Three noons and their importance to determining the shift in circadian rhythms

The circadian rhythm is a biological rhythm in charge of timing many processes in the body, for example melatonin and testosterone secretion, the daily body temperature gradient and the peaks for most efficient SWS and REM sleep. Because these events happen at set timings it is beneficial for people to be able to predict the timing of the circadian rhythm compared to the hours on the clock, and this can be done with comparing the noon to the minimum daily temperature. But first the term “noon” has to be defined. There are three types of noons that can be used when establishing the shift of the circadian rhythm; solar noon, local noon and biological noon.

The solar noon refers to the timing when the sun is in the highest position in the sky. This happens at the same time every day on a specific longitude (ignoring daylight savings time), and is therefore a good event to compare to other set timings. The solar noon is biologically meant to happen at 12:00 pm, which means that an unshifted circadian rhythm would have its natural SWS peak at 9 pm – 12 am, and its REM peak at 6-9 am.

However, if your timezone is not in synchronization with the solar time, and solar noon occurs at for example 1 pm, it results in your peaks being naturally shifted one hour later. In this case the peaks would be between 10 pm and 1 am, and 7 am and 10 am. This would be what is defined as the local noon, or in other words the local time when the solar noon occurs. Some countries have a shifted local noon for economic or cultural reasons, with a couple notable examples being the Netherlands and Spain. Finding timing of your local solar noon can be done on

The third type of “noon” is the biological noon. This is the time of the day that your body treats as noon. The timing of the biological noon is often altered by late-night use of electronic screens in today’s world. Establishing the timing of the biological noon is important to help you assess at what times the optimal sleep placements would be. Unfortunately, determining this timing is not as simple as looking at a clock. Specific timings in the circadian rhythm can however be used as a marker that can then be compared to the natural, solar-based rhythm in order to determine how far the circadian rhythm is shifted. The by-far easiest method to do this is to monitor the body temperature gradient throughout the day. The lowest temperature point happens at around 4:30 am1 on the natural solar rhythm, so by comparing the timing of that event to the local noon, the timing of the biological noon can be established. It should be pointed out that the timing of the noon is not necessary to establish the shift of the circadian rhythm, however it helps some people understand it better. A few examples will be presented to help understand these calculations practically, with all times referring to the local time.

The natural solar rhythm (baseline):
Minimum temperature peak – 4:30 am
Biological noon – 12 pm
SWS peak – 9 pm – 12 am
REM peak – 6-9 am

Example 1:
Minimum temperature peak – 5:30 am
Biological noon – 1 pm
SWS peak – 10 pm – 1 am
REM peak – 7-10 am

Example 2:
Minimum temperature peak – 2:30 am
Biological noon – 10 am
SWS peak – 7-10 pm
REM peak – 4-7 am

Example 3:
Minimum temperature peak – 7 am
Biological noon – 14:30 pm
SWS peak – 11:30 pm – 2:30 am
REM peak – 8:30-11:30 am

When attempting to shift one’s circadian rhythm it can be useful to monitor the temperature gradient to assess when a shift has been successfully completed. Shifting the circadian rhythm is easiest done with the use of light cues for short distances, but continuous use of light, food and higher temperatures for large distances. It is currently unclear for what ranges of shifting these cues apply individually and together, but a fair assessment is that shifting the circadian rhythm by more than 2 hours from the solar noon will require more than just light. When utilizing temperature to shift one’s circadian rhythm, the natural temperature gradient should be shifted in the same fashion as the examples above, so that an earlier shift of the circadian rhythm moves the ambient temperature maximum earlier, and a later shift moves the maximum later.

At this point in time temperature monitoring has not been tested on a large scale. The products listed below seem to be capable of performing the task of continuously monitoring the body temperature, but because they have not been tested it is uncertain whether the products linked below serve their intended functions with good results. When several people have examined the optimal equipment for doing this, the information in this post will be expanded. Buy the products (with paid links) at your own risk, and please research whether they perform the needed task on your own before making the purchase.

  1. Win-health Wireless Body Thermometer
  2. Medisan TM 735 Bluetooth Thermometer
  3. iThermometer Thermometer Kit

In conclusion there are three noons. Solar noon, which occurs at a set time each day; local noon, which determines when the solar noon occurs in local time; and the biological noon, that states how shifted the bodily noon is compared to the other two. This information is useful to utilize when designing a schedule to make sure that adequate sleep quality is achieved, especially on dual core and tri core schedules.

Main author: Crimson

Page last updated: 21 October 2019

Baehr EK, Revelle W, Eastman CI. Individual differences in the phase and amplitude of the human circadian temperature rhythm: with an emphasis on morningness-eveningness. Journal of Sleep Research. 2000;9(2):117-127. doi:10.1046/j.1365-2869.2000.00196.x

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