viii. Warm feet

24 November 2005 at 22:20 | Posted in Circadian rhythm | 3 Comments
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Figure (used by permission) from the Australian study as promised (or threatened?) in my previous post. The study is long and it includes many more types of results than those reported here. 
The horizontal axis shows 4 hours, during which the subjects, 8 women and 8 men, were kept awake. Room temperature was kept between 21 and 23 degrees C. 
The zero on the bottom line represents each individual subject’s natural hour (moment) of melatonin onset. The entire horizontal axis shows the period from 1½ hours before to 2½ hours after that zero point for each person.   
The vertical axis in the upper half of the figure shows Core Body Temperature. The vertical axis in the lower half shows surface temperature on the soles of the feet. 
DIM LIGHT. The solid line with the black dots shows the condition of very dim light which, of course, is the condition which the body considers to be normal/desirable around bedtime. In the top section, the Core Body Temperature (CBT) behaves as expected, beginning its nightly descent. The soles of the feet, at the same time, warm up from about 31 to almost 35 degrees C. 
BRIGHT LIGHT, NO MEDICATION. The dashed line with the black dots shows the condition of bright light, which suppresses the hormone melatonin to very low levels. Here, the CBT stays at about the same level for the entire four hours. The soles of the feet, at the same time, warm up from about 29 to 34 degrees. 
BRIGHT LIGHT + ADMINISTRATION OF MELATONIN SUPPLEMENT AT THE TIME OF ENDOGENOUS (internally produced) MELATONIN ONSET. The solid line with the open squares shows the same condition as the previous one for the first 1½ hours and, as expected, it parallels the dashed line in both the top (CBT) and the bottom (foot soles) sections. After the administration of melatonin, CBT goes dramatically down in spite of the bright light. The feet warm up rapidly a half an hour after the administration of melatonin, from about 31 to 34 degrees C.  
If I am understanding this study correctly, it suggests that melatonin acts directly on peripheral receptors to dilate blood vessels near the skin. That is, melatonin causes diffusion of heat from the blood to the environment. It is this heat loss which, in turn, causes the CBT to decrease. 
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My comments: 

It is surprising to me that the soles of our feet warm up to within a couple of degrees of our normal daytime CBT. 

One and one half hours before the onset of melatonin as measured in saliva, the feet are apparently a couple of degrees cooler in the bright light than in the dim light condition. (The body is not being allowed to prepare for sleep in a normal fashion.)

Even when bright light is suppressing melatonin and the CBT isn’t reduced, the feet do warm up. What signal makes the feet warm up at this time?

It would appear that taking a melatonin supplement at precisely the right moment counteracts the effects of bright light for at least the following 2½ hours. (Moral: eat melatonin, and you can stay at your bright computer screen for another hour or two…)

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See also:  Kräuchi, Kurt, Christian Cajochen, and Anna Wirz-Justice. A relationship between heat loss and sleepiness: effects of postural change and melatonin administration. J. Appl. Physiol. 83(1): 134-139, 1997.


Next post:  ix. Melatonin and the effect of light



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  1. Dear guys,

    I have to congratulate you to the very good work on this blog. It is very helpful and informative.
    But one thing I miss is that you don’t describe your sport habits? Have you ever considered to seek a relation between the DSPD and sport? I am quite new to this subject as I am suspecting to have the disorder too but when I look back in the times when I was doing regularly over a period of 5 years a fair amount of sport (about 15 hours a week) I did not suffer from DSPD so much as I do now.

    Any comments on this subject would be highly appreciated.

    Thanks a lot


  2. Thank you for the kind words about the blog!
    I am not a sporty person, so I cannot answer from personal experience. There have been some studies done with indoor exercise at different times of day and that does not seem to make any difference. But being outdoors in daylight for several hours a day does seem to help.

  3. I have gone through periods of my life when I was relatively inactive and other periods when I exercised regularly (10 hours a week running, weight-lifting and boxing with a punching bag). I found some benefit to my circadian rhythm disorder but the effect was small. It might be enough to alleviate a very mild case of DSPS or to help a more severe case of DSPS or N24 when combined with other treatments, but by itself I did not find it effective enough to make a large difference.

    This tends to agree with the research studies which find that exercise has a small effect on circadian rhythms — much less, for example, than the effect of light.

    If your sport activities were outdoors the light might have enhanced the effect, as my blog partner said.

    Of course everyone is different, and you might find you benefit more from sport. If so, by all means try what works for you.

    On the other hand it’s also possible that the effect was coincidental.
    For example DSPD can worsen at certain ages. If you were much younger when you were involved in sport, it may be that this is what caused the change. Usually DSPD worsens during adolescence, but that can vary. My DSPD became progressively worse from ages 16 to 21,and then became N24 at age 27.

    I hope this helps. I am glad you enjoyed our blog.


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