Ultradian Process

• Less than 24h

• Consists of sleep architecture

• Is modulated by circadian and homeostatic processes

• Sleep stages are differently distributed during early and late night
  

Circadian Rhythm

• Lasts about 1 day

• Rhythm of living organisms

• Dynamic balance between stability of the system and adaptability to demands of the environment

• Balance between modulation of internal organization and external

Circadian System

SCN

• SCN: main oscillator, the circadian master clock

  • It is synchronized by zeitgebers

• If SCN is lesioned, the body’s oscillators become desynchronized

  • We can get very thirsty or hungry during the night and it leads to death in animals

• Transplanting SCN into the hypothalamus restores rhythms to donor’s circadian phase

Non-Image Forming Visual System

Retinal Ganglion Cells

• Characteristics:

  • Contain melanopsin: a light-sensitive protein

  • Control pupil dilation

  • Produces melatonin

  • Projects to SCN and VLPO

Retino-Hypothalamic Tract

• Need higher intensity light than cones

• After the light is off, it keeps firing for some time

IPRGC

• Slow and progressive response to light

  • Because we go from light to darkness all the time in our environment

Blue Light

• Blue light: stimulates retinal ganglion cells → SCN → pineal gland → supresses melatonin

Hierarchical Organization

In Humans

Zeitgebers

• Zeitgeber: meaning time giver

• Participate in circadian entrainment

• Their effect depends on the circadian phase of the organism

• Principal Zeitgebers ranked from most important to less important

  1. Light

  2. Food

  3. Activity-rest patterns

  4. Social cues (ex: everyone going to sleep)

Melatonin

• Characteristics:

  • Hormone secreted by the pineal gland

  • Acts on the SCN

  • Stimulated by darkness and suppressed by light

  • Has an opposing action with cortisol

  • Has different seasonal patterns:

    • Summer = less melatonin produced

    • Winter = more melatonin produced

  • Correlated with body temperature, blood pressure, and growth hormone
    

• Functions:

  • Regulates breeding patterns in animals

  • Involved in immune function

General Information

• In Canada, we sleep more in the winter

• Our biological rhythms are adapted to our planetary movement (day/night)

• If we go to sleep at 5 A.M., we will have a hard time going to sleep in the evening because our circadian rhythm tells us to sleep but our homeostatic pressure is low

• Many organs and peripheral tissues have their own circadian clocks but they are subordinate to the SCN

• At 6 A.M., we have a lot of cortisol and melatonin so that we can be able to wake up eventually

• Cortisol increases in the morning so it could be an explanation for nightmares

• Sleep is really susceptible to the placebo effect

• When the circadian phase is shifted by an advance or delay in light exposure, there is a gradual adaptation over a few days