circadian rhythms

• Circadian rhythms are biological cycles lasting 24hours like the sleep/wake cycle, which is facilitated by time-checks and regular events such as meal times (external cues).
• The main internal biological clock in mammals appears to be located in the hypothalamus, which is responsible for “motivation” and is named the suprachiasmatic nucleus (SCN). 
• The SCN has an inbuilt circadian firing pattern as when damaged in rats the circadian rhythm involving sleeping and feeding patterns has been disrupted (Zucker et al). The SCN regulates the secretion of melatonin in the pineal gland (another endogenous pacemaker which produces melatonin which affects sleep) and is also connected to the retina of the eye through a separate pathway. This highlights the indirect link between exogenous zeitgebers such as light and how melatonin production from the Pineal gland (endogenous pacemakers) works together with the SCN to maintain a rhythm.
• Light can also reach the brain via other means as Campbell et al demonstrated resetting the circadian rhythm through shining light on participants knee’s. This shows other secondary oscillators exist throughout the body maintaining circadian rhythms through the use of exogenous zeitgebers.
• Core body temperature is another circadian rhythm which sees its lowest point at 4:30am (36 degrees) and highest at around 6pm (38 degrees). A slight trough occurs after lunch and this dip occurs even when people do not eat.
• Hormone production also follows a circadian rhythm with cortisol at its lowest around midnight and peaking at 6am. Cortisol plays a role in making us alert and explains why if awaken at 4am we struggle to think clearly. Melatonin and growth hormone also have a circadian rhythm with both peaking at midnight.