March 5, 2025	PSY 340 Brain and Behavior Class 20: 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

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"

• The brain itself generates its own rhythms of change [= biological clock]
• Multiple systems of the body show a 24-hour cycle (e.g., skin, liver, triglyceride levels in blood, reaction time, etc.)
  

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)

Social Jetlag

• "Social Jetlag" = the difference between when we MUST get up (for school or work) and when our body wants to get up

• On days when we do not HAVE to go to work or school, many individuals actually get more sleep.

The Human Circadian Rhythm: Two "chronotypes" (most of us fall in between)

• "morning people" or "larks"
• "evening people" or "owls"

 

C. Mechanisms of the Biological Clock

1. Superchiasmatic Nucleus (SCN)

• Main control for body's internal temperature and sleep

• Intrinsically-photosensitive retinal ganglion cells (RGCs) respond particularly to levels of blue light.

• Totally blind people tend to have significant problems in their circadian rhythms because of the lack of RGCs (= Non-24 Hour Sleep-Wake Disorder)
  

2. Circadian Rhythm Biochemistry

• per [= period]) and tim [= timeless] proteins interact with a third protein (CLOCK) to cause sleepiness.

• In the morning, low levels of per & tim build up over the course of the day.
  

• Bright light tends to stop or, even, break down tim production

• A human PER genetic mutation: circadian rhythm less than 24 hours.
  

3. Melatonin

• The SCN controls the activity level of the pineal gland (an endocrine [hormone-releasing] gland) in the posterior of the thalamus
  
  
• The pineal gland releases melatonin, a hormone that increases sleepiness. In humans, pineal gland begins to secrete melatonin at night about 2-3 hours before we normally go to bed.

• Is melatonin safe to take if you are having sleep problems?
• Probably yes in the short term (1-2 months).
• If taken in the early afternoon, it tends to reset the biological sleep-wake cycle
  
• Shouldn't be used to treat chronic insomnia
• particularly helpful in adults with respiratory, metabolic, and general sleep disorders.
  
• shows little effect in adults who are coping with mental disorders, neurodegenerative diseases, other diseases such as breast cancer, migraine, patients under methadone maintenance treatment, alcohol use disorder, cirrhosis, IBS (irritable bowel syndrome), or brain injury.

---

The first version of this page was posted on March 2, 2005.