6.4.1 Circadian, Infradian and Ultradian Rhythms

Biological rhythms:

Biological Rhythms: patterns of changes in body activity that occur at cyclical periods in time

Types of biological rhythm:

• Circadian – a type of biological rhythm that completes one full cycle every 24 hours
• Ultradian - A complete cycle occurs more than once every 24 hours e.g. stages of sleep, a full sleep cycle takes 90 minutes to complete so can occur multiple times
• Infradian - one full cycle occurs less than once every 24 hours e.g. the menstrual cycle, seasonal affective disorder (SAD)

Ultradian rhythms and stages of sleep

There are five recognised stages of sleep:

• Stages 1 and 2 are the sleep escalator
• Stages 3 and 4 consist of the delta waves of deep sleep
• Stage 5 is REM sleep, the stage in which dreaming occurs according to research

Case study: Siffre

• A case study in which Siffre spent two months in a cave deprived of light and sound determined that his circadian rhythm remained between 24-25 hours. He stayed underground for an extended period of time with no external cues so only his internal clock influenced his behaviour This implies that there was an internal clock that works without the natural day/night cycle, highlighting the role of an endogenous pacemaker in control of his circadian rhythms

Evaluation:

Evaluating biological rhythms:

Circadian:

1. Individual differences- A problem with studying sleep cycles is the differences observed in people, which makes investigating patterns difficult. For instance, although Siffre conducted multiple studies, his results may not be able to be generalised to the wider population. This limits the extent to which the findings represent the experiences of the wider population as they may just represent the experience of Siffre

Ultradian

1. Ecological validity The way research is conducted may show little about ultradian rhythms. This is because when investigating sleep patterns, participants must be attached to monitors that measure rhythms. This may be invasive for the participant, leading them to sleep in a way that is different to their ordinary sleep cycle. Therefore, this makes investigating ultradian rhythms, such as the sleep cycle, extremely difficult as the lack of ecological validity could lead to inaccurate conclusions being drawn.

Infradian

1. Conflicting research - Research by Russell et al (1980) challenges the assumption that the female menstrual cycle is purely controlled by an endogenous (internal) infradian rhythm. Daily samples of sweat were taken from women in one group and applied onto the upper lip of women in another group out of contact. It was found that the menstrual cycles synchronised with the individual donor of sweat despite the fact the groups were kept separate. This suggests that the synchronisation of menstrual cycles is affected by pheromones, which is an exogenous factor