light as an exogenous zeitgeber

• • The biological-clock is reset each day by cues in the environment – like the light cues of sunrise and sunset. This process is called entrainment, (the opposite of free-running where the biological clock works free of any exogenous cues, including social cues like clocks).
• • Light acts an exogenous zeitgeber as it is received from the environment, such as the sun/daylight. It is a social cue which influences our body clock.
• This prompts our bodies to experience chemical changes which are detected by the SCN which is in the hypothalamus in the brain.
• The hypothalamus sends a signal to the pineal gland where melatonin is released in the evening which makes us feel sleepy.
• This is detected by ‘light sensitive cells’ within the SCN. This results in melatonin production ceasing in the morning when there is light, and serotonin being released.