Here is another quote from a scientific article.
Mammalian pineal gland: 7-Day Rhythmic Activity In their
researchon rats at London University in 1975, Vollrath, et al. [1]
confirmed the fact that the mammalian Pineal Gland undergoes
prominent circadian changes in function, which are quite dependent
on environmental lighting conditions. They reported that there are
indications which suggest that the Pineal Gland of rats kept in their
laboratories exhibited in addition to 24 hour cycles, characteristic
7-day cycles. It was unclear to them whether the rhythms were
caused by internal or external factors and whether the rhythm was
just a peculiarity of rats kept under environmental conditions of
their laboratory.
The finding that concerns us for the purposes of this
discussion however, is that HIOMT enzyme activity under natural
lighting conditions was HIGHEST on Saturdays and lowest on
Thursdays; the Thursday values being six times lower than those on
Saturdays. A gradual decrease in enzyme activity was noted between
Saturdays and Thursdays. A striking increase occurred between
Fridays and Saturdays.
https://www.sciresliterature.org/Biology/SJB-ID28.pdf
