Thus, coaches and other practitioners looking to leverage the effects of ambulated body-temperature must schedule training sessions within peak performance hours, or schedule training with enough warm-up time to offset any body temperature-related detriments in physical performance. This highlights the potential for intentional environmental interventions or properly implemented warm-up protocols to modulate body temperature and, consequently, optimize exercise performance throughout the day. Elite athletes showcase elevated performance levels during the evening hours, highlighting the relevance of diurnal fluctuations in optimizing athletic performance outcomes . Prior research suggests that the influence of circadian regulation on neuromuscular function and muscle strength may be sex-specific, where recent studies indicate that males exhibit greater time-dependent force variability than females during isometric contractions . In mammals, the suprachiasmatic nucleus (SCN) of the hypothalamus originates our circadian rhythm, acting as a central pacemaker that entrains peripheral oscillators through neural, humoral, and hormonal signals . At the cellular level, circadian time systems rely on the set of interconnected transcription factors, named clock proteins, that form an oscillating transcription/translation feedback loop spanning a period of 24 h . Prolonged stress not only decreases testosterone but also disrupts circadian rhythms by impairing sleep and increasing nighttime alertness . However, whether and how circadian clock genes regulate male fertility in circannual rhythms requires further exploration. As the two most prominent biological rhythms, day/night alterations (circadian rhythm) and seasonal changes (circannual rhythm) play essential roles in numerous biological functions, including male fertility or sperm quality (24) (Figure 2). While TRT can be a valuable tool in optimizing testosterone levels and circadian rhythms, it’s essential to understand that it’s just one piece of the puzzle. When your circadian rhythms are in sync, you experience a healthy sleep-wake cycle, feel alert during the day, and restful at night. In adulthood, circadian rhythms tend to stabilize around a more traditional pattern of feeling sleepy at night and alert during the day. Sleep is most likely to be refreshing and restorative when circadian rhythms, the natural cycle of daylight and darkness, and sleep patterns align. Siervo et al. revealed that SR for 21 days sharply increased the tert-butyl hydroperoxide-initiated chemiluminescence (CL) curve of the epididymidis, which suggested an enhanced peroxidative attack by ROS (76, 82). Akindele et al. found that SR for 14 days significantly elevated testicular malondialdehyde and glutathione (GSH) in adult rats (82, 83). Some authors thus thought that the altered hormones might not be responsible for decreased sperm quality or infertility (72). Choi et al. reported that SD4 and SD7 significantly decreased testosterone release but increased corticosterone production (32). High DNA stainability (HDS) in the epididymis is an important index of incompletely differentiated sperms (74). The Pregnancy Online Study (PRESTO) with 1,176 couples revealed that the fecundability ratio (FRs) for 8 h/night was significantly higher than that for 73). Regular sleep-wake times, consistent light exposure, scheduled meals, and timed exercise can all help reinforce healthy rhythms. While circadian rhythms are deeply ingrained, they are not immovable. Metabolic syndrome, including obesity, insulin resistance, and type 2 diabetes, has strong associations with disrupted circadian rhythms. When the circadian rhythm is thrown off by travel across time zones (jet lag) or by shift work, sleep quality deteriorates, and health consequences accumulate. Sleep is perhaps the most obvious manifestation of circadian rhythms. Others feel like zombies until midday and come alive after sunset. This phenomenon, known as light pollution, affects urban dwellers most acutely, but its consequences ripple far beyond the city. Morning peaks in blood pressure and blood clotting factors also explain why heart attacks and strokes are more common in the early hours of the day. Cardiovascular risk increases with circadian disruption as well. In the Leydig cells of mice, the cis-regulatory elements controlled by circadian clock genes, including E-box, RORE, and D-box motifs, were discovered in the Star and Cyp11a1 promoters.30 Additionally, analysis of the Star and Hsd-17β promoters in the Leydig cells of rats and goats demonstrated that RORE motifs were also present upstream of their transcription start sites.8,79 BMAL1 and CLOCK directly regulate the expression of Star by binding to the E-box on the Star promoter.7 Changes in the expression levels of BMAL1 and CLOCK can directly affect the expression of genes related to testosterone synthesis. The secretion of testosterone manifests as different rhythms, including entire lifecycle, annual, and daily rhythms.46,47 As men age, their serum testosterone levels exhibit a gradually declining trend.48 The circadian rhythm of serum testosterone levels in healthy elderly men is weakened compared with that in healthy young men.49,50 It was reported that Leydig cells in aged rats produced less cyclic adenosine monophosphate (cAMP) and testosterone under the action of luteinizing hormone (LH) as compared with cells in younger rats.51 Consistent with animal studies, the level of testosterone production stimulated by the administration of human chorionic gonadotropin in elderly men was lower than that in young men,52 indicating a reduced responsiveness of Leydig cells to LH as men age. Although controversy remains over whether circadian clock genes are rhythmically expressed in testicular tissues, their expression in Leydig cells has been demonstrated to exhibit significant circadian rhythmicity. Circadian clock genes are expressed in the SCN of the central nervous system and in various peripheral organ tissues and cells, including the testis.11Per1 was the first biological clock gene discovered to be expressed in testicular tissue, followed by Per2, Per3, Cry1, Cry2, BMAL1, and CLOCK.20 However, there has been controversy over whether the expression of circadian clock genes in the testis manifests circadian rhythmicity. Then, the SCN transmits the phase information to other regions of the brain, as well as cells of peripheral tissues and organs, through a combination of neural, humoral, and systemic signals to drive the peripheral biorhythms.14 Earlier research has shown that circadian clock genes are expressed in the SCN of the hypothalamus, and also widely expressed in a range of peripheral tissues and organs, including the testis, and that the joint regulation of the central and peripheral circadian clocks constitutes a complete clock system.15 At the molecular level, circadian clock oscillations are formed by the interaction between positive and negative feedback transcription and translation loops composed of core genes and their encoded proteins. It is important to note that not all men with disrupted circadian rhythms will experience low testosterone levels or related symptoms. Research has shown that circadian rhythm disruptions, such as irregular sleep patterns, night shift work, and jet lag, can negatively impact testosterone levels in men.
