The glucocorticoid cortisol responds to stress and inflammation, and it is secreted by the hypothalamic-pituitary-adrenal axis [Brown 2020]. In this work, we showed that a rotatory schedule led to delayed circadian phase in serum cortisol during experiment and increased circadian amplitude in serum cortisol during the early stage but not the late stage (Fig. 1B-D), suggesting that circadian rhythm is subject to change and stress occurs owing to the rotatory schedule.
The albumin level and total bilirubin level decreased in experiment #1 but increased in experiment #2, while the triglyceride level increased in experiment #1 but decreased in experiment #2 (Fig. 2,3; supplemental Tables 1,2), which may be owing to individual difference or reflect the different effects of between non-24 schedule and rotatory schedule. Changes in metal elements including sodium, iron were observed in experiment #1. The levels of glucose, cholesterol, urea and creatinine showed changes in experiment #2 (Fig. 3; supplemental Tables 2). These results suggest that these two schedules resulted in the extensive alterations in physiology and metabolism, although most of the changes are still within the normal ranges. However, in experiment #1, the levels of bile acids of several subjects are higher than the normal range (0.1ཞ10.0µmol/L) after the experiment (Fig. 2f; supplemental Table 1), suggesting that risks of dysfunctions in liver or hepatic duct should be taken into consideration [31].
Inorganic phosphate is a vital component of bone mineralization, phospholipids in membranes, nucleotides that provide energy and serve as components of DNA and RNA, and phosphorylated intermediates in cellular signaling [32]. It is also required for skeletal development, mineral metabolism, and diverse cellular. The level of Serum Pi is maintained within a narrow range - the normal plasma phosphate concentration is 0.81–1.45 mmol/L, which is regulated by multiple factors, e.g., intestinal absorption, exchange with intracellular and bone storage pools, renal tubular reabsorption, and their interactions [33, 34].
Plasma phosphate imbalance, including hypophosphatemia and hyperphosphatemia, are associated with effects on any organs. Occurrence of hyperphosphatemia is reported in renal failure, hemolysis, tumor lysis syndrome, and rhabdomyolysis [34]. Both human and animal studies reveal that calcium-phosphate homeostatic abnormalities may be associated with cardiovascular disease, and hyperphosphatemia and hypercalcemia to be cardiovascular events [35]. However, we did not find miRNAs targeting genes regulating phosphate homeostasis, suggesting that the changes in serum phosphate level may be independent of serum miRNAs.
Kidney regulates the maintenance of fluid and ion homeostasis through filtering the blood and blood pressure as consequence [36, 37]. Circadian clocks genes have been demonstrated to govern kidney physiology and function at the molecular level [37]. Therefore, circadian misalignment might affect the kidney function which accounts for the imbalance in serum phosphate.
miRNAs constitute a post-transcriptional layer in the regulation of circadian clock [38, 39]. Insufficient sleep causes changes in the expression amplitude of human blood transcritome [17], suggesting that impairment in circadian rhythms and sleep may cause transcriptomic alterations. In addition to phosphate metabolism, in this work we identified a substantial set of functional miRNAs which are implicated in multiple pathways or physiological processes, including those targeting genes regulating circadian clock and sleep homeostasis, suggesting that these miRNAs may contribute to the misalignment in circadian rhythms and sleep homeostasis caused by abnormal work schedules. The changed miRNAs may be causative or resultant of the desynchronized circadian rhythms. More possibly, these two factors may be tightly associated and mutually influence each other. In addition, miRNAs associated with cancer, neural function and several metabolic pathways were also identified, which suggests that desynchronized circadian rhythms may increase the risks of many disorders through modulating the expression of corresponding miRNAs.
In both experiments, the participants lived in isolation condition, and most of the environmental cues including lighting, noise, ambient temperature and workload were controllable so that we could focus on the effects of circadian misalignment. Instead, in real condition like submarine, as the microenvironment differs between different locations or posts on board, the effects of circadian misalignment might be hard to be addressed due to inconsistency between different micro environmental domains [12].