Rhythms

• What are Circadian Rhythms (CR)?

  - Pattern of behaviour that occurs approximately every 24 hours, set and reset by environmental light levels

  - Driven by body clocks, synchronised by the Suprachiasmatic nuclei (SCN) in the hypothalamus

  - Light sensitive cells in eyes ac as brightness detectors, sending messages about environmental light levels to SCN
• Give examples of Circadian Rhythms


  - Sleep-wake Cycle:

  ○ Dictates when we should sleep and wake up with light and darkness as our external signals

  ○ Dips and rises throughout the day, strongest sleep drive 2-4am and 1-3pm → Intensity dependent on amount of sufficient sleep

  - Hormone Production:

  ○ Prod. and release of melatonin follows CR w/peak levels during hours of darkness

  ○ Activates chem receptors in brain, encouraging feelings of sleep
• Discuss Michael Siffre's Study

  P: Subjected self to long periods of time living underground, no external cues to guide rhythm, woke, slept and ate when he felt like it

  F: First time was underground for 61 days, came out 17th Sept, thought it was 20th August. 2nd time, 6 months in cave, CR settled to just over 24 hours. When he was 60, found that ageing made CR longer with it sometimes settling to 48 hrs

  C: Circadian rhythm without external clues seems to naturally run in a 24-25 hour cycle, though this can vary in diff circumstances and with age
• Evaluate Circadian Rhythms


  ✓ Hughes also found support for importance f light, 4 ppts in Antarctic, in feb cortisol levels reached highest point when ppts woke, 3 months later peak levels at noon → suggests extremes of daylight may result in variations of hormone release

  ✗ Individual differences: cycle length can vary from 13-65 hrs (Czeisler)

  ✗ Research seems to show that artificial light may have more of an impact on CR than initially thought, Czeisler altered ppts CR down to 22 hrs and up to 28 using artificial light alone
• What are Ultradian Rhythms?

  - Cycles that last less than 24 hours e.g. sleep cycle which repeats every 90-100 minutes

  - Basic sleep stages:

  ○ Stage 1: 4-5%, light sleep, muscle activity slows

  ○ Stage 2: 45-55%, breathing and heart rate slows, decrease in body temp

  ○ Stage 3: 4-6%, deep sleep begins, brain prod delta waves

  ○ Stage 4: 12-15%, v. deep sleep, rhythmic sleep

  ○ Stage 5: 20-25%, REM, faster brainwaves + dreaming occurs, muscles relax, heart rate increase, breathing rapid & shallow

  - First 4 stages are NREM (non-rapid eye movement) with final stage consisting of rapid eye movement (REM)
• What is the Basic Rest Activity Cycle (BRAC)?

  - Kreitman suggested the 90 min cycle found in sleep also applies when awake

  - Move progressively through state of alertness to physiological fatigue every 90 mins
• Evaluate Ultradian Rhythms


  ✓ Ericsson found support for BRAC, studied violinists , practise sessions limited to 90 minutes, frequently napping in between. same pattern among musicians, athletes, chess players and writers

  ✗ Individual differences, Tucker suggests that diffs in sleep cycle maybe genetic e.g. ppts studied over 11 days, found large indvid. diffs in sleep duration, time to fall asleep, amount of time in each sleep stage consistently suggesting differences are partially biological determined
• What are Infradian Rhythms?

  - Have duration greater than 24 hours, may last days, weeks, months or years

  - Monthly rhythms: Menstrual cycle lasts about 28 days on avg (between 23-36, Refinetti), regulated by hormones which promote ovulate (oestrogen levels at peak) or stimulate uterus for fertilisation (progesterone levels increase in preparation for possible embryo)

  - Annual rhythms related to seasons e.g. SAD (seasonal affective disorder), with some ppl becoming severely depressed during winter months. Winter also associated w/higher levels of heart attacks, most deaths occur in January
• Evaluate Infradian Rhythms


  ✓ Menstrual cycle can also be governed by exogenous cues, study: daily samples of sweat collected from one group of women and rubbed onto upper lips of second group of women, menstrual cycles synchronised w/'odour donor' (Russell), shows affect of pheromones

  ✗ Some believe infradian rhythm is based on phases of moon, e.g. more babies born during full moon, mental health profession believe full moon can alter behaviour (Vance) tho there's lil evidence for this
• What are Endogenous Pacemakers?


  - Mechanisms within body that govern internal, biological bodily rhythm

  - Suprachiasmatic nucleus

  - Pineal Gland
• What is the Suprachiasmatic Nucleus (SCN)?

  - Main endogenous pacemaker, lies in hypothalamus

  - Imp role in generating circadian rhythm

  - Acts as master clock

  - Neurons within SCN synchronise with each other, so target neurons in body received time-coordinated signals

  - Receives info about light levels via optic nerve, even while asleep

  -Regulates manufacture and secretion of melatonin in pineal gland via interconnecting neural pathway
• What is the Pineal Gland?

  - Produces and secretes melatonin at night and decreases release as light levels increase

  - Melatonin induces sleep by inhibiting brain mechanisms that keep you awake
• Evaluate Endogenous Pacemakers


  ✓ Role of SCN has been demonstrated by animal studies, Morgan bred strain of hamsters so they had circadian rhythm of 20 hours rather than 24 → Their SCN neurons then transferred to 'normal' hamster brains, who started to display same abnormal cycle

  ✗ However, this research is flawed because of its use of hamsters. Humans would respond very differently to manipulations of their biological rhythms, due to biological and environmental differences

  ✗ Seen as biologically reductionist as it considers a singular biological mechanism and fails to consider the other widely divergent viewpoints (could social norms be an influence on our cycles?)
• What are Exogenous Zeitgebers?

  - Environmental cue helping regulate biological clock

  - Light: Receptors in SCN are sensitive to changes in light levels and use this info to synchronise activity of organs and glands. Rods and cones in retina detect light to form visual images, melanopsin gauges overall brightness

  - Social Cues: Aschoff shows that indivi. are able to compensate for absence of zeitgebers such as light by responding to social cues e.g. jet lag adjustment is quicker if travellers got outside more at destination, exposed to social cues of new time zone
• Evaluate Exogenous Zeitgebers


  ✓ Support for role of melanopsin in setting circadian rhythm comes from studies of blind people, Skene and Arendt found that blind ppl with light perception had entrained CR, ppl w/o light perception have abnormal CR

  ✓ Burgess found that exposure to bright light prior to east-west flights decreased readjustment to local time → Ppts exposed to continuous bright light shifted CR by 2.1 hrs, intermittent bright light shifted by 1.5 hrs, dim light shifted by 0.6 hrs, those in first group felt sleepier 2 hours earlier and woke 2 hours earlier (closer to local cond) → shows exogenous zeitgebers have impact