cycles that last for only 24 hours mammals have about 100 circadian rhythms good example is the sleep-wake cycle as it clearly illustrates physiological and psychological processes The consistency of the sleep-wake cycle suggest an internal mechanism endogenous mechanism – biological clock whilst psychological factors such as anxiety can override the consistent pattern.

Endogenous factors are the innate mechanism thought to set the time of biological clocks Supercharismatic nucleus(SCN) is the pacemaker of the sleep wake cycle Generates its own biological clock via protein synthesis It is connected to the optic chiasm so receives input about the light levels which also regulates the rhythm(exogenous zeitgebers) It is connected by neural pathways to the pineal gland, so electrical stimulation of the SCN is passed to the pineal gland causing it to release melatonin Melatonin is inversely proportioned to light It is a hormone which induces sleep as it acts to release the neurotransmitter serotonin which slows the brain’s neural mechanisms decreasing brain activity leading to sleep

Siffre spent 2 months in a dark cave. In the absence of light he developed a sleep–wake cycle of 25 hours. Being close to the standard 24 hours this suggests the influence of endogenous factors, i.e. it supports an innate, biologically determined circadian rhythm because he maintained this rhythm in the absence of external zeitgebers. The man, blind from birth, had a circadian rhythm of 24.9 hours. He had to use stimulants and sedatives to adjust his sleep–wake cycle to the standard 24 hours. This supports a biologically determined, endogenous rhythm because the research shows that his innate biological rhythm was difficult to reset with the use of zeitgebers such as clocks and radios.

When the SCN was removed from hamsters their nocturnal circadian rhythms disappeared. Transplanting with SCN cells re-established the rhythms, including mutant (diurnal instead of nocturnal) rhythms from mutant hamsters, which shows that the SCN is the main pacemaker.. Blake (1967, see A2 Level Psychology page 31) researched naval personnel by asking them to rate their performance on several tasks at five different times of day (08.00, 10.30, 13.00, 15.30, and 21.00). For most of the tasks, the best performance was obtained at 21.00, with the second-best level of performance occurring at

Attempted to establish free-running, biological rhythms, so Siffre was isolated from variables that would affect these, such as daylight, clocks etc. However, there was one key confounding variable—artificial, dim light. It was thought at the time that this would not affect the circadian rhythm, but this is not the case. Also shows the importance of zeitgebers as perhaps the endogenous factors dominated because the blind individual in their study could not access the key zeitgeber, light. Studied submariners whose work schedule consisted of 6 hours on duty followed by 12 hours off duty, thus producing an 18-hour day. In spite of this schedule and their ability to control their own lighting conditions, the submariners had an average circadian rhythm for melatonin lasting just over 24 hours. It was concluded that this was because they had social contacts with people living on a 24-hour schedule, and unlike Siffre they were aware of clock time.

Case study of a young woman, Kate Aldcroft, who was voluntarily isolated for 25 days without any zeitgebers. It was found that though her body temperature rhythm was 24- hour, i.e. circadian, her sleep rhythm was on a 30-hour cycle. This is much longer than our innate 24.9-hour rhythm and so shows that this is subject to individual differences and the importance of exogenous zeitgebers in entraining our endogenous rhythms to the timing of the 24-hour clock.

More than 24 hours e.g. Menstrual Cycle Controlled by hormones oestrogen and progesterone and the target organs ovaries and the womb the hormones cause the egg to be released and the thickening of the womb If the egg doesn’t get fertilised then the lining of the womb pulls away with the cycle ending with menstruation Menstruation is an endogenous cycle but can be influenced by exogenous factors

Found that women who live or spend considerable time together, such as girls in a boarding school, can have synchronised menstrual cycles. Twins can menarche at exactly the same time, including the time of their first menstruation. This may be due to pheromones, biochemical substances that act like hormones but are released in the air rather than the bloodstream. These are “chemical messengers” and so may co-ordinate the synchronisation. Participants had donor pheromones rubbed onto their upper lips, daily, for 5 months. A control group went through the same procedure as the experimental group but was not exposed to pheromones. At the end of the 5 months four of the five women in the experimental group had menstrual cycles that synchronised with the donor. Women who work with men have much shorter menstrual cycles and so McClintock (1971) concluded that male pheromones might reset a woman’s biological clock, increasing ovulation, which has an evolutionary advantage because females who reproduce more often will have more offspring and therefore their genetic line is likely to become dominant.

