Umm. I hope I'm not misinterpreting your comment, but time keeping is innate to the body. It does not need any "conscious effort". And almost all multicellular life on earth does it. Most unicellular organisms also have (I think) some equivalent of a circadian rhythm. [1] There's a large body of evidence that the circadian rhythm is self generated, starting from this classic self experiment in 1938 [2].

Sure, you need external cues to synchronise / entrain it, but the body does keep its own time. As do animals, plants, insects..

[1] https://en.m.wikipedia.org/wiki/Circadian_rhythm

[2] https://www.google.co.in/amp/s/www.the-scientist.com/foundat...

Will "the brain", which is in question here, not loose the attained rhythm once the stimuli are removed?

Even if making a stronger claim not focused on the brain specifically, plants especially need light for like everything.

That's not really true. Take a look at the experiments performed all the way back in the 18th century [1], where mimosa plants followed a rhythm even when they were kept in an absolutely dark room.

[1] https://en.wikipedia.org/wiki/Jean-Jacques_d%27Ortous_de_Mai...

More work since then extends this idea. Pubmed has a ton of good papers.

I think the prime example where uncouncious rhythm is concerned would be the period, the menstruation cycle. The correlation with the moon phase is probably no coincidence I guess, and I somewhat doubt it's magnetic, but suppose it's gravitational - which is an external factor. Anyhow that's not experienced time keeping. Rather it may be used for time keeping, or rather other externals like hair growth for example or feeling tired. "I'm tired, it must be late huh" is what I was talking about more than "It's late, I must be tired" though the later should be imperative for me, right now (here near the Greenwich Line).

And for the 1930s exp you posted, i suppose its just the energy reserves of whichever crucial part for the older were either fine tuned or not extendable, whereas a 20 year old (risk inclined) college student can still pull allnighters twice a weak and other endurance exercises. Physical fitness peaks before the 30s. And a prof is someone working with the mind so the experiment itself without intense physical strain is taxing enough. They self tested, that alone is probably a huge warningsign.

But i was mainly refering to plants that can be brought to flower by light cycle manipulation. Now I kind of wonder how mimosa reacts circadian anyway ... and I guess it's due to relative changes in the air humidity.