It’s finally time to connect the pieces of the puzzle. Many studies have been conducted monitoring how thermoregulation is controlled by body temperature. We know a lot about what happens. Sadly, despite many efforts by scientists all over the world, we still seem to only have a vague notion of how this interaction occurs. Without the ability to really trace neuronal pathways and easy detect what signaling molecules are being used (neurotransmitter, hormones, etc.) it’s difficult to be given definitive answers. This blog entry will focus on describing what we observe about the relationship between thermoregulation and circadian rhythms.
While measuring someone’s internal body temperature throughout the day sounds like a simple enough task actually measuring the circadian effect is quite difficult. This is due to an effect called “masking.” Lots of things we do can affect our internal body temperatures: our innate behaviors to find comfortable temperatures, our level of physical activity and alertness, if one is standing or lying down, and others can be confounding factors. Only after controlling for all of these can one claim to demonstrate a circadian pattern. Several scientists have been able to accomplish this and discovered some very interesting findings. In its purest form our body temperature rhythms bottom out at around 5:00 AM (a little after halfway through sleep) and come to a maximum at about 5:00 PM 12 hours later. In fact, the researchers were able to fit the cycle to a cosine curve quite well. In another study, this curve was shown to be altered by two other factors which, in my opinion, seemed a little random. Physical activity and age can have very strong effects on circadian rhythms. The more a person exercises or engages in other physical activities the more efficient they become at removing excess body heat, making the slope between the two peaks steeper with regards to cooling. It is theorized this has to do with vasoconstriction and vasodilatation. This is the closing and opening, respectively, of peripheral blood vessels in the body. If vessels are opened more blood flows to the surface allowing heat to escape while closing them traps heat inside. This is one of the largest thermoregulatory mechanisms and it is thought that physical activity somehow alters this process. The pattern with age is one you would expect. When you are very young, you haven’t developed all of your bodily functions entirely so circadian rhythms don’t function as well. Throughout adulthood they are fairly consistent, but as you become very elderly they begin to deteriorate and it becomes harder for a person to adjust to changing schedules.
A question, however, still remains how does the thermoregulatory cycle function? One chemical that keeps being tested is melatonin. The concentration of melatonin in the bloodstream rises and falls throughout the day and is associated very strongly with circadian rhythms. Melatonin levels peak at night and go down throughout the day as you are exposed to sunlight. We know that melatonin can have a least some sort of thermoregulatory effect. When given melatonin pills orally test subjects experienced a hypothermic effect. Their body temperatures were decreased. Clearly melatonin could be a major culprit in what we see, but frankly we’re not even sure how it works. In a study done here at the College of William and Mary researchers tried to see the effects of melatonin directly on PO/AH neurons. What they got were very mixed reviews. Some cells were melatonin sensitive some weren’t. Some increased their firing rates, others did not. The data came close to showing the response that Hammel’s model would have predicted just the data were just barely insignificant. It has been theorized that melatonin could act on the thyroid or some other part of the body or brain in order to produce the observed effect, more research simply needs to be done.
I am coming out of this project with a much greater understanding of the relationship between thermoregulation and circadian rhythms. I have a few project ideas, and with some luck perhaps further research of my own can shed some light on this field.