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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