xx. Entrainment

14 December 2005 at 06:45 | Posted in Circadian rhythm | 3 Comments
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“To entrain”, according to my old Webster’s, means to board a train.  I need a new dictionary.
 
When biologists say that an organism is entrained, they mean that one or more of its circadian (about a day) periods is reset to coincide with cues from the environment.  In the laboratory they can entrain animals or other organisms to various periods, within limits.  Light and darkness are the most oft-used cues; others are physical and social activities, temperature and drugs or chemicals.
 
When a healthy entrained human subject is chosen for a study, this means that he (seldom she) at least once daily resets his central pacemaker to conform to the 24-hour period of the natural light/dark cycle.  As almost all of us do.
 
When biological clocks are not entrained, as when, for example, existing in constant darkness without time cues, they are said to be freerunning.  The average freerunning period for human adults is, as noted, 24 hours and 11 minutes.  Our own clock, as well as that of rodents, fruit flies and others, “tells time” in order to be able to program activities at appropriate times.  Its key function is “to provide an internal estimate of the external local time” (Johnson, Elliott and Foster, 2003).
 
As far as I can see, the researchers have plenty left to do.  There are unanswered questions about effects of different wave lengths, duration and intensity of light exposure as well as gradual changes in light intensity, how the light-sensitive cells “work” and whether their sensitivity varies throughout the day and night.  And, of course, I’m waiting for some explanation for Delayed Sleep-Phase:  why/how one can adjust to waking at the same time every day, just not the right time every day!
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Next post: xxi. Is it life-long?
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