So what is the SCN?
Mammals have a pineal gland in the brain which releases the hormone melatonin.
Studies show that melatonin induces sleep, for example Abraham (2000) who found that sparrows fell asleep when injected with the hormone. Melatonin production decreases, however, when it is light.
In birds, the pineal gland is near the skull. As the skull in this area is very thin, light can get through to the pineal gland directly and activate a decrease in melatonin production. In humans, and more evolved mammals, however, the pineal gland is in the centre of the brain. This means that no light can reach it directly.
This is when the SCN comes in!
The suprachiasmatic nucleus is, as discussed before, above the optic chiasma. Light reaches the eyes and this triggers reactions down the optic paths to the visual cortex, passing through the optic chiasma. The SCN picks this up and 'signals' to the pineal gland.
Hence why light can effect our sleep/wake cycles :)
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