I recently read something about a major earthquake in Japan having possibly shortened Earth's day due to an increase in rotation. Here's an example:


When I read some more about Earth's day length, I learned that the length of Earth's day is believed to have been shorter in the past and getting longer over time. Here's an example from this site:

What was the length of the solar day 73 million years ago?

As some may know from an introductory psychology course, humans tend to settle into a day that is longer than 24 hours when placed in an environment with no external time or light cues. For most people, their circadian system is able to adjust their day to the 24-hour Earth day. Some blind people lose this ability, as seen from commercials about medication to help them gain a 24-hour rhythm. Here is some background about that:


At the risk of asking an unacceptable "opinion" question, how convincing is the evidence of historical Earth day lengths? Is the evidence at a point where shorter Earth days in the past are an accepted certainty? Or are things still at a point of being a best guess without really knowing? It would seem rather odd for our nervous systems to have evolved with a preference for days longer than 24 hours. It is certainly possible, but I would appreciate some input here before spending time contemplating why evolution might have worked that way. I'm only a hobbyist, so there would be no need to go into too much depth for me specifically. Thanks in advance.

Edit: The current answers and comments are interesting and appreciated, but to clarify, the intended focus is the extent and quality of the astronomical evidence that could be used in a discussion (elsewhere) about circadian issues.

  • $\begingroup$ Very interesting question! $\endgroup$
    – Max0815
    Apr 22, 2019 at 1:08
  • 1
    $\begingroup$ "It would seem rather odd for our nervous systems to have evolved with a preference for days longer than 24 hours." I think part of the answer is that we don't have a preference for days longer than 24 hours; the mean human circadian rhythm is about 24.2 hours, and is clearly meant to be continually synchronized by day/night cycling. $\endgroup$ Apr 22, 2019 at 11:26
  • $\begingroup$ @Peter Erwin, my intro psych course and some additional modest study of circadian issues was before the studies claiming to debunk what seemed a well-founded, roughly 25-hour cycle, including in lab animals. I’m sceptical of the later studies, though this is not the space-time for that sort of discussion. :-) $\endgroup$ Apr 23, 2019 at 3:33

4 Answers 4


From a physics point of view, there's no obvious mechanism that could provide significant, consistent increases in the earth's rotation speed.

The moon's gravitation appears to be the main driver of changes to the earth's rotation. And since it revolves more slowly than the earth's rotation, it tends to pull energy from the rotation, lengthening the day. This is completely consistent with the evidence that points to ancient years on earth having more (and therefore shorter) days than is true in the modern era. This is also consistent with the observed gradual recess of the moon away from the earth.

It is also possible to directly measure the length of a day and see that days on earth are significantly longer than they were in 1900. Again, consistent with a slowing rotation.

The implications of this for the evolution of sleep cycles would indeed be better in another location (perhaps Biology SE). But I'd suggest that this is a very slow process.

Remember that this is something that increases the length of the average day. But there are lots of other variations between days during a year. An organism might for instance synch to something like sunrise. But in spring, sunrises come more than a minute earlier than 24 hours apart each day, and in fall they come more than a minute later than average each day.

Meanwhile, it was probably more than a million years ago that the average day was shorter by a full minute. So the increase in day length over time is definitely happening, but at a rate that is almost irrelevant to biology.

  • $\begingroup$ +1 The last sentence sums it up nicely $\endgroup$ Apr 24, 2019 at 12:31

Systemic geophysical investigations into past variation of the earth's LOD (length of day) date at least from the 1960's (Wells, J. W. 1963. Coral growth and geochronometry. Nature. 197:948–950.) Growth indicators--persistent lines or layers deposited at intervals ranging from 6-hour tide cycles to annual--in mollusks, corals, and other invertebrates, both modern and fossilized, give us an objective record of major aspects of the Earth-Moon system, including LOD. Geology offers other corroborating techniques that suggest a monotonic but slightly varying deceleration of the Earth's rotation, and therefore lengthening of day, of about 20 minutes per 100 million years.

Given the relatively slow changes in LOD, the OP can reasonably wonder why mammals don't adhere more closely to a 24-hour or slightly longer daily cycle. The only good answers for the variability of circadian rhythms in mammals are empirical.


Our biological clocks are ancient. One theory about the >24 hour period is that the clocks evolved in the sea, where the tidal period can be more important than the solar day length.

The tides are primarily caused by the Moon, and in the modern era the Moon rises roughly 50 minutes later each day, plus or minus 20 minutes, due to the eccentricity, inclination, and general complexity of the lunar orbit. The actual tidal changes at a given location are quite complex, but that lunar period gives a reasonable first approximation. So it's actually a good strategy to have a clock with a period of 24h 50m but which can easily be adjusted to be in synch with sensory input.

That 24h 50m lunar period would have been different hundreds of millions of years ago, when our ancestors were in the sea. But I'm not sure how different, since although the solar day length was shorter, so was the lunar orbital period, and so the Moon' daily motion relative to the Sun was greater.

  • $\begingroup$ I think that's very unlikely. This study from 2010 mentions that the endogenous human period "has a reported population range of 23.47–24.64 [hours] in laboratory conditions", so it's pretty close to 24 hours and sufficiently variable that you shouldn't conclude there's some deeper meaning behind it. $\endgroup$ Apr 22, 2019 at 11:07
  • $\begingroup$ The simplest explanation is that endogenous circadian cycles don't need to be precise, since they are designed (by evolution) to be continually reset by daily day/night and/or temperature cycles. The fact that, on average, human circadian cycles are slightly longer than 24 hours probably doesn't mean anything. $\endgroup$ Apr 22, 2019 at 11:07
  • $\begingroup$ @Peter Fair enough. BTW, this isn't my theory, but I can't remember where I read about it. FWIW, biology has evolved some pretty nifty clocks, including one that a unicellular organism can pass onto its daughter cells, without resetting the clock. Of course, any evolved mechanism doesn't have much pressure to improve further once it performs well enough in its current environment. $\endgroup$
    – PM 2Ring
    Apr 22, 2019 at 11:15

It can be observed. The astronomical constants at 2000AD_500AD _ 1604AD are given below. The Àryabhatíya of Àryabhata, the oldest precise astronomical constant?

Here are some tables.This table is from Jacobs.

Table 1. Comparison of The Àryabhatiya of Àryabhata and Astronomic values.(orbit times are slowing down ever so slightly) Astronomy Constants AD2000 _ AD 500 _ 1604 BC

Rotations per solar orbit 366.25636031_ 366.2563589_ 366.25635656

Days per solar orbit 365.25636031_ 365.2563589_ 365.25635656

Days per lunar orbit 27.32166120_ 27.3216638_ 27.32166801

Rotations per lunar orbit 27.39646289_ 27.39646514_ 27.39646936

See the number of decimal places. It goes on like this to the Rg veda. India had a passion for astronomy and was very good at it!

Other links. Amartya Kumar Dutta — Aryabhata and Axial Rotation of Earth

Orbits decay ever so slightly in 1000 year time scales!





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