Recent studies have proved that in mammals, molecular oscillators are present in every cell of the body. They comprise a group of core circadian genes forming a transcription–translation feedback loop to confer circadian programming in physiology, metabolism, and behavior. Mutations of the gene and physical destruction of the suprachiasmatic nucleus (SCN) proved that the master clock is responsible for the circadian rhythm. Multiple reports explain the tissue-specific altered circadian clock gene expression in various malignancies, including head and neck, breast, colon, and lung cancers. Extensive lines of emerging evidence collected for over two decades convincingly demonstrated that proper functioning of circadian rhythms is crucial for well-being and survival. There is highly credible evidence that circadian rhythm dysregulation results in a multitude of pathologies, including cancer. The circadian rhythms of melatonin, cortisol, TSH, growth hormone, core temperature, and rest–activity and sleep–wake cycle are disrupted due to shift work, jet lag, and light at night in cancer patients. These rhythm alterations are studied with the help of parameters, such as 24-h average or mesor, amplitude, phase advance or delay, and so on. This chapter addresses the prospects of applying the principles of chronobiology to treating cancer malignancies. Cancer cells exhibit a distinct circadian rhythm compared to normal healthy cells, necessitating a more nuanced treatment approach. Cancer chronotherapy, a promising field in clinical oncology, underscores the importance of administering anticancer medications in alignment with the patient’s circadian rhythms to optimize therapeutic efficacy.