ultradian and infradian rhythms

• ultradian rhythms: in humans daily cycles of wakefulness and sleep follow a circadian rhythm however within the sleep portion of this cycle another type of rhythm exists - an ultradian rhythm
• ultradian rhythms: a biological rhythm is referred to as ultradian if its period is shorter than 24 hours, in humans a classic example of such a rhythm would be the 5 stages that make up a typical night's sleep
• sleep stages: the ultradian rhythm found in human sleep follows a pattern of alternating REM (rapid eye movement) and NREM (non-rapid eye movement) sleep, which consists of stages one through to four
• sleep stages: this cycle repeats itself about every 90-100 minutes throughout the night w/ different stages having different durations
• sleep stages: a complete cycle consists of a progression through the 4 stages of NREM sleep before entering a final stage of REM sleep then the whole cycle starts all over again
• sleep stages: most of what we know about these different stages of sleep comes from recording the electrical activities of the brain w/ each stage showing a distinct EEG pattern, as the person enters deep sleep their brainwaves slow and their breathing and heart rate decreases
• sleep stages: during the 5th stages (REM sleep) the EEG pattern resembles that of an awake person and it is in this stage that most dreaming occurs
• stage 1: 4-5% light sleep, muscle activity slows down, occasional muscle twitching
• stage 2: 45-55% breathing pattern and heart rate slows, slight decrease in body temperature
• stage 3: 4-6% deep sleep begins, brain begins to generate slow delta waves
• stage 4: 12-15% very deep sleep, rhythmic breathing, limited muscle activity, brain produces delta waves
• stage 5: 20-25% rapid eye movement, brainwaves speed up and dreaming occurs, muscles relax and heart rate increases, breathing is rapid and shallow
• the basic rest activity cycle: Kleitman (1969) referred to the 90 minute cycle found during sleep as the Basic Rest Activity Cycle or BRAC, however Kleitman also suggested that this 90 minute ultradian rhythm continues during the day even when we are awake
• the basic rest activity cycle:the difference is that during the day rather than moving through sleep stages we move progressively from a state of alertness into a state of physiological fatigue approximately every 90 minutes
• the basic rest activity cycle:research suggests that the human mind can focus for a period of about 90 minutes and towards the end of these 90 minutes the body begins to run out of resources resulting in loss of concentration, fatigue and hunger
• the basic rest activity cycle: the operation of the BRAC in wakefulness is not as obvious as it is in sleep but argued Kleitman everyday observations provide evidence of its existence e.g. the familiar 10:30am coffee break allows workers to divide the 9am to noon morning session into 2, 90minute phases
• the basic rest activity cycle: this pattern is repeated in the afternoon w/ cat-naps more likely in mid-afternoon
• infradian rhythms: infradian rhythms are rhythms that have a duration greater than 24 hours i.e. longer than circadian rhythms, they may cycles lasting days, weeks, months or may even be annual
• infradian rhythms: they include female menstrual cycle in humans (monthly cycles) and the organisation of human activities into weeks (weekly cycles)
• weekly rhythms: the grouping of 7 days into a unit called a week is common in most areas of the world and there are obvious (and sometimes less obvious) differences in human behaviour that conform to this weekly cycle
• weekly rhythms: e.g. although male testosterone levels are elevated at weekends and young couples report more sexual activity at weekends than on weekdays, the frequency of births at weekends is lower than on weekdays
• weekly rhythms: it is tempting to look for underlying biological cycles that would dictate these differences e.g. Halberg et al (2002) reported 7 day rhythms of blood pressure and heart rate in humans but the evidence for weekly infradian rhythms in humans remains sketchy at best
• monthly rhythms-the human menstrual cycle: a woman's reproductive cycle is known as a menstrual cycle because it lasts about 1 month (mensis is latin for month)
• monthly rhythms-the human menstrual cycle: there are considerable variations in the length of this cycle w/ some women experiencing a relatively short 23 day cycle whereas others have a cycle as a long as 36 days (Refinetti, 2006)
• monthly rhythms-the human menstrual cycle: the average appears to be around 28 days, the menstrual cycle is regulated by hormones which either promote ovulation or stimulate the uterus for fertilisation
• monthly rhythms-the human menstrual cycle: ovulation occurs roughly halfway through the menstrual cycle when oestrogen levels peak and usually last for 16-32 hours, after the ovulatory phase progesterone levels increase in preparation for the possible implantation of the embryo in the uterus
• sex hormone cycle: oestrogen is a quick high peak, progesterone is a longer less steep peak
• annual rhythms: in most animals annual rhythms are related to the seasons (e.g. migration as a response to lower temperatures and decreased food sources in winter) but in humans the calendar year appears to influence behaviour regardless of changes in temperature
• annual rhythms: research suggests a seasonal variation in mood in humans especially in women (Magnusson, 2000) w/ some people becoming severely depressed during the winter moths (seasonal affective disorder)
• annual rhythms: the winter is also associated w/ an increase in heart attacks which varies seasonally and peaks in winter, in fact there is a robust annual rhythm in human deaths w/ most deaths occurring in January (Trudeau, 1997)
• evaluation S: Ericsson et al (2006) provide research support for the BRAC in a study of elite performers, Ericsson's analysis also indicated that the violinists frequently napped to recover from practice w/ the very best violinist napping more than their teachers, consistent w/ the predictions of the BRAC Ericsson discovered the same pattern among other musicians, athletes, chess players and writers
• evaluation S: the menstrual cycle is normally governed by an endogenous system - the release of hormones by the pituitary gland however it can also be controlled be exogenous cues, suggesting that the synchronisation of menstrual cycles can be affected by pheromones which act in a similar way to hormones but have an effect on the bodies of people close by rather than the body of the person producing them
• evaluation S: despite empirical evidence to the contrary the belief in an infradian rhythm based on the phases of the moon remains strong, occasional studies have found correlations between the phase of the moon and various aspects of human behaviour but there is no evidence of a causal relationship (Foster and Roenneberg, 2008)
• evaluation infradian L: there are large individual differences in the duration of rhythms which may be due to biological causes or to external or exogenous factors that affect the onset of rhythms e.g. the onset of menstruation can be affected by exposure to pheromones released in the sweat of women at another stage of their cycle
• evaluation ultradian S: there is strong evidence for the sleep cycle, EEGs monitor brainwaves as people sleep and show distinct stages in the sleep cycle that repeats several times a night
• evaluation ultradian L: Tucker et al (2007) although broadly supportive of this finding showed large individual differences in the duration of stages