last updated:

March 26, 2021

[Brain Image]    

PSY 340 Brain and Behavior

Class 22: Rhythms of Waking and Sleeping (Outline)


Chronobiology = how time and seasons affect the biological functioning of living organisms


Consider what the rhythms of your life are like in this time of your life.
  • What time do you usually go to bed?
  • What is sleep like for you? Do you toss & turn? Awaken at night? Dream?
  • What time do you usually get up?
  • How do you feel when you get up?
  • What time of day are you usually the most awake & alert?
  • What time of day are you usually the least alert & able to concentrate?
  • What season of the year do you like the most? Why?


A. Endogenous Cycles

Circadian rhythm of human body temperature What cycles do you experience? 

  • Each year = circannual (circum = "about" + annus = "year)
  • Each month = circamensual (circum = "about" + mensis = "month")
  • Each day = circadian (circum = "about" + dies = "day")

Are there any other ways in which you are affected by the change of seasons or time of the year?

  • If changes happen because of mechanisms in the body = endogenous ("born from within")
  • If changes happen because of mechanisms outside the body = exogenous ("born from outside") 

B. Setting & Resetting the "Biological Clock"

Human circadian rhythms

Protein & circadian rhythm

[Jet Lag diagram]Zeitgeber ("time giver" or "synchronizer") is an environmental stimulus which resets biological clock. The process of resetting the biological clock is called entrainment

  • Light is the primary zeitgeber in humans
    • Internal body temperature tends to set rhythms for different organs in the body
  • Also (less important) noise, exercise, and ambient/environmental temperature
  • New data suggest that food might also serve as a zeitgeber, esp. in the liver.

Jet Lag = disruption of circadian rhythms due to travel across time zones
  • Most people find it easier to travel from east to west across time zones rather than west to east.

Shift Work

  • Associated with higher level of accidents

Jet lag & shift work as possible factors in metabolic disorders (such as obesity, Type II diabetes)

Thais et al.

The Human Circadian Rhythm: Two "chronotypes" (most of us fall in between)
  • "morning people" or "larks"
  • "evening people" or "owls"
  • "Social Jetlag" = the difference between when we MUST get up and when our body wants to get up
sleep & social jetlag
C. Mechanisms of the Biological Clock

[Diagram of Light-Brain-Sleep mechanism]1. Superchiasmatic Nucleus (SCN)

2. Circadian Rhythm Biochemistry

Nobel Prize foe
            2017 in Physiology or Medicine

[Is Melatonin Safe?]
3. Melatonin

This page was first posted March 2, 2005.