Biological rhythms

• Biological rhythms

  A biological rhythm is a change in the body processes or behaviour in response to cyclical changes within the environment.
• Biological rhythms

  All living organisms - plants, animals and people - are subject to biological rhythms and these exert an important influence on the ways in which the body system behaves. 

  All biological rhythms are regulated and governed by two things:

  • Endogenous pacemakers - The body’s internal body clocks that regulate biological rhythms e.g. suprachiasmatic nucleus - detects light, even when eyes are closed. 

  Exogenous zeitgebers - External factors in the environment which reset our biological clocks e.g. light, social cues.
• Circadian rhythms - 24 Hour cycles

  Biological rhythms lasting about 24 hours adapt the body to meet the demands of the day/night cycle

  Examples of circadian rhythms:

  • Sleep/wake cycle 
  • Core body temperature 
  • Hormone production
• What does the sleep-wake cycle dictate?

  When we should be asleep and awake
• What is the main external cue for the sleep-wake cycle?

  Light
• How is light detected in the sleep-wake cycle?

  By the eye
• Circadian rhythms - 24 Hour cycles

  • The strongest sleep drive is usually between 2-4 am and 1-3 pm. This sleepiness is more intense if we are sleep deprived.
  • This “free-running” internal circadian “clock” maintains a cycle to 24-25 hours even in the absence of external cues. It is disrupted by major changes in sleep schedules such as jet travel or shift work.
• What role do the suprachiasmatic nuclei (SNC) play in the sleep-wake cycle?

  They coordinate the activity of the entire circadian system
• Besides circadian rhythms, what other factor influences sleeping and waking?

  Homeostasis
• What does homeostasis indicate regarding the need for sleep?

  It indicates a need to sleep after being awake for a long time
• When does the homeostasis drive for sleep reach its maximum?

  In the late evening
• What are the key components of the sleep-wake cycle?

  • Circadian rhythms
  • Homeostasis
  • External cues (mainly light)
  • Suprachiasmatic nuclei (SNC) in the hypothalamus
• Circadian rhythms - 24 Hour cycles

  Core body temperature is lowest (about 36 degrees) about 4.30am and highest (about 38 degrees) around 6pm. It also dips between 2pm and 4pm.

  Hormone production also follows a circadian rhythm.

  For example, melatonin production by the pineal gland peaks during  the hours of darkness promoting sleepiness
• Circadian rhythms - 24 Hour cycles

  Demonstrates a free-running circadian cycle of about 25 hours.

  Folkard et al (1985): 12 pts lived in a dark cave for 3 weeks, going to bed when the clock said 11.45pm and waking up when it said 7.45am.

  The researchers gradually speeded up the clock so an apparent 24-hour day eventually lasted only 22 hours.

  Findings: only 1 pt adjusted to the new regime.

  This shows the existence of a strong free-running circadian rhythms that can’t be overridden by changes in the external environment.
• AO3 - Evaluation (Individual differences)

  The cycle length can vary from 13 to 65 hours (Czeisler et al).

  Duffy et al (2001): people display a natural preference for sleeping and rising early. Morning people (“larks”) prefer to rise early and go to bed early, whereas evening people (“owls”) prefer to wake and go to bed later.

  = So ind. seem to have innate preferences in their cycle length and onset. Researchers should also focus on this.

  = Findings from sleep/wake cycle studies may not fully represent individual differences within the population.
• AO3 - Evaluation (Chronotherapeutics)

  (+) Real world application of circadian rhythms: the study of how timing affects drug treatments.

  To be more effective drugs need to be released into the body at the optimal time

  For example, the risk of heart attack is greatest in the early morning.

  = this has prompted the dvpt of novel drug delivery systems so that the drug is released into the bloodstream during the vulnerable period.
• AO3 - Evaluation (Extraneous variables)

  …than light in setting the body clock.

  It seems the SCN transforms info about light level into neural messages that set the body’s temperature.

  Buhr et al: fluctuations in body temperature cause issues to become active or inactive.

  = so also the SCN responds to light, the circadian fluctuation of body temperature may actually control the other biological rhythms.
• Infradian rhythms

  = cycles with a duration longer than 24 hours

  Examples?

  Female menstrual cycle

  Annual rhythms
• AO3 - Infradian rhythms

  Around ovulation women prefer more “masculinized” faces.

  This may represent good genes for short-term liaisons, with more likelihood of conception.

  In contrast, some women generally prefer “slightly feminised” male faces when picking a partner for a long-term relationship, as they may represent kindness and cooperation

  = this shows how a hormonally controlled rhythm may also impact behaviour
• AO3 - Infradian rhythms

  The menstrual cycle can also be controlled by exogenous (external) cues

  Such as what?

  Synchronisation of menstrual cycles

  Why might this be this be the case HINT - think from an evolutionary standpoint
• What are infradian rhythms related to?

  Annual rhythms related to seasons
• How do annual rhythms affect human behavior?

  They may cause seasonal variations in mood, especially in women
• What is Seasonal Affective Disorder (SAD)?

  A type of depression that occurs in winter months
• What is the circannual rhythm associated with melatonin secretion?

  Melatonin is secreted for a longer period due to lack of light
• How does melatonin affect serotonin levels?

  Increased melatonin is thought to affect serotonin, creating depressive symptoms
• What health issue is associated with winter months?

  Increase in heart attacks
• What is notable about human deaths in January?

  There is a robust annual rhythm with most people dying in January
• What are the key effects of infradian rhythms on human health and behavior?

  • Seasonal variations in mood, particularly in women
  • Seasonal Affective Disorder (SAD) linked to winter months
  • Increased melatonin secretion affecting serotonin levels
  • Higher incidence of heart attacks in winter
  • Peak in human deaths during January
• Ultradian rhythms

  = cycles lasting less than 24 hours.

  Examples?

  Sleep stages sleep involves a repeating cycle of 90-100 minutes, with 5 stages including REM sleep
• Ultradian rhythms

  The sleep cycle follows a ultradian rhythm of 90 minutes where we move between NREM sleep and REM sleep.

  Our understanding of sleep comes from EEG recordings (Electroencephalogram).

  Each of the sleep stages shows a characteristic EEG

  pattern.
• AO3 - Ultradian rhythms

  Dement and Kleitman (1957) measured brain activity using electroencephalogram (EEG), controlling for the effects of caffeine and alcohol.

  REM sleep was highly correlated with dreaming, the more vivid the more active. Waking people up during the REM stage, people could recall very clearly what they dreamed.

  This finding has been replicated on multiple occasions.
• AO3 - Ultradian rhythms

  Tucker et al: large difference between individual’s sleep patterns, which were consistent over 11 nights in a controlled sleep lab.

  Ind. also responded very differently to 36-hour period of sleep deprivation

  = this suggests that sleep patterns may be at least partially determined by genes
• What condition is treated with phototherapy according to the study?

  Seasonal Affective Disorder (SAD)
• How does a lack of sunlight affect some people's mental health?

  It creates depression in some individuals
• What was the brightness level of the light treatments used in Eastman's study?

  10 to 20 times brighter than ordinary indoor lights
• How many patients were randomly assigned to the bright-light treatments in Eastman's study?

  96 patients
• What were the three treatment groups in Eastman's study?

  Morning bright light, evening bright light, and morning placebo
• How long did patients receive bright light treatment each day?

  One and a half hours
• What was the outcome of the morning light treatment group compared to the placebo group?

  Significantly more people showed complete or near-complete remission of symptoms
• How did the response to evening light treatment compare to the placebo group?

  The response was better than placebo, but not at a level of significance