From: skill@qucis.queensu.ca (David Skillicorn)
Subject: Resetting Circadian Rhythm
Summary: light at appropriate time and intensity resets rhythm
Keywords: jet lag, sleep disorders
Organization: Computing & Information Science, Queen's University at Kingston
Date: Fri, 6 Nov 1992 17:38:56 GMT

                                                                             
Many people seem unaware of the interesting results obtained recently
about the human circadian clock. Since these results have direct
application to handling jet-lag, and possibly to sleep disorders as
well, I thought I would briefly summarize.

Details can be found in Jewett, Kronauer, Czeisler, Nature 350, 7 March
1991, pp59ff and Winfree's commentary in the same issue.

The phase diagram for the human circadian clock is (partly) shown
below. It is symmetric about the midline.
                                                                             

                                    *             *    *    
                                    *             *    *   
                                    *             *    *    
                                    *             *    *     
                                    *             *     *    
D                                   *            *      *                 
O                                   *           *       *                
S                                   *           *        *        
A                                   *          *         *              
G                                   *         *          *             
E                                   *        *           *
                                    *       *             *                  
                                    *      *              *                  
                                    *     *               *                  
                                    *    *                *                  
                                    * ***                  *                 
                                                           *                 
                                    A *******              *                 
                                             **B*          *                 
                                    * *          *         *                 
                                    *  **         *        *                 
                                    *    **        **      *                 
                                    *      **        *     *                 
                                    *        *        **   *                 
                                    *         *         *  *                 
                                    *          *        *  *                 
                                                *       *  *                 
    Noon                        Midnight                      Noon              
                                                                             
                 Subjective Time (the time your body is on)

Here's how to read it. If your body is subjectively at time x, look
along the horizontal axis to that time. An exposure to light at
a dosage read from the vertical axis will reset your circadian clock to a 
time obtained by following one of the starred lines to a horizontal axis.

Example: You've flown overnight from the east coast of North America
and land in London at 7 in the morning. Your body time is 2 a.m.
so you read up from 2 a.m. and find a line that cuts the horizontal
axis at about local time (7 a.m.) -- point B. Reading from the
vertical axis you need to have a moderate dose of light to reset
your internal clock to local time.

A moderate dose seems to be 7000-12000 lux for 8-9 hours, centered
around the nominal time of exposure.
There is some evidence that greater amounts for less time can be
substituted. 7000 lux is only about the outside intensity at dawn or dusk.
Thus 2 or 3 hours of full sun may suffice.

The point A is a singularity i.e. if you get a quite moderate dose
of light at subjective midnight you can completely flatten your
circadian rhythm. This is not a good thing to do.

There is a missing component in the diagram, the 
amplitude of the resulting circadian rhythm which I can't 
figure out how to draw. Roughly speaking, the closer to point 
A your exposure, the flatter will be the rhythm after exposure.


                                 -david skillicorn
                                  skill@qucis.queensu.ca

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