Mitochondria Need Their Sleep: Redox, Bioenergetics, and Temperature Regulation of Circadian Rhythms and the Role of Cysteine-Mediated Redox Signaling, Uncoupling Proteins, and Substrate Cycles

**Background:** Circadian biorhythms of mitochondria and cells are highly conserved and crucial for well-being, yet there is a paucity of studies on the reciprocal interactions between oxidative stress, redox modifications, metabolism, and thermoregulation in sleep-wake cycling. The authors note that over 90% of mammalian oxygen consumption is by mitochondria, which are also the major cellular source of reactive oxygen species (ROS). Enzymatic antioxidants (catalase, GPX, PRX, SOD) usually peak during the light phase in humans, while nonenzymatic antioxidants (melatonin, vitamins C and E, GSH) peak during the dark phase. Core body temperature in humans drops during sleep by about 1 °C, lowest 3 h before wakening, and rises during the day to a peak in the evening. The authors hypothesize that circadian/ultradian interaction of the redoxome, bioenergetics, and temperature signaling strongly determine the differential activities of sleep-wake cycling in mammals and birds.

**Methods:** This is a narrative review synthesizing published human and animal studies on circadian rhythms, mitochondrial biology, redox signaling, and thermoregulation. The authors integrate findings from proteomic studies (e.g., 82% of primate protein-coding genes exhibit 24 h rhythms in gene expression across 64 tissues; ~38% of the mitochondrial proteome oscillates in mouse liver), studies of posttranslational modifications (S-glutathionylation, S-nitrosylation), measurements of redox couples (GSH:GSSG, NADPH:NADP+), respiratory quotient (RQ) studies, and temperature regulation experiments. Key data sources include isolated mitochondria experiments, human plasma measurements, and animal models of sleep deprivation and hibernation.

**Key Results:** The authors report that protein S-glutathionylation and S-nitrosylation are overrepresented in mitochondrial proteins compared to the whole-cell proteome—almost 30% of all altered cysteine sites displaying S-glutathionylation modifications were located in or associated with skeletal muscle mitochondria. Non-ETC ROS sources (α-KGDH, PDH, BCKDH) can produce 2- to 8-fold more ROS than complex I. The GSH:GSSG ratio in human plasma is at its lowest (most oxidized) in the early afternoon (~13:30) and highest (most reduced) in early waking hours (~8:00) and evening (17:30–21:30). Lowering mitochondrial temperature by 2 °C (from 37 °C to 35 °C) increased O2•− and H2O2 production in respiratory states 4 and 3 by 62% and 98%, respectively, in isolated murine brain mitochondria. The human core temperature drops to 36.5 °C at 4:00 during sleep and rises to 37.4 °C at 20:00 when awake. The respiratory quotient (RQ) is ~25% greater in the biological morning than at its nadir in the late evening, indicating greater carbohydrate oxidation during wakefulness and greater fatty-acid oxidation during sleep. The GSH:GSSG ratio in human plasma is stable until about 45 years of age, then declines linearly. Healthy older adults have a core body temperature oscillation amplitude that is reduced by a third compared to younger adults. In hibernating Arctic ground squirrels, UCP1/3 are several-fold more active than during euthermia, and the GSH:GSSG redox state in ground squirrel intestines was fivefold lower (more oxidized) during hibernation. Cell-free mtDNA in astronauts was elevated ~2- to 355-fold post-spaceflight, and astronauts develop a persistently high body temperature of 1 °C above normal (37 °C) over 2.5 months.

**Clinical Implications:** The hypothesis provides a framework for understanding how disrupted circadian redox-bioenergetics-temperature cycling contributes to aging, metabolic syndrome, neurodegenerative diseases (Alzheimer's, Parkinson's, Huntington's), SIDS, and spaceflight health effects. The authors suggest that impaired circadian entrainment in newborns (first 3–6 months) may increase SIDS vulnerability, and recommend interventions like light therapy and thermoregulation to enhance circadian development. For aging populations, the reduced amplitude of core body temperature and instability of redox couples (1.8-fold greater diurnal variation of cysteine:cystine ratio in persons ≥60 years vs. ≤40 years) may contribute to disease susceptibility. The hypothesis also explains how sleep disruption in night-shift workers increases risk of metabolic syndrome and cancer, and how spaceflight hazards (radiation, microgravity) perturb all components of the tripartite-interactome signaling. The authors propose that maintaining robust circadian fluctuations of the redoxome, bioenergetics, and thermoregulation promotes 'nucleorestorative' wakefulness and 'mitorestorative' sleep, and that therapeutic strategies targeting these interactions could mitigate age-related and disease-related circadian dysfunction.