circadian rhythms

• our circadian rhythms are driven by our bodyclocks and synchronised by the master circadian pacemaker
• the master circadian pacemaker must be constantly reset so we are in synchrony with the outside world
• light acts as the primary input and sets the body clock to the right time by photoentrainment
• in mammals, light centres in the eye act as brightness detectors, sending messages about brightness to the SCN
• the SCN uses info about brightness levels to coordinate the activity of the whole circadian system
• light and darkness are the external signals that determine when we feel the need to sleep
• the circadian rhythm dips and rises at different times of the day
• our strongest sleep drive occurs in 2 dips, one at 2-4am and one at 1-3pm
• sleep and wakefulness are also under homeostatic control
• when we have been awake for a long time homeostasis tells us that the need for sleep is increasing because of the amount of energy used up
• the circadian system keeps us awake as long as there is daylight
• the internal circadian clock is free running and will maintain a cycle of about 24 hours even with the absence of external cues
• the circadian system is intolerant of any major alterations in sleep and wake cycles
• core body temperature is one of the best indicators of the circadian rhythm
• core body temperature is at its lowest at around 4:30am and at its highest at 6pm
• in a normal rhythm, sleep begins occurs when core temperature starts to drop
• body temperature begins to rise in the last few hours of sleep to promote a feeling of alertness
• a small drop in body temperature occurs between 2pm and 4pm
• hormone release follows a circadian rhythm
• the release of melatonin follows a circadian rhythm with peak levels during hours of darkness