ABSTRACT
As the earth rotates, all species on the surface of the planet are exposed to 24-hour patterns of light and darkness. In response to these regular, daily oscillations to the natural light-dark cycle, these species have evolved endogenous circadian rhythms that repeat approximately every 24 hours [1,2]. Examples of circadian rhythms include oscillations in core body temperature [3], hormone secretion [4], sleep [5], and alertness [6]. Circadian oscillations also exist at a cellular level, including cell mitosis and DNA damage response [7,8]. These oscillations are a result of a small group of clock genes inside the cell nuclei creating interlocked transcriptional and post-translational feedback loops. The timing of these circadian clock genes is generally orchestrated by a master biological clock located in the suprachiasmatic nuclei (SCN) [9] of the hypothalamus of the brain
[10]. The master clock in the SCN provides precise time cues throughout the body to regulate these diverse physiological, hormonal, and behavioral circadian patterns. However, in total darkness the timing of the SCN will become asynchronous with the solar day because in humans the period of the master clock is slightly longer than 24 hours [1]. To maintain synchrony with the external world, the light-dark pattern incident on the retina resets the timing of the SCN, so that as we travel across time zones, we can entrain our biological functions to the local environment. If the period of the light-dark pattern is too long or too short, or if the light and dark exposures become aperiodic, the master clock can lose control of the timing of peripheral circadian clocks.
