Which gland plays a very important role in the regulation of the $24$-hour (diurnal) rhythm of our body?

"Pinealocytes" are found in which of the following?

The diurnal rhythms are regulated by . . . . . . .

The pineal gland is associated with:

Daily rhythms are usually associated with

$P$ is a small,round,reddish structure located on the dorsal side of the forebrain. It contains a stalk and releases a hormone $Q$ which controls the diurnal rhythm of the body. $P$ and $Q$ are