What makes me tick…tock?June 2012
Lesson 6: What happens to humans when normal rhythms are disrupted?
General Information about Circadian Rhythms
What are Circadian Rhythms?
Research conducted in caves, as well as in modern sleep laboratories where the environment is completely time-free, has proven that the human body is governed by built-in time setters, commonly referred to as an internal body clock. Over 100 body functions fluctuate between their minimum and maximum values once a day. Because such fluctuations in human functioning take about a day's time to complete—roughly 25 hours—the term circadian rhythms was coined (circadian means "about a day" in Latin). Dr. Franz Halberg of Germany first used this term in1959 to describe these changing body functions.
Body Temperature, Light and Zeitgebers
One of the most easily measured of these circadian rhythms is the body temperature. Healthy humans experience rhythmic variations in their body temperature throughout the course of each day. For most people, the difference between high and low values is about two degrees Fahrenheit (97° to 99°), with the lowest value typically occurring in the early morning hours (2:00 a.m. to 5:00 a.m.) and the highest values commonly occurring in the evening (7:00 p.m. to10:00 p.m.). Studies in which the body temperature has been monitored in a time-free environment have shown that our temperature level fluctuates in the same 25- to 26-hour pattern, no matter when we sleep or when we are awake. In short, our body temperature cycle operates independently of our sleep/wake cycle.
Under natural conditions, our "body clock" or circadian rhythms are linked or "entrained" to an external clock time by the synchronizing effects of the physical world in which we live. The continual resetting of our circadian rhythms to a 24-hour sun cycle is accomplished by environmental time cues, known aszeitgebers (which is German for "time givers"). These zeitgebers may be physical or social in nature. The most powerful time cue for our bodies is the changing light levels in our environment. When disruptions occur in our natural time patterns, these zeitgebers either act upon the circadian system to bring it into synchronization with the new time pattern or, in cases such as an assignment to the night shift,zeitgebers actually discourage adjustment to the new routine.
Sleep Patterns and Circadian Desynchronization
One disruption in natural conditions which shift workers often experience is a change in their sleep/wake cycle. Coming onto an evening or night shift can require a delay in our sleeping time of as much as 3 to 12 hours. Because the body temperature rhythm is independent of the sleep/wake cycle, changing our sleeping time does not automatically produce a change in the temperature rhythm or other associated body functions, like locomotor activity (body coordination) and immune responses.
When the sleep/wake and body temperature cycles are no longer in phase or "in sync" with each other, we experience a condition known as internal desynchronization. This desynchronizationusually is aggravated further by the influence of environmental time cues. So, an employee placed on the night shift must suddenly ignore the light-dark patterns in the external environment, and try to sleep at a time when the body temperature clock is telling him or her to be most alert and active.
It should be noted that all shift workers do not have the same degree of problems when their bodies experience circadian desynchronization. Some employees are greatly disturbed by these internal disruptions and others experience only mild symptoms. Yet, like a machine which eventually breaks down when its operating specifications are continually ignored, the human body also defaults under unnatural conditions. If employees' internal clocks are repeatedly desynchronized, some negative effects on health, safety and/or performance will occur.
1