Before civilization it was just important to be up in time - you needed to wake up in time to use what daylight hours you had. The specific time of the day wasn't as important - sure, you needed to know to some degree how late it was to plan your day, but before civilization there was simply almost no need to meet someone at a specific time when people were living in small tribes.

And there definitely is some sense of time, although it might be very subjective. In my every day life I have trained to keep track of time in my head and I can usually estimate it within 15-30 minutes.

The circadian rhythm is driven by a hormone called melatonin. When we have little melatonin we are awake but also start producing melatonin. When we have a lot of melatonin we get tired but also start breaking down the melatonin. If left completely alone this rhythm will repeat itself roughly every 24 hour. In some people less but in most people more. But things like light and meals will cause the melatonin levels to go down in order to tune the circadian rhythm to an even 24 hours which matches the day and night cycle.

In addition to the circadian rhythm the entire brain does not fall asleep. Part of your brain never sleeps and is able to wake up other parts on demand. This is how we have been able to stay alive for millions of years without getting eaten by predators or taken by landslides even though we need sleep every night. So even when sleeping you are constantly listening out for cues that it is time to get up and might even open your eyelids to scan the environment before falling back asleep. This means that things like a clock in the room, the sound of garbage men outside, light through the window, the neighbors alarm clock, etc. can cue your brain onto the idea that it is now time to wake up.

Your body has various clocks and mechanism. So various hormones such as melatonin and just stuff like temperature control your circadian rhythm.

Basically you have biological clocks that control you.

Like mechanical clocks, it's not perfect and can drift and get longer.

So your body uses things like getting bright light in the morning, exercise, meal times, lack of bright light in the evening, etc. to set and calibrate your circadian rhythm.

A minor correction to your comment.

Left alone the cycle repeats roughly every 25 hours for the average person. In a cave that would be the default sleep pattern. Not 24 hours.

The rest of what you say is pretty spot on but it's important to realise we have a clock that needs active sync, not a 24 hour clock that can look after itself.

A number of factors like blue light hitting your retina, body temperature, when you eat and when you exercise can effect this.

But light is the most powerful cue. The circadian clock in everyone is essentially one that runs at a slow rhythm. But light inhibits melatonin production and darkness accelerates it.

As well as melatonin there's cortisol triggered to wake you up. There's seratonin released in response to light when you're meant to be awake.

This is a concern as we moved away from incandescent lighting as the wavelength of light we're meant to see when the sun is out is more prevalent at home, from screens, in street and car lighting. Etc

https://www.news-medical.net/health/How-Does-the-Suprachiasmatic-Nucleus-(SCN)-Control-Circadian-Rhythm.aspx

Messing with light can get our melatonin, cortizol, and seratonin out of sync with each other. Leading to poor sleep and all the other effects from that.

For good sleep get a good light cycle, then clean air, then temperature, then noise right. So many people think of those in reverse.

I think I touched on most of this but thanks for going more in depth on it.

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apparently almost everyone can do that