Outline and evaluate research into infradian and/or ultradian rhythms (16 marks)
Infradian rhythms are biological rhythms that last over 24 hours. Examples of this include
the female menstrual cycle and seasonal affective disorder (SAD). The menstrual cycle is
endogenous, controlled by the hormones oestrogen and progesterone which stimulate
ovulation and thicken the uterus in preparation for a fertilised egg. On average, this cycle
lasts 28 days although can range from 24-35. SAD is a form of depression which occurs in
the winter months when it is darker due to an increased production of melatonin. The
increased hours of darkness stimulate the pineal gland to produce more melatonin, the
hormone associated with sleep. This is thought to counter-act the levels of serotonin,
therefore producing depressive symptoms such as low mood and a lack of energy.
McClintock’s 1998 study challenged the idea that the menstrual cycle is purely endogenous
through the use of pheromones. Out of a group of 29 women with irregular periods, 9 wore
a cotton pad under their armpit for 8+ hours. The pad was treated with alcohol and frozen
before being rubbed on the upper lip of another woman for each day of her cycle. The
results showed that 68% of the women had their period and menstrual cycle sync closer to
their ‘odour donor’. This suggests that the menstrual cycle is not just endogenous and can
be influenced by exogenous zeitgebers such as pheromones. This would’ve been
advantageous to our ancestors as women could be fertile at the same time, meaning
offspring would grow up at the same time. However, criticisms of McClintock’s study
include the fact that a small sample size was used, meaning low population validity, so
results cannot be generalised universally. Furthermore, the results were not 100% for each
individual case and are therefore not completely accurate.
Schank (2004) questioned the validity of the ‘evolutionary advantage’ deducted from
McClintock’s study. They argued that if all females were fertile at the same time, the
competition for a fit, suitable mate with desirable characteristics would be higher. This
would result in the production of multiple lower-quality offspring, rather that ones who
hold advantageous genes for survival. This may discredit the evolutionary basis of
McClintock’s 1998 study.
A strength of seasonal affective disorder (SAD) is that it has led to practical applications,
this being in the form of phototherapy. The aim of this therapy is to counter-act the levels
of melatonin in the body by sitting in front of a bright white light for a period of time.
Eastmann’s study into phototherapy found that it worked for 60% of participants/patients
with SAD, proving it to be somewhat successful. However, in another part of their study,
the placebo effect was used, with 30% claiming the therapy to be successful despite using
a ‘sham negative-ion generator’. This placebo effect lowers the validity of phototherapy
as a treatment to maintaining SAD as people may claim to feel better just because they
are being treated for something, so their body and brain tricks them into believing that
they are ‘cured’, when in reality nothing about their physiological state has changed.