Disruption Of Biological Rhythms Essay About Myself


The internal circadian clock adapts slowly, if at all, to rapid transitions between different shift schedules. This leads to misalignment (desynchrony) of rhythmic physiological systems, such as sleep, alertness, performance, metabolism and the hormones melatonin and cortisol, with the imposed work–rest schedule. Consequences include sleep deprivation and poor performance. Clock gene variants may influence tolerance of sleep deprivation. Shift work is associated with an increased risk of major disease (heart disease and cancer) and this may also, at least in part, be attributed to frequent circadian desynchrony. Abnormal metabolism has been invoked as a contributory factor to the increased risk of heart disease. There is recent evidence for an increased risk of certain cancers, with hypothesized causal roles of light at night, melatonin suppression and circadian desynchrony. Various strategies exist for coping with circadian desynchrony and for hastening circadian realignment (if desired). The most important factor in manipulating the circadian system is exposure to and/or avoidance of bright light at specific times of the ‘biological night’.

Body clock, cancer, circadian rhythm, heart disease, light, melatonin, metabolism, shift work


Many reviews have been published regarding the subjective perceptions, health, performance and psychosocial aspects of shift work [1–11]. There is little doubt that shift work is associated with a number of health problems, for example poor sleep, gastrointestinal disorders, abnormal metabolic responses and increased risk of accidents. A longer term risk of major disease such as heart disease and cancer is beginning to be appreciated. This review will concentrate on shift work in relation to biological rhythms since disturbed rhythms appear to underlie many of the short- and long-term health problems of shift workers [12–14]. To this end, an introduction to the subject is provided.

Literature search

A literature search with the keywords ‘shift work’ and ‘circadian’ gave 1034 references in PubMed. Since the pineal hormone melatonin is currently used as the primary output marker of the internal clock (as well as its actions as a chronobiotic), the search was then restricted to ((shift work) and (circadian) and (melatonin)). This provided 189 references, which together with the author's personal collection formed the basis of this review.

Importance of biological rhythms to health

Biological rhythms serve to align our physiological functions with the environment. We are a diurnal species and thus, we normally sleep at night and are active during the daytime. The timing of functions with prominent rhythms such as sleep, sleepiness, metabolism, alertness and performance in a normal environment is such that they are optimal during the most suitable phase of the day (Figure 1). Abrupt deviations from ‘normal’ timing of work and sleep can lead to problems, for example sleep taken during the day is usually shorter and of worse quality than when taken at night [6,15]. Alertness and performance reach their nadir at night during peak sleep propensity and fatigue [13,16,17] and close to the low point of core body temperature and the peak of melatonin secretion. The health problems and increased risk of major disease in long-term shift workers are ascribed largely to working out of phase with the internal biological clock. It is likely that many perceptions of the detrimental effects of clock disruption or abnormal timing derive from observations in shift workers.

Figure 1.

Diagrammatic examples of circadian rhythms, from Rajaratnam and Arendt, Lancet 2001 [13], by permission.

Figure 1.

Diagrammatic examples of circadian rhythms, from Rajaratnam and Arendt, Lancet 2001 [13], by permission.

Characteristics of circadian rhythms

Basic properties

Everything is rhythmic unless proved otherwise [18]. Biological rhythms of various periodicity are ubiquitous. The frequency displayed varies from fractions of a second (for example the firing of neurones) to years (for example population variations). By far, the most information is available concerning daily rhythms [18,19]. They are either externally imposed, internally generated or more frequently a combination of these two factors. Internally generated rhythms with approximately a 24 h period are known as circadian, from the Latin ‘circa diem’, ‘about a day’). Circadian rhythms serve to temporally programme the daily sequence of metabolic and behavioural changes. By definition, they persist in the absence of time cues such as alternating light and darkness and are coordinated by an internal biological clock (pacemaker, oscillator) situated in the suprachiasmatic nuclei (SCN) of the brain hypothalamus [20]. The basis of circadian rhythm generation is a negative feedback loop of clock gene expression [21,22].

