Infradian rhythms and Ultradian rhythms Flashcards
Infradian rhythms:
Rhythms that have a duration of over 24 hours, and may be weekly, monthly or even annually.
Ultradian rhythms:
Cycles that last less than 24 hours, such as the cycle of sleep stages that occur throughout the night.
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.
This cycle repeats itself about every 90-100 minutes throughout the night, with different stages having different durations.
A complete cycle consists of a progression through the four stages of NREM sleep before entering a final stage of REM sleep, then the whole cycle starts all over again.
Most of what we know about these different stages of sleep comes from recording the electrical activities of the brain, with each stage showing a distinct EEG pattern.
As the person enters deep sleep, their brainwaves slow and their breathing and heart rate decreases.
During the fifth stage (REM sleep), the EEG pattern resembles that of an awake person, and it is in this stage that most dreaming occurs.
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 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.
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 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.
For example, the familiar 10:30 am coffee break allows workers to divide the 9 am to noon morning session into two 90-minute phases.
This pattern is repeated in the afternoon, with cat-naps more likely in mid-afternoon.
types of INFRADIAN RHYTHMS:
Weekly rhythms:
Monthly rhythms: the human menstrual cycle:
Annual rhythms:
Weekly rhythms
The grouping of seven 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.
For example, 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.
It is tempting to look for underlying biological cycles that would dictate these differences.
For example, Halberg et al. (2002) reported seven-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
A woman’s reproductive cycle is known as a menstrual cycle because it lasts about one month (mensis is Latin for month).
There are considerable variations in the length of this cycle, with some women experiencing a relatively short 23-day cycle whereas others have a cycle as long as 36 days (Refinetti, 2006).
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.
Ovulation occurs roughly halfway through the menstrual cycle, when oestrogen levels peak, and usually lasts for 16 to 32 hours.
After the ovulatory phase, progesterone levels increase in preparation for the possible implantation of an embryo in the uterus.
Annual rhythms:
In most animals, annual rhythms are related to the seasons (eg. 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.
Research suggests a seasonal variation in mood in humans, especially in women (Magnusson, 2000), with some people becoming severely depressed during the winter months (seasonal affective disorder).
The winter is also associated with an increase in heart attacks, which varies seasonally and peaks in winter.
In fact, there is a robust annual rhythm in human deaths, with most deaths occurring in January (Trudeau, 1997).
EVALUATION/DISCUSSION
Individual differences in sleep stages:
Research support for the BRAC
The menstrual cycle - the role of exogenous cues:
Individual differences in sleep stages:
Differences in the sleep patterns are usually attributed to differences in non-biological factors (e.g. sleep hygiene) but a study by Tucker et al. (2007) suggests that these differences are in large part biologically determined and may even be genetic in origin.
Participants were studied over 11 consecutive days and nights in a strictly controlled laboratory environment.
The researchers assessed sleep duration, time to fall asleep and the amount of time in each sleep stage.
They found large individual differences in each of these characteristics, which showed up consistently across the 8 nights. For deep sleep (stages 3 and 4), the individual differences were particularly significant.
This meant that differences between participants were not driven by circumstances, but were at least partially biologically determined.
Research support for the BRAC:
Ericsson et al. (2006) provide research support for the BRAC in a study of elite performers.
They studied a group of elite violinists and found that, among this group, practice sessions were usually limited to a duration of no more than 90 minutes at a time, with practice systematically distributed during the day in these 90-minute segments.
This supported Kleitman’s claim that fatigue was a characteristic of the end of the BRAC cycle.
Ericsson’s analysis also indicated that the violinists frequently napped to recover from practice, with the very best violinists napping more than their teachers.
Consistent with the predictions of the BRAC, Ericsson discovered the same pattern among other musicians, athletes, chess players and writers.
The menstrual cycle - the role of exogenous cues:
The menstrual cycle is normally governed by an endogenous system - the release of hormones by the pituitary gland.
However, it can also be controlled by exogenous cues.
When several women of childbearing age live together and do not take oral contraceptives, their menstrual cycles tend to synchronise.
In one study, daily samples of sweat were collected from one group of women and rubbed onto the upper lips of women in a second group.
The groups were kept separate yet their menstrual cycles became synchronised with their ‘odour donor (Russell et al., 1980).
This suggests 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.
