Biological rhythms are cyclical behaviours, ones repeated periodically. These are controlled by endogenous pacemakers, functioning as internal biological clocks to regulate biological functioning, and exogenous zeitgebers in the form of external environmental cues. Circadian rhythms last around 24 hours, like the sleep—wake cycle, a free-running cycle controlled by an endogenous pacemaker operating as a body clock and facilitated by exogenous zeitgebers, such as time checks and regular mealtimes. Infradian rhythms last longer than 24 hours, for instance the menstrual cycle, regulated by hormonal secretions and controlled by the hypothalamus operating as an endogenous pacemaker and also facilitated by exogenous zeitgebers, like pheromones. Ultradian rhythms last less than 24 hours, for instance the cycle of brain activity reflected in the stages of sleep occurring through the night. The main endogenous pacemaker involved in the circadian sleep—wake cycle is the superchiasmatic nucleus (SCN), a small group of cells in the hypothalamus generating a circadian rhythm reset by light entering the eyes. A rhythm is generated from several proteins interacting to form a biological clock. Exogenous zeitgebers help reset and synchronise the sleep—wake cycle, sunlight being the main one, with endogenous pacemakers responding to such zeitgebers to help regulate sleep behaviour in response to the external environment.
Czeisler et al. (1999) criticised early sleep—wake cycle studies, where participants were generally kept in isolation with no time cues, as being negatively affected by exposure to high levels of artificial light, which may have continually re-set participants’ internal body clocks. In their study 24 participants were kept in conditions of constant low-level light for 1 month with no clues as to the passage of time and were put on an artificial 28-hour sleep—wake cycle. Measurements were recorded in the form of regular body temperature readings and biochemistry levels through the analysis of blood chemicals. The findings showed that participants had adopted a sleep—wake cycle of 24 hours and 11 minutes, which differed from the earlier criticised studies that found a sleep—wake cycle closer to 25 hours. This suggests that the human sleep—wake cycle is close to the 24 hours that would logically be expected.
• Aschoff & Weber (1962) found that participants isolated in a bunker with no natural light formed sleep—wake cycles between 25—27 hours, suggesting endogenous pacemakers control the cycle in the absence of light cues.
• Russell et al. (1980) found that female participants’ menstrual cycles synchronised after a donor’s underarm sweat was applied to their upper lips, suggesting that pheromones act as an exogenous zeitgeber.
• Dement & Kleitman (1957) from EEG readings found that sleep consists of stages characterised by different levels of brain activity, with dreaming occurring in REM sleep, implying sleep is an ultradian rhythm.
• Stephan & Zucker (1971), by removing the SCN from rats, found that the usual rhythmic cycles of sleep and activity disappeared, suggesting that the SCN is the crucial endogenous pacemaker in the sleep—wake cycle.
• Luce & Segal (1966) found people in the Arctic Circle still slept 7 hours nightly, even though it was continually light in the summer, suggesting that social cues act as zeitgebers to regulate sleep.
Research suggests that endogenous pacemakers do exist and are regulated by exogenous zeitgebers.
Turke (1984) argues that there is an evolutionary advantage to women synchronising periods, in that it allows women living together to synchronise pregnancies and therefore share childcare duties. Also women working close to men have shorter menstrual cycles, giving them an evolutionary advantage in having more opportunities to get pregnant.
The development of EEG readings gave psychologists an objective means of investigating sleep.
There is an adaptive advantage to animals having endogenous pacemakers reset by exogenous zeitgebers, as it keeps them in tune with seasonal changes etc.
Isolation studies of circadian rhythms have few participants, making generalisation difficult.
Yamakazi et al. (2000) found that circadian rhythms persist in isolated livers, lungs, etc. grown in culture dishes without the influence of the SCN, suggesting that cells other than the SCN act as exogenous pacemakers.
Findings from sleep studies are conducted in sleep laboratories with participants linked up to EEG machines, which implies findings lack external validity due to the artificial environment.
One practical application of research into biological rhythms is that of the phase-delay system of rotating work shifts forward in time to reduce negative effects upon health and therefore improve output. There are also melatonin supplements to reduce the negative effects of jet lag.