Most animals (including humans) have a natural circadian rhythm. Although the sun wakes us up in the morning, if the sun didn’t rise we’d still wake up. Our bodies have an internal rhythm that would keep us waking and sleeping with a regular rhythm. However, our natural rhythm is not exactly 24 hours.

Blind mole rats, for example, maintain a circadian rhythm even though they are, well, blind, and live underground pretty much all the time.

It’s close, but not exact. In fact, it’s the regular rising and setting of the sun that provides the stimuli to make sure we wake and sleep on a 24 hour cycle. In technical language, our circadian rhythm is entrained by visual cues.

Of course the actual story is a lot more complicated than these simple statements, but this is close enough for now.

The natural circadian rhythm can be measured by observing what an animal does in the absence of entrainment by visual cues. In other words, you take away the day/night cycle and put a mouse into an environment that is always dark (or put a human into an environment that is always light; remember that mice are nocturnal) and see what time the mouse gets up every morning to make a cup of tea. By measuring the activity of Well, not that mice make tea, but you get the idea. the mouse (usually on some sort of running wheel) scientists can map visually the periods of waking and sleeping; a typical example, called an actogram, is shown in Fig. 5.25. In this experiment (Nakamura et al, 2016) a mouse was first put into a regular dark/light cycle. After a few days of this, the light was removed and the mouse was put into darkness. You can clearly see how, on each subsequent day, the mouse started its activity just a little bit earlier. This is because the natural circadian rhythm of this mouse was just a bit less than 24 hours.
We know; a long introduction to a short question, but it’ll take some careful thinking to find the answer.
Hint: you can’t do it by measuring the time between when the activity stops and when the activity begins again.
This is because the activity doesn’t stop at a consistent time. You’ll have to think of another way of working it out.
So, finally, here’s the question. What was the natural circadian rhythm of the mouse in this experiment?