xviii. [CHAPTER TWO] Phase Response Curve4 December 2005 at 14:59 | Posted in Circadian rhythm | 7 Comments
Tags: Core body temperature, Coturnix, Phase response curve
“Chapter” One ended, appropriately I think, with a break. A coffee break… On to Chapter Two!
A half a year ago, I was struggling to understand what a Phase Response Curve (PRC) is. Now I’ve drawn one and am going to try to explain it. In the illustration you can see, obviously, the 24 hours of a day / night on the horizontal, and a good night’s sleep for the person whose PRC is shown. It’s not mine, not yours, but someone’s Phase Response Curve, similar for all humans. (Source ) (Another example)
The hours are clock hours, only if this person’s circadian period happens to be exactly 24 clock hours long. Most people have a period a bit longer than 24 hours, and will need to advance the phase, shift it earlier, each day.
The human PRC for light
This curve shows what light administration does for the timing of the Core Body Temperature in humans. In general, several other circadian rhythms follow the body temperature rhythm.
The numbers on the vertical axis represent hours of phase shift. Very bright light to the eyes of this person, immediately upon awakening, will shift her phase two hours earlier, a two-hour advance. Just a few hours later, light won’t have any effect (in the “dead zone”, where the curve follows the zero-line). Toward bedtime, exposure to light has the opposite effect; the negative numbers represent a phase delay. Very bright light just before bedtime will cause a two-hour delay, cancelling out the morning’s advance. However, the light we usually are exposed to in the evening hours will be considerably less intense.
While the “dead zone” in midday can last for several hours, the “dead zone” in our subjective night lasts for only a moment. Light exposure shortly thereafter, on the rising portion of the curve, can create a mess according to this chronobiologist. Exposure to light too many minutes before our spontaneous wake-up time is thus an uncertain business.
Those of us with Delayed Sleep-Phase Syndrome probably have a circadian period which is longer than normal. We need to phase advance quite a bit each day, just to adjust to nature’s 24 hours. Advancing even more, in order to awaken at a “normal” time, is impossible without treatment such as that described in my entry no. xvi.
Have I misunderstood something? Corrections are welcome!