I guess instead of "natural", if you say "inherently biological" or "circadian" it conveys the sense better. I think the author intends to refer to the natural

I guess instead of "natural", if you say "inherently biological" or "circadian" it conveys the sense better. I think the author intends to refer to the natural cycles inherent within the human body without the aid of devices. In that sense, sunlight and the day-night cycle, and the cycle of seasons may be a better indicator since they have a large enough effect on the human body even in the absence of other mechanical aids.