Circadian (~ 24-hour) rhythm has been observed in all living organisms. In humans, the circadian system governs different physiological functions such as metabolism, sleep-wake cycle, body temperature, hormone secretion, and cellular proliferation. The suprachiasmatic nucleus (SCN) of the anterior hypothalamus is the principal circadian pacemaker. The SCN receives input signals primarily from the retinohypothalamic tract (RHT), sends output signals to different parts of the hypothalamus, pineal gland, and the peripheral clocks through the neural or humoral network. The functions of the circadian clock are mediated by the rhythmic expression of the core clock genes through a complex feedback loop. Disruption of clock functions influences the development of several pathologic conditions, including cancer, shift work, chronic or acute jet lag, and light-at-night affect the circadian activity, leading to development of several physiological disorders, more specifically cancer. Circadian dysfunction alters the expression of core clock genes that promote the deregulation of the cell cycle, increase cell proliferation and survival, decrease apoptotic activity, alter metabolic functions, increase metastatic property, collectively induces cancer progression.