Biological Rhythms 1

Outline Circadian rhythms 

• Circadian rhythms last 24 hours.
• The sleep wake cycle is one circadian rhythm.
• An endogenous factor of this is the SCN, the body's masterclock, which works with the pineal gland to produce melatonin as night approaches.
• An exogenous factor is sunlight which entrains the body's 24 hour clock. Hormones and neurotransmitters cause the body to respond to different levels of light. 

Outline ultradian rhythms 

• Ultradian rhythms last less than 24 hours, including the different stages of sleep.
• There are 5 sets of 90 minute cycles.
• The first four stages are NREM sleep and the final stage is REM sleep which is also the stage in which we dream.
• The first stage has EEG activity characterized by alpha waves.
• The second stage has EEG activity characterized by theta waves.
• The third and fourth stages have EEG activity characterized by delta waves.
• There is slow wave sleep during the fourth stage.
• The endogenous factor is the RAS which prevents muscle movement.
• The other endogenous factor is the thalamus which prevents sensory information entering the brain.
• The exogenous factors are sunlight and temperature. 

Outline Infradian rhythms 

• One example of an infradian rhythm is the menstrual cycle which lasts longer than 24 hours.
• The endogenous factor is the pituatry gland which releases FSH and LH and these cause oestrogen and progesterone to be released in the ovaries. 
• The exogenous factor is pheromones, which are also known as external hormones, that cause the synchronisation of others' infradian cycles.