Yeah, I recall that also. Apple itself published a biorhythm program.[1] (A pic and WAV file are there!) Many 8-bit computer demos showed off their hi-res graphics with a classic biorhythm plot of three overlapping sines.

We know now though, as we basically did back then, that biorhythm theories have never been validated[2] but circadian rhythms are verifiable and appear to be a real feature of animal sleeping and eating patterns.

[1] https://mirrors.apple2.org.za/Apple%20II%20Documentation%20P...

[2] https://en.wikipedia.org/wiki/Biorhythm_(pseudoscience)