Biological rhythms

• Distinct patterns of changes in the body that

  relate to cyclical time periods.

  Three types:

  Ultradian - Have a duration of less than 24 hours

  so there is more than one cycle every 24 hours.

  2) Circadian- Have approximately a 24 hour duration

  From the Latin terms circa = about and diem = day

  3) Infradian - Have a duration of more than 24

  hours so there is less than one cycle every 24 hours.
• Distinct patterns of changes in the body

  Relate to cyclical time periods
• Types of biological rhythms

  • Ultradian
  • Circadian
  • Infradian
• Ultradian rhythms


  Have a duration of less than 24 hours, so there is more than one cycle every 24 hours
• Circadian rhythms


  Have approximately a 24 hour duration
• Infradian rhythms


  Have a duration of more than 24 hours, so there is less than one cycle every 24 hours
• Ultradian rhythms - Stages of sleep

  1. EEG monitoring shows clear differences in brain activity during the different stages

  2. Stages 1 and 2 - light NREM sleep

  3. Stages 3 and 4 - slow wave NREM sleep

  4. Stage 5 - REM sleep

  5. Each sleep cycle lasts approximately 90 minutes and continues throughout the night
• Infradian rhythms - The menstrual cycle

  Monthly cycle that is controlled by changes in hormone levels which regulate ovulation<|>Typical cycle lasts 28 days<|>During each cycle there is an increase in oestrogen (linked to egg development/release) and progesterone (linked to thickening of the womb)
• Control of the sleep/wake cycle

  Relies on the interaction of endogenous pacemakers (our body's internal body clocks) and exogenous zeitgebers (external changes in the environment)
• Zeitgebers


  German for 'time giver', essential for entrainment of biological rhythms
• Entrainment


  Alignment of the internal body clock rhythm to external cues
• Circadian system


  • Suprachiasmatic nucleus (SCN) - bundle of fibres found in the hypothalamus, connected to the visual cortex and receives information about light
  • Pineal gland - Receives information from the SCN, releases melatonin which induces sleep
  • Adrenal gland - Releases cortisol which induces wakefulness
• Effect of exogenous zeitgebers on the sleep/wake cycle

  1. Light - key in providing information for the SCN, when it gets dark the SCN signals the pineal gland to secrete melatonin, generating sleep, when the SCN detects light it signals the pineal gland to stop producing melatonin and generates wakefulness

  2. Other cues - e.g. meal times, exercise, external temperature fluctuations and regularly timed social interaction
• Disruption of these rhythms can lead to health problems such as jet lag, shift work disorder, seasonal affective disorder (SAD), and insomnia.
• Circadian rhythms can also be influenced by external factors such as light exposure or meal times.
• The circadian rhythm is controlled by an internal biological clock which regulates physiological processes such as sleep/wake cycles, hormone release, eating habits and body temperature.
• The circadian system is important for regulating various physiological processes including hormone production, metabolism, and immune function.