I know many Anons knows this, but, it's worth reminding and maybe put more pieces of the puzzle together. See it from another angle possibly.
.
A circadian clock, or circadian oscillator, is a biochemical oscillator that cycles with a stable phase and is synchronized with solar time.

Such a clock's in vivo period is necessarily almost exactly 24 hours (the earth's current solar day). In most living things, internally synchronized circadian clocks make it possible for the organism to anticipate daily environmental changes corresponding with the day–night cycle and adjust its biology and behavior accordingly.
.
What is really pineal gland? Think of it like some sort of Antena that connects to the Cloud (of some Higher possibly collective knowledge). During sleep (while dreaming) we seem to be able to connect with the "Cloud".

Energy, frequency, vibration.

Some may explain it in other, more spiritual terms. I'm more technical.

Also, Organs have their own rhythm throughout each day. If you're having trouble sleeping at a certain time it could be caused by the organ associated with that time being overworked.

- Lungs = when Buddhists wake up for meditation
- Large Intestine time = morning toilet trip
- Stomach time = Bfast is most important meal of the day etc.