Tags: Australian study, Core body temperature, Melatonin
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…)
Tags: Australian study, Circadian rhythm, Core body temperature, Cortisol, Jet lag, Melatonin, Serotonin
- 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.