Historical Day Lengths on Earth 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:

https://www.nasa.gov/topics/earth/features/japanquake/earth20110314.html

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:

https://en.wikipedia.org/wiki/Non-24-hour_sleep%E2%80%93wake_disorder

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.
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.