ultradian and infradian rhythms

• A biological rhythm is referred to as ultradian if it is below 24 hours. For example, the sleep stages which follows patterns of alternating REM (rapid eye movement) and NREM (non-rapid eye movement) sleep, which consists of stages 1-4. The cycle repeats every 90-100 minutes. A complete cycle consists of a progression through the 4 stages of NREM before the final stage of REM sleep, then the cycle restarts.
• Most of what we know from the stages of sleep is due to the recording of electrical activities in the brain, each stage shows a distinct EEG pattern. As deep sleep is reached, brainwaves slow and breathing and heart rate decreased. During the 5th stage, EEG shows that of an awake person and this is where dreaming occurs.
• . Kleitman referred to the 90 min cycle as the basic rest activity cycle (BRAC). He said that this then continues even in the day, but we go from alertness to psychological fatigue every 90 mins. The human mind can focus for 90 mins, then it runs out of resources, so results in a loss of concentration. This has been shown in everyday life, such as a coffee break at 10.30am which divides the morning.
• . Infradian rhythms are those which are greater than 24 hours. It can be days, weeks, months etc. Weekly rhythms have differences in behaviour over this 7 day period. For example, male testosterone levels are elevated on weekends and young couples report more sexual activity on weekends however, frequency of births at weekends is lower.
• An example of a monthly rhythms is the menstrual cycle. There are differences in this cycle, but an average was found of 28 days. The menstrual cycle is regulated by hormones which promote ovulation or stimulate the uterus for fertilisation. Ovulation occurs roughly halfway through. An example of annual rhythms is migration of animals, who respond to the changing seasons. Another is in humans which is the seasonal affective disorder. Where some people become depressed in winter months. Winter is also associated with a higher amount of heart attacks.
• The cycle is usually controlled by endogenous factors, but it can be influenced by exogenous factors.

  when several women of childbearing age live together and don’t take oral contraceptives, their menstrual cycles tend to sync up. In one study, daily samples of sweat from one group of women were then rubbed on the upper lip of another group. The groups were kept separate yet their cycles synched up. 

  this suggests that the cycle can be affected by pheromones, which act in a similar way to hormones but can have effect on someone else, not just the one producing them. 
• Research by Penton-Voak 1999 suggests that human mate choice varies across the menstrual cycle, an infradian rhythm, with different preferences at stages of the cycle.

  They found that women generally expressed a preference for 'slightly feminised' male faces when picking partner for a long-term relationship, however in the ovulatory phase of their menstrual cycle, women show preference for more masculine faces. 

  this suggests that there is a preference for kindness over the long term, but 'good genes' at the time of menstrual cycle so they're passed down to their offspring.
• As this was a controlled study in a strict lab setting, other variables were controlled.

  however, the participants may have been sleeping differently due to being in a controlled setting while they knew their sleep was being recorded. this may affect their sleep due to the change and strictness of this environment.

  therefore, this may decrease the validity of the findings.

   
• Differences in the stages of sleep patterns of individuals are usually attributed to differences in non-biological factors but a study suggests different. 

  pt were studied over 11 days and nights in a strictly controlled lab environment. Researchers assessed sleep duration, time to fall asleep and amount of time in each stage. They found large pt variables in each of these across the 8 nights. Sleep stages 3 and 4 the variables were particularly significant. This meant that differences between pt were not driven by circumstances but were partially biologically determined.