AO3

• There is research to support the idea that some ultradian rhythms are biologically determined. For example, Tucker et al found large differences between individuals' sleep patterns, which were consistent over 11 nights in a controlled sleep laboratory. This suggests that sleep patterns may be at least partially determined by genes
• exogenous zeitgebers cues: Russell has shown that when several women live together and do not take oral contraceptives they tend to menstruate at the same time every month. He did this by rubbing sweat taken from one group onto the upper lips of women in another separate group. They synchronised cycles! This was due to the presence of Pheromones which act like hormones and have an effect on people close rather than the person.
• Synchronisation is adaptive. There is an evolutionary advantage to synchronised menstruation, as synchronised conceptions would mean that childcare could be shared (Bentley, 2000). Thus, synchronisation is an adaptive behaviour. This supports the research findings and thus validity.
• SAD occurs in sunny countries. Fredda et al. found a similar rate of SAD to that found in the Arctic circle sample in a retrospective study of seasonal mood disorders in Italy, a Mediterranean country known for its sunshine both in summer and most of the winter! There are weaknesses of this research as it was retrospective. If SAD does occur in sunny countries then it may well be that this condition is far more complex than just the amount of natural light.
• Biological rhythms are incredibly consistent for the individual but there are big differences between individuals! Even though we are led to believe that

  individual differences are down to differences in non-biological factors such as sleep routine temperature etc …Tucker conducted a sleep study over 11 consecutive days.
• Tucker conducted a sleep study over 11

  consecutive days.

  • He assessed for sleep duration, time to fall asleep and the amount of time in each stage. He did find large variations between different individual’s particularly in terms of stage 3-4 sleep but consistency for that person over the time. Suggestion is that differences are due to biological/genetical factors rather than environmental ones.
• Universality. There are some universal characteristics of sleep as stages 3 and 4 occur only in the first two cycles and REM sleep always increases in duration with each successive cycle.