AO3

• Evidence supports the role of melatonin in SAD. Terman (1988) found that the rate of SAD is more common in Northern countries where the winter nights are longer. For example, Terman found that SAD affects roughly 10% of people living in New Hampshire (a northern part of the US) and only 2% of residents in southern Florida. These results suggest that SAD is in part affected by light (exogenous zeitgeber) that results in increased levels of melatonin.
• The problem with studying sleep cycles is the differences observed in people, which make investigating patterns difficult. Tucker et al. (2007) found significant differences between participants in terms of the duration of each stage, particularly stages 3 and 4 (just before REM sleep). This demonstrates that there may be innate individual differences in ultradian rhythms, which means that it is worth focusing on these differences during investigations into sleep cycles.
• Tucker's study was carried out in a controlled lab setting, which meant that the differences in the sleep patterns could not be attributed to situational factors, but only to biological differences between participants. While this study provides convincing support for the role of innate biological factors and ultradian rhythms, psychologists should examine other situational factors that may also play a role.
• The way in which such research is conducted may tell us little about ultradian rhythms in humans. When investigating sleep patterns, participants must be subjected to a specific level of control and be attached to monitors that measure such rhythms. This may be invasive for the participant, leading them to sleep in a way that does not represent their ordinary sleep cycle. This makes investigating ultradian rhythms, such as the sleep cycle, extremely difficult as their lack of ecological validity could lead to false conclusions being drawn.
• There is an evolutionary basis for synchronising menstural cycles - if women mensurate together they could fall pregnant at the same time. This means new borns could be cared for collectively which increases the chance of survival.
• Other researchers have suggested that synchronised cycles would produce competition for high quality males and not synchronising would be more advantagous. This shows that there is a biological theoretical basis for infradian rhythms.
• Much of the evidence on pheromones effects on infradian rhythms comes from animal studies and it is difficult to generalise findings from animal studies to humans.
• Practical applications to treatment of SAD - a light box which stimulates very strong light in the morning and evening. It is thought to reset melatonin levels in people with SAD and has relieved symptoms by up to 60%.
• Counter evidence - using a placebo effect, they told ppts a 'negative ion generator' was another form of treatment when it wasn't and found a 30% improvement in symptoms. This casts doubt on the usefulness of the practical application in infradian rhythms.
• Strong evidence to support the distinct stages in sleep and thus the ultradian rhythms. A key study by Dement and Kleitman monitored sleep patterns of 9 ppts in a sleep lab. They controlled extraneous variables such as caffeine and alcohol and woke subjects when they were in REM sleep and brain activity varied depending on how vivid the dreams were.
• Despite having small samples, studies have been replicated and the same results have been found, increasing the reliability and validity of ultradian rhythms.