xviii. [CHAPTER TWO] Phase Response Curve

4 December 2005 at 14:59 | Posted in Circadian rhythm | 7 Comments
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“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! 
Next post:  xix. Phase Response Curve (2) 




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  1. This is very interesting. All of this site is. I have learnt a lot here and I haven’t even read it all. My diagnoses are DSPS, Non-24 and Irregular… I usually always wake up at lunchtime but when I go to sleep wanders from 01 to 09am. The day I get to little sleep (like if I go to sleep at 9am and then wake up only some hours later) it resets and I’m of to sleep already at 01am again. And the cycle continues… Why I comment on exactly this post is because I would like to see my own PRC. Very often if I take my light therapy lamp when I naturally wake up, I go to sleep again after or under the therapy. 🙂 I’m probably seriously messed up but I’m starting to accept that and also I think at least some of my problems are going to get cured by my sleep doctor. Thanks again for all of this information!

  2. Hei Tobias. Eg har prøvd å lese det laaaange innlegget ditt, så plager eg deg ved å tvinge deg til å lese norsk. Svensk kan eg ikkje.
    You are very welcome; info is what this blog tries to be for. When you usually wake at lunchtime, it sounds more like DSPS than Non-24. Are they planning to do an overnight sleep study? That usually isn’t necessary for a circadian rhythm disorder, unless they want to rule out other sleep disorders. Yours does sound a bit complicated 🙂
    You should be a member (if you’re not already) of ‘niteowl’ mailing list. See the ‘Circadian Disorders’ link in my blogroll. We are mostly DSPS, but also a fair number of Non-24 members there.
    Your own PRC isn’t easy to construct. Here’s one which shows outliers all over the place from several studies: http://i36.tinypic.com/ivhjk4.jpg
    Thanks for commenting here, and good luck!

  3. Hej igen!
    I have missed we’re country neighbors 🙂

    I’ve done a sleep study and it’s from that sleep lab I got diagnosed with DSPS. It seemed more complicated with my tiredness and I got to a Sleep specialist. He was the one saying also non-24 and irregular sleep wake. He’s probably the only one in Sweden working with circadian rhythms. I also find it a bit strange that I wake up lunchtime although it differ some hours. Not as much as sleep time though.

    I am a member of the mailing list but I find it a bit too old school and would prefer a forum 🙂 Paused my reading of it at the moment.

    Thanks for the PRC picture. Strange strange. Mine to probably 🙂

    I’m soon up for some medication I have not read anywhere about so we’ll se how that goes and i can report back!

    Thanks again!

  4. So a mail list is too old-fashioned for you, eh? I found it a bother when I was on yahoo.com, but gmail.com’s “conversations” work just like a forum. Recommended.

    Do update us on new medications. I’ve tried Circadin instead of Melatonin. Melatonin (cheap) works and Circadin (expensive) doesn’t. Haven’t tried Rozerem nor the new one Agomelatine (an antidepressant which is said to work by correcting misaligned circadian rhythms). Those are the evening ones. Then there are dozens of wake-up meds.

    That PRC was a composite of several.

  5. Yeah I’ve heard about gmails function. I selected the lists collecting option and haven’t tried changing. Will do!
    I’ll update but I haven’t got the meds jet. I have the expensive circadin on prescription. To get regular Melatonin here in Sweden you have to get a license but thats going to get sorted out. My first tests with it made me hung over. But I figured if I chew them I take away the time release and it worked better. But then I get 2mg. Should try smaller doses but I think I cannot sort this out myself anyway. The medicine I think is originally for epilepsy.
    And by strange I meant the outliers in the PRC 🙂

  6. My Norwegian pharmacy could *only* get 3 mg tablets or capsules, they said. I told my specialist that I wanted liquid, so I could control the dosage myself. So he wrote the application/prescription for “liquid”. After trying 3 brands I settled on Natrol, which he now writes on the annual application/prescription, and the same pharmacy manages to get ‘hold of it, when it’s specified so clearly. (He even writes the name/address of the lab in California.) I never take more than 1/4 mg. (Tastes like raspberry, nam.)

  7. The way I visualise the PRC is thus:
    There is a reset point, where your bodyclock says, ok that was yesterday, and now it’s tomorrow. You can measure it if you want by placing a thermometer where the sun don’t shine, and this reset point is the time where your temperature reaches minimum. For people keeping normal hours it’s around 5am.
    The reset point will MOVE AWAY FROM LIGHT TOWARDS DARKNESS. So if you apply bright light up to just before the reset point it will move away to a later time (and vice versa, light after the reset point moves it earlier). Bright light exposure which starts before the reset point and finishes after will cancel the effects out. The jargon isn’t great: moving to an earlier time is called Advancing, moving to a later time is called Delaying
    If the light exposure starts just after the reset point it will have a greater (advancing) effect, or similarly if it finishes just before it will have a greater delaying effect.

    It’s logical. Animal bodyclocks need to be tweaked to keep us in sync with the daytime because they don’t run on a perfect 24.0000 hour day. If they are slightly out they need a little tweak, if they go way adrift they need a big push. For those of you who recognise this pattern, yes it’s a feedback loop.

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