As the term "circadian" might imply, circadian rhythms are those rhythms that have a period of approximately 24 hours (circa diem ... about a day). Perhaps the most familiar of the human circadian rhythms is the natural sleep/wake pattern we follow, that is closely tied to our core body temperatures (also a rhythm with a 24 hour period). Over the last two decades, scientists have been able to quantify what they already believed to be true ... the influence of light on our circadian rhythms.
But what happens when our work schedules don't conform to the typical daylight hours? This is just one of the many problems that face shift workers.
This model allows us to compare the circadian rhythms (as reflected through the core body temperature) typically experienced by a person with a "typical" lifestyle, and those of a person placed on shift work duty.
The light that affects us can be thought of as coming from two different sources ... indoor lighting and direct sunlight. These two types of light are factored into the "brightness" level that effects the models equations for our circadian rhythms.
The temperature oscillator consists of two parts (core body temperature and it's complementary temperature). Light influences both of these through the brightness function, which is in turn effected by the core body temperature. [See the flow diagram of the model below.]
The picture below shows the Stella Model that describes the dynamic relationship between the core body temperature and the influence of light. Note, the typical daylight exposure is effected by the workers shift schedule, as are the brightness function (indirectly) and the complementary temperature variable. The first flow chart describes what will be referred to as the "benchmark" temperature rhythms. These are the temperature rhythms experienced by someone with a typical lifestyle ... awake during the hours of 8 a.m. and midnight (think of your weekend schedule) ... without the outside influences of unusual work schedules. The work influenced model takes into account the effects of shift work as seen through both the change in lighting and the "zeitgeber" function, meant to indicate an addition of energy or stress to the system.
Check out the applet that simulates the circadian rhythms, created using Stella. You can compare the temperatures experienced by the shift worker to those of the typical daytime worker (as well as the "illumination" they're exposed to). What happens to the rhythms as you adjust the workers starting time, or the percent of total light exosure due to the indoor lighting (rho)? What type of work schedule would you rather have?
For more information on human circadian rhythms, biological timing and its applications check out the Biological Timing Tutorial website.