So far, we have only discussed rhythms for multi-cellular organisms. Do individual cells have rhythms? Yes!! In the 1950's, Beatrice Sweeney, a pioneering plant clock biologist, was able to measure rhythms of oxygen release in individual Gonyaulax (an alga) cells. Gonyaulax also exhibits a rhythm of luminescence. Various rhythms have been detected and studied in other single-celled organisms such as Paramecium, Euglena, Acetabularia,
cyanobacteria and Chlamydomonas (see Activity section).

In multi-cellular organisms, scientists often study the rhythms of individual cells. In the sea hare, Aplysia californica , optic nerves in eyes removed form the animals exhibit circadian activity rhythms. Cells from a variety of organisms are being studied with respect to rhythms in ion exchange, hormone release, and genetic activity.