As day turns into night, and night turns into day, the vast majority of living organisms follow a fixed circadian rhythm that controls everything from sleep needs to body temperature.
In the brain, clock genes are particularly active in the so-called suprachiasmatic nucleus.
It sits just above the point where the optic nerves cross and sends signals to the brain about the surrounding light level.
From here, the suprachiasmatic nucleus regulates the rhythm of a number of other areas of the body, including the cerebellum and the cerebral cortex.
However, these three areas of the brain are not directly linked by neurons, and this made researchers at the University of Copenhagen curious.
'In humans, the hormone is known as cortisol, and although the sleep rhythm in rats is the opposite of ours, we basically have the same hormonal system', says Associate Professor Martin Fredensborg Rath of the Department of Neuroscience.