The infradian rhythms are rhythms that last longer than a day for example the human menstrual cycle, which occurs over a 28-day cycle. Research into infradian rhythms includes a study by Reinberg (1967); this study involved a woman spending 3 months in a cave with only the light of a miner’s lamp. As a result of this her days lengthened to 24.9 hours and her menstrual cycle shortened to 25.7 days. This study shows that the levels of light in the cave could have affected the woman’s menstrual cycle. After the study it took her body a year to readjust her menstrual cycle. This research shows how infradian biological rhythms can be influenced by external zeitgebers such as light. This research was also supported by findings that menarche’s is reached earlier in blind girls than sighted girls.
There is also research to show that other factors than just light may affect the human menstrual cycle, these include pheromones released by other women. This was shown by a study of girls in a boarding school, these girls tended to have synchronised menstrual cycles. It has been suggested that the production of pheromones which synchronise menstruation may be evolutionary as it would have meant that women in the same groups had a greater chance of becoming pregnant at the same time this would have meant that if a mother had died another mother could breast feed her baby and bring it up.
It has also been found that women have a shorter menstrual cycle if they are around men a lot. This could be thought to have an evolutionary advantage, because if you are around males a lot, then a woman who ovulates more often is like to produce more offspring where as if there are not the no men around a woman would be wasting energy producing eggs more than she needs to. Ultradian rhythms are rhythms that are shorter than a day, for example oral habits, renal excretion, heart rate and hormone release.
There has been the most research into ultradian rhythms that occur while we are sleeping. There is said to be a link between these types of rhythms and brain size. Aserinsky 1952, connected his son to EEG monitors and put electrodes on his eyes while he slept. He noticed that there were cycles of about 90 minutes when his son would show a lot of brain activity and his eyes showed movement. When he woke his son he said that he was dreaming. The same patterns were recorded in adults.
Rechtschaffen and kales (1968) monitored brain waves over a nights sleep they noticed 4 stages of sleep and then a 5th stage of REM sleep. They also recorded REM sleep to occur around every 90 minutes. A criticism of sleep studies is that they are all conducted under very artificial conditions and lack ecological validity because of this. Friedman and fisher studied oral habits of people. They did this by placing them in luxurious isolation. They didn’t know the time of day and had no cues, they were able to help them self to food and drink as the pleased. They found that their oral behaviours were repeated every 85 to 100 minutes. They also found that this time seemed to shorten under stress.