[danarmak]

Measuring regular changes in lighting (the day/night cycle) is actual time. (So would be heartbeats, although heartrates vary so much that measuring them would be very inaccurate on a daily scale.)

All definitions and measurements of time rely on counting events that are believed to happen at regular intervals. A second was defined for most of history as 1/246060 of a solar day, in other words, measuring changes in lighting.

[xmprt]

I agree but I don't know if that explains how humans can tell time. If you put a human in a completely isolated room with no lighting changes then I doubt they'd be able to count the number of days.

[danarmak]

Humans can't tell time accurately without external cues about the day and night cycle. We also can't tell time accurately to the minute if we can see outside but don't know how long the day is, i.e. latitude and time of year.

We can tell time inaccurately by timing somewhat regular internal biological events. That pretty much follows from the definition of time as the interval between regular events (which are assumed for physical reasons to be equally spaced). The only question is which underlying events we use and how.

https://en.wikipedia.org/wiki/Time_perception