Time Is Running Out: The Circadian Clock Suggests Sex and Aging Differences in Human Epidermis

Day and night have long guided daily activities in our lives. Before time-keeping innovations such as sundials and clocks, humans maintained schedules by merely opening their eyes as the sun rose in the sky. Unbeknownst to them, their internal biological clock was maintaining regulation of their bodily functions throughout the course of the day through the trillions of cells that comprise the human body. This endogenous cellular process is referred to as one's circadian or biological rhythm and oscillates over an approximately 24-hour period, often portrayed as a cosine wave.