Rhyme and reason: the role of circadian rhythms in skin and its implications for physicians

The study of the day and night cycle and its effects on the human body has been a field of interest for many decades. In more recent years, the effects of these diurnal variations have been looked into with respect to human skin. To encourage future medical interest in this subject as well as to discuss the futuristic concept of transdermal drug delivery incorpo­ rating the effects of circadian rhythms, we have summarized recent research into this topic. The day and night cycle creates widely dif­ ferent environments for the skin. Depending on the time of day, there is variation in the risk of UV exposure, transdermal water loss and exposure to extremes of temperature. Diurnal rhythms are patterns that recur every 24 h, whereas circadian rhythms or biorhythms persist with a period of ∼24 h in the absence of external cues but remain responsive to environmental conditions such as light and exercise [1]. The circadian clock, a primor­ dial system, which regulates human physiol­ ogy based on diurnal variation, has a natu­ ral impact on the way our skin functions. In plants, the circadian control of development is well established. Research in recent years has focused on variations of the circadian rhythm in human skin and its impact on its integral complex components [2]. In the fol­ lowing report we summarize recent research on circadian oscillations and its effect on the skin. Human circadian cues originate in the suprachiasmatic nucleus (SCN). The SCN regulates the timing of body clocks via auton­ omous nervous control of hormones such as glucocorticoids or by sending indirect signals via body temperature. The SCN is itself syn­ chronized to light via the retinohypo thalamic tract; the result is a flexible system of clocks, each with an intrinsic period of about one day that is constantly readjusting to the timing of environmental light [3].