cavity [49]. In the literature, there are previous reports about daily changes in antioxidant enzymes, but mainly in the serum of patients with systemic diseases or animals [33, 50]. Apart from the frequency of measurements, the undoubted advantage of our study is the homogeneity of the study group consisting of 10 healthy men, mainly in the third decade of life. Moreover, their sleep parameters, activity and eating habits did not differ significantly between summer and winter.

Only a few studies have discussed the daily MPO variations, not the rhythms. Dominguez-Rodriguez et al. [51] observed significant differences in MPO levels comparing the dark and light periods (2 a.m. vs 9 a.m.) in STEMI patients with higher levels in the dark phase, suggesting the existence of diurnal differences in these patients. In the healthy subjects, MPO concentrations were lower in both dark and light periods, without day/night variations. Also, in the study by Uluoglu et al. [52], the activity of MPO in the rat liver was significantly higher in the diabetics than in the controls, however, without a circadian variation.

Surprisingly, our analysis suggests the 12-hour ultradian rhythm for MPO; however, it is known that circadian rhythms are fundamental regulators of neutrophil receptor expression, counts, and tissue homing. Although inflammatory processes can also reprogram rhythms, it seems that rhythmicity-related treatment should be more effective in therapeutic response [53].

Light is an essential factor influencing circadian and seasonal rhythms [54]. A physiological link between the light/dark cycle and MPO variability may be related to diurnal rhythmicity in neutrophils and endothelial activity, which is known to occur in the morning hours [55]. The involvement of these parameters and MPO in atheromatous lesions, resulting in CVD events, is well established [56]. In the large population study, the highest incidence of acute myocardial infarction occurred at 9 a.m., with a second lower incidence peak at 8 p.m., and the lowest incidence during the night [57]. Thus, diurnal rhythm in MPO can modulate susceptibility to CVD events.

In turn, Tenovuo and Valtakoski [58] suggested that the circadian variation in peroxidase activity and redox potential positively correlated with the variations in the salivary flow rate during the day. Decreased salivation around noon and late evening was associated with decreased peroxidase activity. This tendency is similar to our ultradian rhythmicity. The higher peroxidase activity should result in the lowered redox potentials of the oral environment by decomposing hydrogen peroxide. Also, the differences found in peroxidase activity and salivary flow rate did not correlate with the clinical conditions of the oral cavity.

In the literature, previous studies have been concerned on diurnal variations of antioxidants, mainly in plasma. Budkowska et al. [59] suggest that increased levels of antioxidants at night may be associated with the intensification of processes responsible for removing oxygen free radicals.

In particular, more studies have focused on the daily rhythms of plasma antioxidants in selected diseases. Kushwaha et al. [60] determined that the renal stone formers demonstrated significantly lower MESOR of SOD, GPx, GR and CAT. Also, the patients differed from the healthy subjects in terms of the circadian amplitudes and acrophases for almost all determined enzymes (except amplitude for GPx and acrophase for SOD). In cirrhotic patients, circadian amplitudes of plasma CAT, GPx and GR, as well as MESOR for plasma SOD were lowered, indicating a decrease in antioxidant defensive mechanisms [61]. Similar findings were observed for patients with gynaecological malignancies and pulmonary tuberculosis [62, 63]. In the recent study, Singh et al. [64] found lower circadian amplitudes and MESOR for plasma SOD, GPx and GR in patients suffering from peptic ulcer disease. The reduced activity of these primary defence enzymes could be caused by the greater involvement of reactive oxygen species in the pathogenesis of gastric ulcers, interfering with the pro- vs antioxidant ratio.

Interestingly, Goncharova et al. [65] observed a tight correlation between the diurnal changes in the SOD activity and the cortisol levels in the monkeys’ blood plasma. Despite the flattening of circadian rhythms with ageing, the correlation between these parameters remained. It suggests that corticosteroids may play a role in regulating SOD activity in non-human primates, which decreases with ageing.

In contrast to the rhythms of antioxidants, the daily cortisol rhythm is much better known, but there is no agreement in the literature as to seasonal rhythmicity. Based on the CIRCORT dataset, Miller et al. [35] observed elevated levels of salivary cortisol in winter and spring, with accompanying delayed acrophases. In the same study, the authors found that men between 20 and 30 years of age had significantly earlier demonstrated a daily increase in cortisol levels in saliva, compared to the age-matched women. Also, these analyses described the expected circadian variations of salivary cortisol levels with an immediate increase about an hour after awakening and a gradual decrease during the day, leading to a nadir around bedtime.

Previously, Hadlow et al. [66] stated that the sunrise time might be the most crucial zeitgeber for the seasonal variations in the cortisol acrophase. Similarly, Vondrasová et al. [67] investigated that exposure to a natural bright light photoperiod in summer advanced the morning salivary cortisol rise compared with exposure to a combined artificial and natural light photoperiod in winter. However, in the study by Persson et al. [68], the diurnal concentration pattern for salivary cortisol was similar across months, with higher levels in February, March and April, and lower ones in July and August.

In contrast, Kanikowska et al. [36] presented a significantly higher MESOR of salivary cortisol without the differed amplitudes for the volunteer women in summer compared with winter. The acrophase occurred about 4 hours delayed in summer than in winter. The participants with higher scores on the seasonal affective disorder scale demonstrated a tendency toward lower relative cortisol amplitude in summer. Moreover, Thorn et al. [69] concluded that only the salivary cortisol response to awakening was attenuated in participants with self-assessed SAD during the shortened photoperiod in winter. In summer, no differences in psychological and cortisol profiles were found.

Our observations suggest the possible occurrence of the ultradian rhythm for MPO in saliva, with two peaks during the day, regardless of the season. On the other hand, our male volunteers presented the salivary cortisol diurnal rhythmicity and the potential variability of this rhythm depending on the season. Considering individual factors, such as seasonal affective disorder, is also important. As mentioned, the potential daily variations of the oxidative status markers seem to be promising factors in planning more effective treatment models for systemic diseases. However, further research is needed to verify these findings.

Data are available on request from the corresponding author.

KN is responsible for conceptualisation, methodology, formal analysis, investigation and resources, writing the original draft, review and editing, visualisation. RR is responsible for investigation and resources. DG is responsible for investigation and resources. JW is responsible for conceptualisation and methodology. AS is responsible for conceptualisation and supervision. DK is responsible for conceptualisation, methodology, writing review and editing, and supervision. All authors contributed to editorial changes in the manuscript. All authors read and approved the final manuscript. All authors have participated sufficiently in the work and agreed to be accountable for all aspects of the work.

The study was conducted according to the guidelines of the Declaration of Helsinki, and approved by the Bioethics Committee of Poznan University of Medical Sciences (223/19). Informed consent was obtained from all subjects involved in the study.

Not applicable.

This research received no external funding.

The authors declare no conflict of interest. Given his role as Guest Editor, Kacper Nijakowski had no involvement in the peer-review of this article and has no access to information regarding its peer-review. Full responsibility for the editorial process for this article was delegated to Graham Pawelec.