Ultradian rhythms are those which are less than 24 hours Example is the sleep cycles which occur at 90 minutes at a time in the following stages Sleep isn’t a loss of consciousness but a reduced state of consciousness

Stage 115mins – EEG = alpha waves EOG = slow rolling eye movements EMG = reduced muscle tension Stage 220mins – EEG = theta waves, spindles and K complexes EOG = little eye movement EMG = muscles relaxed Stage 315mins – EEG = delta waves, spindles EOG and EMG are the same as stage 2 Stage 430mins – EEG = delta waves(SWS) EOG and EMG show very little activity Stage 510mins – EEG = beta waves, very active(REM) EOG = rapid eye movement EMG = paralysed

Cycle 11,2,3,4,3,2,5 Cycle 22,3,4,3,2,5 Cycle 32,5 Cycle 42,5 Cycle 52,5 EEGs EOGs EMGs have been used to distinguish the stages and cycles of sleep. EEG readings are normally used to distinguish the sleep stage as the frequency and amplitude differ depending on the stage. Artificiality of the sleep laboratory. A significant weakness is that such physiological measures are gathered in a sleep laboratory, where the artificial conditions and the fact that research participants are wired up to machines may affect sleep patterns. The sleep laboratory is reductionist as it does not reflect the range of factors that can influence sleep in real life. The research lacks mundane realism and as a consequence the findings may lack generalisability to real-life sleep patterns and so ecological validity may be lacking. Consequently, the meaningfulness of the findings may be limited.

Individual differences. Most people have five sleep cycles, which last approximately 90 minutes each. However, there is great variation in sleep patterns, as evidenced by individual differences in the total amount of sleep. There is also diversity in the pattern of NREM (non-REM) to REM sleep between people and within the individual, as the pattern may vary in the same person over time.

Disrupting the biological rhythms goes against the inbuilt endogenous pacemakers which as psychological and physiological consequences

One significant disruption is the changing of the seasons which can lead to severe depression. Case of Pat Moore who had a history of suffering from unipolar depression before she realized that her depression was seasonal. The effect of day light on her mood was tested through phototherapy for a few hours on waking. The bright white light used was as similar as possible to natural light. Pat showed remarkable improvement within just a few days of beginning therapy. Found an incidence of about 9% in Alaska. Alaska lies across the Arctic Circle and so will have very restricted day-length and also low quality of light through the winter months.

Light therapy involving dawn simulation is very effective. Light intensity is increased towards the end of the sleeping time, which mimics natural dawn, and reaches a bright light level at the desired waking time. Found a similar rate of SAD to that found in the Arctic circle sample in a retrospective study of seasonal mood disorders in Italy. 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.

Causes major disruption to biological rhythms as the individual is working when the body should be sleeping. Given enough time the biological rhythm can re-adjust itself and reduce disruption. Most major accidents e.g. Chernobyl, and 3 mile island occurs between 1 -7am. Most lorry accidents occur in the early hours of the morning. Showed that people on shiftwork slept 1–4 hours less than when they could sleep normally at night. He also showed that they had a particular deficit in REM sleep. Found that night shiftworkers had disrupted circadian melatonin production, and that this was associated with disrupted sleep, fatigue, increased risk of accidents, and even chronic disease. Found that because phase delay (staying up later) needs less adjustment than phase advance (getting up earlier) it is better to rotate with the clock than against it. Forwards rotation works with rather than against the innate 24.9-hour rhythm. Czeisler et al. tested this in a chemical plant where workers reported feeling better and less tired and managers reported increased productivity and fewer errors.

They identified two major types of shiftwork: (1) rapid rotation where the worker changes shifts approximately every third shift, and (2) slow rotation where the worker changes shifts every week or month. Rapid rotation seems preferable as it may be possible for the body to maintain a fairly constant circadian rhythm Studied accident rates in an industrial chemicals plant and found higher accident rates in shiftworkers compared to day workers. Explains car accidents involving shift workers Due to momentary falling asleep, sometimes for a few seconds only, because an absence of skid or braking marks indicated that the driver had not been aware of the impending accident! This suggests a lapse of consciousness such as when asleep, rather than a lapse of attention.