Resonance, the PRC, and adaptation

Recently on the polyphasic google group, someone posed the question:

Are there accounts of people who've attempted to adapt to any of these low-sleep schedules with what's generally agreed upon as the correct steps and still didn't adapt, or all evidence indicates anyone can theoretically adapt?

A few months earlier (from this posting), I tried forcing myself on ~6 hours monophasic sleep a day, largely based on an alleged second-hand story of a guy forcing himself to do 4 hours sleep, claiming to adapt after a month. I stopped after 5 weeks when there was no long-term improvement in wakefulness. I'll hopefully not do such experiments based on flimsy evidence next time :-) I figure now that the story's either not true or that there was something special about the guy's body, the latter of which leads me to inquire on the subject in relation to polyphasic sleepers.

While I don't think there is a definitive answer to this question, I think we can gain some insight into what might be going on by examining how the body adapts to changes in the external clock (for example, at daylight savings time, or when traveling across time zones). The body's internal clock can be thought of as an oscillator with its own natural frequency (typically in the neighborhood of, but not exactly 24 hours). This frequency varies from person to person. How is it, then, that we stay in sync with the 24 hour day? Well, the circadian clock gets a kick every time we see bright light (among other things) which brings the frequency into harmony with its external stimulus (in this case, the rising and setting of the sun)[1].

How much the internal clock changes, and in which direction, depends on when the person is exposed to the bright light. A simple analogy helps us to understand why this is so: if you are pushing a child on a swing, you can make the child go higher by pushing at just the right time. If you push at the wrong time, however, it is possible to damp out the swing's motion. For example, if you push on the child while she is moving toward you, you will reduce the amplitude of the motion, whereas a push when the child is moving away will tend to increase the amplitude.

The analogy is not perfect; the circadian clock is not a simple oscillator like a swing. The basic principle still holds, though, which is that the response of the clock to an outside stimulus depends on exactly when that stimulus is applied. The mathematical relationship between the time of the cycle when the external stimulus is applied and its result is called the phase response curve, or PRC.

More broadly, in physics this kind of phenomenon is referred to as resonance. The idea is that a given system will have one or more natural modes of operation, and if one wants to put energy into or take energy out of the system, it is easiest to do near to one of these natural modes.

What does this have to do with the question at the top of this post? Perhaps nothing. My guess, however, is that each of the three major things which affects one's ability to adapt to a new sleep schedule (circadian phase, sleep effectiveness, and sleep/wake thresholds) has some sort of variable response curve to those things which affect it. If I want to sleep less, I need to increase my sleep effectiveness, and the details of how I go about trying to do that are probably important. In particular, a gradual transition may be easier than an abrupt one. Also (and this may be an even stronger effect) when you put your 6 (or 4, or whatever) hours of sleep in your day can make a big difference in how you feel, and likely in to what extent and how quickly you adjust to the new sleep schedule.

As far as I'm aware, the experiments to determine whether what I've been saying in this post is actually true or not haven't been done, and I'm not the guy to design or perform them (I'm a computational physicist, not a physiological psychologist). If I had to guess, though, and I didn't have any more information than was originally given, I would say that the questioner's problem in adapting to a 6 hour monophasic schedule stemmed from some combination of trying to take the 6 hours of sleep at the wrong time of day, and not using an appropriate adaptation schedule to get from 8 hours to 6.

References

[1] J. Duffy and K. W. Jr, "Entrainment of the human circadian system by light," Journal of Biological Rhythms, vol. 20, iss. 4, pp. 326, 2005.
[Bibtex]
@article{Duffy:2005p184,
author = {JF Duffy and KP Wright Jr},
journal = {Journal of Biological Rhythms},
title = {Entrainment of the human circadian system by light},
number = {4},
pages = {326},
volume = {20},
year = {2005},
date-added = {2010-08-27 18:58:04 -0400},
date-modified = {2010-08-27 19:07:13 -0400},
pmid = {1820236591755455448related:2GvAQonHQhkJ},
local-url = {file://localhost/Users/cmckay/Documents/Papers/2005/Duffy/Journal%20of%20Biological%20Rhythms%202005%20Duffy.pdf},
uri = {papers://FA38B055-1173-4624-8A1D-F12C6F8B1027/Paper/p184},
rating = {0}
}

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