Individuals kept in a time-free environment (or at least with very weak time cues) manifest their own endogenous periodicity referred to as ‘free-running’. The free-running period is individually variable and is an inherited characteristic. On average, the human endogenous period (or tau) is about 24.2–3 h although this does depend on previous experience of time cues [18,23]. Synchronization or entrainment of the circadian clock to 24 h is dependent on suitable time cues, also known as ‘zeitgebers’. In circadian literature, synchronization means that rhythms display a 24 h period but may not necessarily be in the right phase, for example, abnormally delayed or advanced. Entrainment means that the rhythms are synchronized with the appropriate phase. When entrained to the 24 h day, a short endogenous tau is associated with morning diurnal preference (larks) and a long tau with evening preference (owls) [18].

Circadian response to time cues

Because the circadian clock period is not exactly 24 h, it must be reset regularly (phase shifted) to maintain a 24 h period. The most important time cue for maintaining a 24 h period is the light dark cycle acting partly via a novel retinal photoreceptor system and a novel photopigment melanopsin (circadian photoreception) [24]. Recent evidence indicates that short wavelengths of light (460–480 nm, blue) have the most powerful resetting effects [25]. Blind people with no conscious or unconscious light perception frequently display free-running rhythms, underlining the importance of light. The timing of sleep also has an influence together with minor ‘non-photic’ zeitgebers such as exercise, social cues, clock time and food ingestion. Specific manipulation of food timing in animals influences a so-called food entrainable oscillator, which is independent of the SCN [26]. The content of meals in humans may also have a minor influence.

The circadian response (change in timing or phase shift) to light exposure, and indeed to other time cues, is dependent on the strength and timing of the stimulus. It can be described by a ‘phase response’ curve (Figure 2) [27,28]. The central clock adapts slowly, and with considerable individual variability, to a rapid shift in work time or time zone. After a time zone change, the average rate often approximates to 1 h of adaptive shift per day. After an abrupt shift in work time, the change is very variable as discussed later [29–31]. During the process of adaptation, endogenous rhythms are out of phase with the external environment (external desynchronization). They may also be out phase with each other, i.e. assume a transitory abnormal phase relationship (internal desynchronization). This condition is often referred to as ‘circadian desynchrony’. Time cues or zeitgebers are all important in controlling the circadian response to such changes. In general, it is easier to delay the clock than to advance it in view of the >24 h period of most people. During a period of desynchrony, for example, a single night of night shift in a sequence of days, workers are attempting to sleep at a time of maximum alertness and to work at the nadir of alertness and performance. If adaptation of the clock to a new work schedule occurs, the problems of desynchrony resolve [32–35].

Figure 2.

Circadian response (‘phase response curve’, shift of the melatonin rhythm, advances are positive, delays are negative) to a 1–2 h light pulse, ca 300 lux, 500 nm, at different times of night. DLMO = dim light melatonin onset, on average at ∼2100 h, thus 8 h after DLMO = 0500 h clock time. From Paul et al. [28], by permission.

Figure 2.

Circadian response (‘phase response curve’, shift of the melatonin rhythm, advances are positive, delays are negative) to a 1–2 h light pulse, ca 300 lux, 500 nm, at different times of night. DLMO = dim light melatonin onset, on average at ∼2100 h, thus 8 h after DLMO = 0500 h clock time. From Paul et al. [28], by permission.

Genetic basis of circadian rhythms

Many of the genes concerned with circadian rhythm generation in mammals and other species have now been identified, e.g. CLOCK, PER1, PER2, PER3, TIM, CRY1, CRY2, BMAL1, REV-ERBALPHA. The mechanism is similar in all species investigated and substantial homology exists between, for example, Drosophila and mammals. Oscillation of clock genes also occurs in peripheral structures, and in general, they are considered to be coordinated through SCN activity. However, it is possible to shift the timing of some peripheral oscillations (for example, in the liver by timed feeding), independently of the SCN [36]. Investigation of polymorphisms in human clock genes in relation to occupational health and disease is in its infancy. Some polymorphisms have been identified and associations are emerging with phenotypic characteristics such as diurnal preference (larks–owls), intrinsic period, vulnerability to disease and response to sleep deprivation [37–39].

The circadian clock influences hormones, behaviour, cognitive function, metabolism, cell proliferation, apoptosis and responses to genotoxic stress [22]. There is new strong evidence concerning the importance of circadian control for health in that disruption of circadian clock gene expression can lead to increased incidence or progression of cancer (in animals) [22,40].

