Our fairly consistent sleep pattern suggests an internal or endogenous mechanism”the biological clock. But this can be overridden by psychological factors such as anxiety. An endogenous (internal) clock (or set of clocks) is proposed as some sort of innate mechanism that sets the timing of biological rhythms. Their tick may be a 24-hour one, which is the case for the sleepwake cycle, an 8-hour one, which is the case for the rhythms of sleep or even a 12-month one, as in patterns of hibernation.
The suprachiasmatic nucleus (SCN) is the pacemaker of the biological clock in the sleepwake cycle. The SCN generates its own innate biological rhythm due to protein synthesis. It is connected to the optic chiasm and so receives input about the amount of light (an exogenous or external factor), which also influences the rhythm. Therefore, light stimulates nerve impulses which travel from the eye, along the optic nerve to the brain.
The pineal gland is linked to the SCN by a neural pathway and so the electrical stimulation of the SCN is transferred to the pineal gland and this causes it to release melatonin. Light and melatonin are inversely proportional, i. e. when light levels are low high amounts of melatonin are released. Melatonin is a hormone that induces sleep because it enhances production of the neurotransmitter serotonin and this causes the nervous system to slow down, brain activity consequently falls, and sleep begins.
With regards to exogenous factors, light is the key external factor which influences our circadian rhythm. Without light, our natural sleep/wake cydle would be longer than 24 hours, at between 25 and 27 hours. Cave men many years ago would have slept longer than modern man, with the reason being due to improved technology, such as electric lights and alarm clocks. These days, even after sunset, streets and houses are lit and we are able to produce light 24 hours a day.