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Argument favouring learning:
A newborn sleeps for 20-22 hours. But overtime (s)he learns to focus sleeping time to night time, according to his or her needs and family needs. Some sleep from 1 am to 7 am, some from 11pm to 5 am. So it is learnt.

Argument favouring a genetic origin:
Somewhere I read (I don't exactly remember the article) where it was told that the biological clock is not of 24, but of 25 hours. This means that the synchronisation is not maintained with the earth. If it would be learnt, then why is it not in synchrony with the world?

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  • $\begingroup$ The circadian clock is hardwired at ~25h and reset according to various cues, mainly day/night, but also time of food supply, parents who put you to bed at a certain time, among other things. And kids definitely sleep longer than 6 hours :) $\endgroup$ – AliceD May 26 '15 at 5:14
  • $\begingroup$ The sleeping time was just an example... Also AliceD, can you give me more details of circadian clock you are speaking of? May be some article. Also, it is not clear what is the answer to the question. $\endgroup$ – Mitradip Das May 26 '15 at 5:33
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    $\begingroup$ related question: How do humans perceive time? $\endgroup$ – Luigi May 26 '15 at 13:49
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Short answer
The biological master clock is neurologically and genetically hardwired. The biological clock is not affected by learning. Instead, it is constantly entrained mainly by daylight.

Background
The biological clock is hardwired in the human body and can be traced back to the master clock in the suprachiasmatic nuclei (SCN) of the hypothalamus (Fig. 1).

SCN
Fig. 1. The SCN is entrained by environmental light captured by the retina in the eyes. Source: Neuroscience News.

The SCN is an intrinsic oscillator that governs sleep and wake timing, rhythms of temperature, hormones, mood and cognitive acuity etc. etc. These rhythms are entrained to 24 hours by the environmental light-dark cycle primarily via a subset of photosensitive retinal ganglion cells that project directly to the SCN (Fig. 1). Using hormones and neuronal signals, the SCN entrains peripheral clocks of similar molecular mechanism present in many tissues (Pagani et al., 2010).

In humans and other organisms, the timing of 24-hour behavior is governed by the period length of the circadian oscillator. This period is approximately, but not exactly, 24 hours long (circa diem), and ranges from 23.47–24.64 across people (23 hours 28 min to 24 hours 38 minutes) in laboratory conditions (Pagani et al., 2010).

Why is it circa diem and not exactly 24 hours? Note that the absolute length of a day depends on the definition used. For example, one complete rotation of the earth around its axis (the sidereal day) actually takes 23 hours, 56 minutes and 4.1 seconds. The solar day is 24 hours and is defined by the time it takes the sun to make an apparent circuit across the skies. More importantly, there is apparent solar time, sometimes called true solar time, which is determined by the daily apparent motion of the observed sun. It is based on the interval between two successive returns of the sun to the local meridian. The length of a solar day varies throughout the year, and the accumulated effect of these variations (equation of time) produces seasonal deviations of up to 16 minutes.

The length of a day can also be defined in the number or hours from sunrise to sunset, which varies where you are on earth. At the equator every day of the year is exactly 12 hours long. At 50° latitude (e.g., England & Canada) the longest day is 16.5 hours and the shortest day 7.5 hours. At 35° latitude (e.g. many US states and Greece) the longest day is 14.5 hours and the shortest day 9.5 hours. Given that light is the most effective way to entrain the circadian rhythm, it is obvious why the absolute length of the intrinsic circadian rhythm is not important. As long as it is circa diem, it will do.

Reference
- Pagani et al., PloSONE (2010); 5(10): e13376

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    $\begingroup$ Good answer. Note however that the complete rotation of Earth on itself is not the definition of a solar day: you want the duration between two occurrences of the sun being at the zenith, and this is 24h. (23h56'4.1'' is the duration before two occurrences of a given star being at zenith) $\endgroup$ – Joce May 28 '15 at 6:22
  • $\begingroup$ @Joce - thanks for your suggestion - I will look into this later. Are you saying my information is incorrect then? If so, I will make some edits. Thanks again! $\endgroup$ – AliceD May 28 '15 at 6:27
  • $\begingroup$ It's not incorrect, but I'd say that the duration of earth revolution upon itself is not relevant : 23h56min is the time until stars are back at the same position, for circadian rhythm it is the time until the sun is back at the same position which is relevant. (Then on top of that you have the "true solar time" correction) $\endgroup$ – Joce May 28 '15 at 22:35
  • $\begingroup$ @Joce - I adapted the answer $\endgroup$ – AliceD May 29 '15 at 2:45
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Is the human biological clock genetically programmed or learnt?

It is genetically programmed.

A newborn sleeps for 20-22 hours. But overtime (s)he learns to focus sleeping time to night time, according to his or her needs and family needs. Some sleep from 1 am to 7 am, some from 11pm to 5 am. So it is learnt.

Sleep clears the brain of toxic metabolic byproducts. A newborn have different sleep requirement than an adult (it would be an interesting question why). The connection with the age is something like this:

sleep requirements by age

- Figure 1. - Sleep requirements by age - ref

So it is not learnt, if they wouldn't sleep enough, they could have health problems or they could even die because of brain damage.

Somewhere I read (I don't exactly remember the article) where it was told that the biological clock is not of 24, but of 25 hours. This means that the synchronisation is not maintained with the earth. If it would be learnt, then why is it not in synchrony with the world?

It is in sync with the world, every morning the blue light resets the clock, that's why we tend to use red light at night.

There is a clock in the hypothalamus (brain), which is entrained directly by the daily light/dark cycle. There are peripheral clocks in the liver, heart and in other tissues (probably in every single cell), which are weakly affected by the light/dark cycle, and strongly affected by the food intake (probably by other factors as well). The sleep/awake rhythm depends on all of these clocks. So it depends on the actual light/dark pattern and the metabolic (feeding) pattern of the individual.

When food is plentiful, circadian rhythms of animals are powerfully entrained by the light-dark cycle. However, if animals have access to food only during their normal sleep cycle, they will shift most of their circadian rhythms to match the food availability.

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  • $\begingroup$ If they [newborns] won't sleep they would die because of brain damage? Do you have any reference to that? The linked NIH website doesn't mention that. +1 for the reference to food-based resetting $\endgroup$ – AliceD May 29 '15 at 3:06
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    $\begingroup$ @AliceD I just applied logic, people die because of too much sleep deprivation, but you are right, it is exaggeration, because less, but chronic sleep deprivation can cause just health issues, but not instant death. I'll edit that part, thanks! :-) $\endgroup$ – inf3rno May 29 '15 at 4:12
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    $\begingroup$ @AliceD Edited. Not the best references, but I think it is enough in this case. $\endgroup$ – inf3rno May 29 '15 at 4:27
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    $\begingroup$ Better for sure, great. Can't +1 you again unfortunately :) $\endgroup$ – AliceD May 29 '15 at 4:28

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