The human body possesses an internal timekeeping system known as the biological clock, which regulates changes that repeat approximately every 24 hours. These changes include sleep and wakefulness, body temperature, hormone secretion, and metabolic processes. The central clock is located in the suprachiasmatic nucleus of the hypothalamus in the brain. It receives light information from the eyes and helps synchronize clocks present in most body tissues with the day-night cycle. This synchronization allows organs to adjust their functions according to the daily time. The genetic regulation of this rhythm relies on successive cycles of gene activation and inhibition. At the heart of this mechanism, the proteins CLOCK and BMAL1 combine to form a complex that activates the expression of the PER and CRY genes. After the proteins of these genes are synthesized and accumulate, complexes are formed that enter the nucleus and inhibit the activity of CLOCK and BMAL1, gradually reducing their production. As the inhibitory proteins are broken down, the inhibition is lifted, and a new cycle of gene expression begins. This process is known as the negative feedback loop, and it is the basis for generating the circadian rhythm at the cellular level. The rate at which clock proteins are synthesized, modified, and broken down contributes to regulating the timing of the cycle. Environmental cues, primarily light, also influence the timing of the internal clock, while mealtimes and physical activity help regulate the rhythms of peripheral tissues. This system is linked to the regulation of melatonin secretion, which typically rises in darkness and helps signal the arrival of biological night. Disruption of synchronization between the internal clock and the environment can occur when working shifts or traveling across multiple time zones, affecting sleep, wakefulness, and daily functioning. Understanding the genetic regulation of the biological clock reveals how interactions between genes and proteins translate into a chronotype that coordinates bodily functions and helps explain individual differences in the tendency to sleep and wake early or late. Al-Mustaqbal University is ranked first among Iraqi private universities.