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


vii. Sleep and OTHER daily cycles

24 November 2005 at 20:52 | Posted in Circadian rhythm | 8 Comments
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I wrote:  “We have several circadian rhythms which are determined by our body clocks.  The most obvious of these is the sleep/wake cycle.”
Yes, and how many others are there?  Probably dozens, depending upon how one classifies them.  I’ve not gone looking for information on all of them.  (Although I am a bit curious as to why/how the liver is the last organ to adjust after jet-lag.  It takes up to a month!)  Though cycles of serotonin and cortisol are related to the sleep/wake cycle, I may never understand enough about those to dare to write about them. 
I do want to try to explain a bit here about the rhythms of:
  • Core Body Temperature (CBT) and, incidentally, the temperature of the soles of our feet
  • Melatonin, “the hormone of darkness”

During our day, CBT bobs up and down.  It’s up into the fever range when we exercize or take a hot bath.  It drops quite low if we lie down for a minute or two.  Charting CBT throughout one’s waking hours while living normally is difficult to interpret and thus not very meaningful.

Those who volunteer for studies where they must recline almost motionless and stay awake in near-darkness in order to have their temperatures measured continually, are usually paid for their trouble.  There are many such charts to be found.  The illustration above is from this lecture slide by Dr. Bjørn Bjorvatn in Norway.  It is easy to follow, with degrees celsius on the vertical axis and the time from 6 p.m. to 6 p.m. on the horizontal. 

If one is sleeping at the time one’s body prefers to sleep, CBT declines very steadily and evenly for hours as shown in the illustration, from a while before bedtime until about two hours before awakening.  Then, very abruptly, it starts climbing on a somewhat steeper slope than the decline. 

Each person’s body “knows” this curve and when it should occur in relation to the 24 hour day.  Each person’s body has its own rule about when awakening should occur:  precisely  X minutes after the temperature minimum.  If your temperature minimum occurs at 5 a.m. and you awaken spontaneously at 7 a.m., you are very normal and about average. 

Researchers don’t agree on what makes this long temperature decline start each day.  It is clearly related to the secretion and blood level of the hormone melatonin, but which triggers which, or does something else trigger both?

In any case, all that body heat has to go somewhere.  If you’ve noticed that your feet get hot just as you’re getting sleepy, you’re very observant.  The heat which will be excess as you “hibernate” for the night is escaping.  That’s OK, just let it.

According to an Australian study from 2001 we are sleepy when our feet are at their warmest, an average of 4 degrees C. above normal.  This occurs shortly after onset of melatonin in the blood, when the core body temperature has just started its decline.

Just for fun, see a figure from that study (and a link to it) in my next post.

In the post after that: more about melatonin and the effect of light.


Next post:  viii. Warm feet


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