Examples of rhythms relevant to human disease

Some examples of human rhythms in disease processes include night time asthma, early morning increases in blood pressure, death rate from cardiovascular disease and stroke, disrupted menstrual cycles, abnormal cortisol rhythm in Cushing's syndrome, sleep disorders for example delayed sleep phase syndrome, advanced sleep phase syndrome, non-24 h sleep wake cycles (especially in the blind), some psychiatric disorders. Numerous aspects of human biochemistry show rhythmicity, even urinary creatinine. Thus diagnostic tests should be aware of these rhythms. Measurement of a given rhythmic variable in someone who has just crossed several time zones, or worked a series of night shifts, can give false-negative or false-positive results. Moreover, many drugs have a rhythmic variation in both pharmacokinetics and efficacy (chronopharmacology).

The melatonin rhythm

A darkness hormone

Melatonin (N-acetyl-5-methoxytryptamine) in an indolic hormone whose principal physiological function is to provide a humoral time cue for the organization of seasonal and circadian rhythms [41]. The pineal gland secretes melatonin with a marked circadian rhythm, peaking at night—it has been called the ‘darkness hormone’ and the duration of its secretion is directly related to the length of the night. In animals which depend on day length to time their seasonal physiology, the length of melatonin secretion signals the length of the night. In humans, its circulating concentrations are high from ∼2100 to 0700 h with large individual variations. This period can be used to define ‘biological night’. The peak secretion occurs ∼0400 h, closely associated with the nadir of core body temperature, alertness and performance (Figure 1). In specific circumstances, humans may also show changes in the duration of secretion [41].

Melatonin as a chronobiotic

Melatonin is not only a so-called ‘hand of the clock’, it has the ability to induce sleepiness or sleep, change circadian phase and to entrain free-running rhythms when administered in suitable doses and timing [42

Hi guys,

I’ve had a lot of people request my sleep and biological predictions for PSYA3 2017 so here they are. Just to clarify this is for retake students sitting the old specification and not the new linear specification. You should be sitting these exams this June 2017 as part of your retakes from the old spec.

Download The Sleep and Biological Rhythms Model Essay Answers Here!

Sleep was one of the topics I studied for this paper (the other topics were Aggression and Relationships) and I scored 100 UMS (full marks) in all three of the 24 mark sections. I’ll be completely open and honest here and say Sleep and biological rhythms was my most hated topic out of all of them and this includes the PSYA4 topics too. I chose this topic mostly because it seemed easier to learn than the others but it became such a chore to learn and I was too invested revision wise to suddenly change ! So I stuck with it and thankfully managed to do well using my essays I had created.

If you are studying this topic and need help (and I dont blame you if you did) I would highly recommend you check out my sleep and biological rhythms ebook by clicking the image on the right. It covers all your potential essay questions you can get in this exam and it is the same essays I memorised myself to score full marks. The ebook becomes available instantly to download and print so you can begin revising with it straight away after purchase. If you are a retake student then this is your last opportunity to retake this PSYA3 exam and improve your grades so don’t miss out just incase.

2017 Predictions For Sleep and Biological Rhythms

Last year we had a broad question come up focusing on the role of endogenous pacemakers and exogenous zeitgebers which helps me rule these two out. This then leaves us an interesting selection of potential big essay questions.

  • Circadian, Infradian and Ultradian rhythms – these havent ever come up as sole essays and rather a smaller question is more likely I believe considering the lack of focus they have had. I would prep these just to grab those smaller marks if it does come up but if an essay does come up it may want you to discuss research into biological rhythms so its best to go in with some grasp of all 3 regardless.
  • The disruption of biological rhythms (shift work/Jet-lag) – This is something thats a possibility considering its been 3 exam windows. My gut says to know this going in.
  • Explanations for sleep disorders; sleepwalking, insomnia, narcolepsy – I think a question on insomnia is more likely personally but the whole section on sleep disorders is well overdue. It appeared only as a 4 marker in June 2015 for Narcolepsy and that was it! So I would brush up on these considering the lack of focus this chapter has had historically.
  • The nature of sleep: Stages of sleep – a bit of an outsider but its not really appeared since June 2011 and that was a broad question.

So these are my thoughts on what I would personally prepare for – what are your thoughts? Always be sure to revise and learn everything to be safe and use these predictions merely as a guide.

Drop me a comment below and let me know if you have any questions!


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