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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  1. Hey this is Manuel, we talked on Brian Armstrong’s a little while ago.

    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.

    Do you think that warming the feet might help to fall asleep? If nothing else just because it would fire off an anchor (if you’re familiar with NLP)?



  2. Hi Manuel, and welcome to my blog!

    (NLP: no, I wasn’t familiar with it but Google is my friend.)

    When my feet are chilly at bedtime, soaking them in warm/hot water helps me. I suspect it helps anyone.

    It’s suggested (in that Australian study, next post) that melatonin acts directly on peripheral receptors to dilate blood vessels near the skin, causing the core body temperature to fall as it is supposed to just before sleep. I’ve wondered if the cooling off of artificially warmed feet leads to (hooks on to) the same thing. Seems like it might, but I dunno.

    I think my feet are chilly usually when I’ve stayed up past the magic moment when I should have gone to bed. Then I can stay up ’til 5 a.m. or even all the next day if I’m not well-disciplined.

  3. This hasn’t happened to me in a while (beneficial effects of darkness therapy yadda yadda yadda, but I remember that in the bad old days when I’d be up half the night or more, my feet and lower calves would end up absolutely freezing. I don’t think it happened just when I missed the sleep gate (your “magic moment”), I think it took quite a while to get that far. Dawn, perhaps?

  4. Would that be between 2 and 4 hours before your natural waking time? If so, it’s likely your natural lowest body temperature. Shortly after that is the most effective time to use light therapy (if you happen to be staying up anyway…).

  5. It quite probably was, and that explains it. I never really went for the approach where I would stay up all night, it didn’t work with the ME, but that’s useful to know for other people.

  6. I “suffer” for the disorder as well and I have two questions that I could not find the answers to no matter how much research I did online. Wondering if you can help.

    1. If the person with DSPS can get a job that will allow him to get the full hours of sleeps he needs when he needs them, is everything else about his sleep “normal”. Are the various hormones (like growth-hormone) still being produced at same rate as for a person that doesn’t have the syndrome? Is their production suppressed or only delayed as well?

    2. I have a 2 yo now. She has been adjusted to my schedule and although she is asleep by 10-10.30 pm she wakes up with me at noon. I sometimes (almost every day really) feel very guilty about that. Is the syndrome something you either have or not have in you genes? Or could I be “training” her to have it is I keep this up until she needs to go to school?

  7. Very good questions, E.
    1. In theory, everything should be delayed the same amount. At least the core body temperature usually is. Research is, unfortunately (usually) done on normal people, so it’s hard to know to what degree it may apply to us.
    2. DSPS does (sometimes) run in families. I suspect that, if it turns out she does have it, it won’t be something you are responsible for. If she doesn’t have it, she’ll probably be waking at 6 in a year or two.
    Sorry I’ve not really given you answers to your questions. Good luck!

  8. […] Sleep and Other Daily Cycles (delayed2sleep – DSPS & N-24 Blog) […]

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