xix. Phase Response Curve (2)12 December 2005 at 15:46 | Posted in Circadian rhythm | 2 Comments
Tags: Circadian rhythm, Core body temperature, Light therapy, Melatonin, Phase response curve
I can’t yet leave the subject of the PRC, even if it is a bit technical. Not all researchers say that we have a long “dead zone” mid-day in regard to our response to bright light. The figure above shows the phase response of melatonin in the blood to light administered at various times. From a study at Harvard published in 2003, it is somewhat simplified here.
21 carefully chosen persons were kept in dim light, then just once exposed to nearly 7 hours of bright light at different points in their personal cycles on the 6th day of a 10-day study. The timing of each individual’s melatonin secretion was determined at the beginning and again at the end of the experiment. All results are combined in the curve shown.
An interesting point: the midpoints of the individual melatonin profiles were found to be within a range of 2½ to 6 hours prior to these persons’ habitual awakening times.
The horizontal line, about a half an hour below zero, shows how great the delay would have been anyway during the course of the experiment, due to the built-in pacemaker’s longer-than-24-hour period, had there been no light exposure at all.
Phase shifts, from the beginning to the end of the study, are shown in clock time (vertical axis; phase delays are the negative values). Here, the falling slope in the daytime is only slightly slacker than the rising slope in the night, and the true “dead zone” is only momentary, twelve hours before (or after) the core body temperature minimum.
It is not difficult to explain the long mid-day “dead zone” found by earlier research. Since bright light has only slight effect in the middle of the day, the effects of light before and after that magic moment will cancel each other out. Unless, one could imagine, someone consistently is exposed to darkness for an appreciable period just before or just after. How about someone working in darkness in the projection room of a cinema for the afternoon show, for example — might a phase advance be the result?
The authors state: “These data provide a comprehensive characterization of the human PRC under highly controlled laboratory conditions.” So this is perhaps the last word on that matter.
Honors for inventing the Phase Response Curve in 1960 go to Patricia DeCoursey. The “daily” activity rhythms of her flying squirrels, kept in constant darkness, responded to pulses of light exposure. The response varied according to the time of day — that is, the animals’ subjective “day” — when light was administered. When DeCoursey plotted all her data relating the quantity and direction (advance or delay) of phase-shift on a single curve, she created the PRC. It has since been a standard tool in the study of biological rhythms.