The suprachiasmatic nucleus is a bundle of nerve cells in the hypothalamus which helps maintain circadian rhythms.
Nerve fibres from the eye cross at the optic chiasm on their way to the right and left visual areas. The SCN is positioned above the optic chiasm and receives information about light from this structure.
DeCoursey destroyed SCN connections in 50 chipmunks which were returned to their natural habitat + observed for 80 days. Their sleep - wake cycle disappeared completely and many were killed by predators.
Ralph bred 'mutant' hamsters with a 20 hour sleep-wake cycle. SCN cells were transplanted from the foetal tissue of these hamsters into the brains of normal hamsters, which then developed the 20 hour cycle.
The SCN passes information on day length to the pineal gland which increases production of melatonin during the night. Melatonin is a hormone that induces sleep and is inhibited during periods of wakefulness. It’s also been suggested as a causal factor in seasonal affective disorder.
