Three Noons and Their Importance in Determining Circadian Shifts


It is likely uncommon to hear of the existence of “three noons” and their importance for circadian shift. The circadian rhythm is a biological rhythm in charge of timing many bodily processes. Examples include:

  • Melatonin
  • Testosterone secretion,
  • The daily body temperature gradient
  • The peaks for most efficient SWS and REM sleep

Because these events happen at fixed timings, it is beneficial for people to be able to predict the timing of the circadian rhythm compared to the hours on the clock. This is possible with comparing the noon to the minimum daily temperature. But first, the term “noon” needs a specific definition. There are three types of noons to establish the circadian rhythm’s shift: solar noon, local noon and biological noon.

Solar noon

The solar noon refers to the timing when the sun is at the highest position in the sky.

  • This happens at the same time every day on a specific longitude (ignoring daylight savings time). Thus, it is a good event to compare to other set timings.
  • The solar noon intends to biologically happen at 12:00 pm. This means that an unshifted circadian rhythm would have its natural SWS peak at 9 pm – midnight; the REM peak will be at 6-9 am.

Local noon

However, your time zone may not be in sync with the solar time.

  • For example, if solar noon occurs at 1 pm, it naturally shift your sleep peaks by 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 the local noon; in other words, the local time when the solar noon occurs.
  • Some countries have a shifted local noon for economic or cultural reasons. For example, Netherlands and Spain.
  • You can find the timing of your solar noon here.

Biological noon

The third type of “noon” is the biological noon.

  • This is the time of the day that your body treats as noon.
  • Late-night use of electronic screens nowadays often alters the timing of the biological noon.
  • Establishing the timing of the biological noon is important to help you assess optimal time for sleep. 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 for comparison with the natural, solar-based rhythm. This would serve to determine how big the circadian shift is.

So far, the easiest method for 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. Thus, by comparing the timing of that event to the local noon, it is possible to determine the timing of the biological noon. 
  • Additionally, the timing of the noon is not necessary to establish the shift of the circadian rhythm. However, it helps some people understand it better.


Below are a couple examples to help understand these calculations practically. All times refer to the local time.

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Natural solar rhythm

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 you attempt to shift your circadian rhythm, it can be useful to monitor the temperature gradient to assess when a shift has been complete. Shifting the circadian rhythm is easiest with the use of light cues for short distances. However, continuous use of light, food and higher temperatures would suit large distances.

It is currently unclear for what ranges of shifting these cues apply individually or together.

  • Nevertheless, 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 you shift your circadian rhythm using temperature, the natural temperature gradient should shift in the same fashion as the examples above. This would help an earlier shift of the circadian rhythm move the ambient temperature maximum earlier.  A later shift would then move the maximum value to later hours. 

Body temperature monitoring

At this point in time, temperature monitoring testing on a large scale has been limited. The products below seem to be capable of continuously monitoring the body temperature. Nonetheless, they have not been tested. Thus, it is uncertain if these products would serve their intended functions with good results.

When several people have examined the optimal equipment for this, there will be more information in this page. Buy the products (with paid links) at your own risk; please research if they perform the necessary task for you before making the purchase.

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

Shifting the circadian rhythm

A shifted circadian rhythm in relation to the natural day-night cycle may be desirable to better accommodate for a particular sleep schedule. Alignment of sleep blocks with the sleep stage peaks is what matters the most. However, this is generally not advisable when working a day job or participating in daytime activities; it requires extra care and planning.

Regardless, many people have to resort to this to be able to handle a third shift job or voluntarily participate in late social events, etc. There is only one main way to shifting the circadian rhythm; in addition, it is going to take some time.


Stop the dark period at the desired new circadian morning by using light and food to start it.

  • This method is also useful for people who are adapted to a polyphasic sleeping schedule and want to rotate their schedule. Confusing the circadian rhythm is not something you want while relying on a fixed circadian rhythm. This is the best and preferred way to adapt to a new circadian time.
  • Academic researchers created an app called Entrain to reduce jet lag by designing optimal schedules for circadian rhythm shifting. It is compatible with slow circadian shift, as it can adjust the circadian rhythm optimally. However, polyphasic sleepers should not submit any of their results through the app. The researchers are interested in results related to jet lag; they will use the results for different purposes and skew the data.
  • To maintain this rhythm, using light and food to wake up and having a fixed dark period should be satisfactory. However, if the circadian rhythm is shifted a lot compared to the local day/night cycle, temperature cycle is going to be useful. Exercising during the circadian morning could also be beneficial.

Other notes

If the circadian goes through an unintentional shift, for example by an overly short dark period, it can reduce SWS or REM in the core. This is because the sleep peaks have moved. As a result, people will have difficulty adapting. They will also experience feelings of tiredness, sleep deprivation and get SWS rebounds. All in all, these detriments increase the chance of oversleeping drastically.

Especially, an increased amount of sleep deprivation will cause difficulties when you shift the circadian rhythm. Experiments with phase shifts of the circadian rhythm with increased sleep homeostatic pressure suggest that the circadian clock is less susceptible to phase shifts with high sleep pressure. What this means is that if you aim to shift your circadian rhythm, you should preferably not combine it with a polyphasic sleep adaptation. Instead, spend time shifting the rhythm first. Then, start the polyphasic adaptation after your circadian shift.


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.
  • Biological noon, that states how much bodily noon shifts compared to the other two.

This information is useful when designing a schedule. The purpose is to secure adequate sleep quality, especially on dual core and tri core schedules.

Main author: Crimson

Page last updated: 28 December 2020

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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