Biological Rhythms - Circadian

Define “endogenous pacemakers”

Internal body clocks that keep biological processes to time

Define “exogenous zeitgebers”

External cues that entrain internal body clocks, meaning altering body clocks to match the environment

How long does a circadian rhythm last?

24 hours

Explain how the sleep wake cycle is a circadian rhythm

The sleep wake cycle is the pattern of sleep and wakefulness that repeats every 24 hours

There must be an internal body clock (endogenous pacemaker) that keeps this pattern and we are aware of conflicts between the endogenous pacemaker and exogenous zeitgeber like light and social cues in situations like jet lag. The endogenous pacemaker will be entrained by the exogenous zeitgeber’s to the new time zone, but it can take several days

What is the suprachiasmatic nucleus (SCN)?

The suprachiasmatic nucleus (SCN) is the endogenous pacemaker for the sleep wake cycle (also known as the master clock). Located at the point on the optic nerves where they cross. When light is detected by the SCN it sends light to the pineal gland, stopping the production of melatonin, a hormone that results in a sleep response

Exogenous zeitgebers in sleep/wake cycle:

Light is thought to be the primary exogenous zeitgeber in the sleep wake cycle. Social cues, like when we time our meals, or exercise, when others go to bed or environmental cues like clocks are also thought to act as EZ’s

AO3 - Siffre (1975)

Spent 179 days (6 months) in a cave with no natural light or other potential EZ’s such as clocks or social cues. Siffre’s body clock (EP) maintained a regular cycle of around 25 hours without EZ’s, longer than the expected 24 hour cycle. Suggesting the EP for the sleep wake cycle is free running but needs entrainment to keep the 24 hour day night cycle

AO3 - Czeisler (1999)

Psychologists have criticised the use of artificial lights in isolation experiments on the sleep wake cycle, suggesting the artificial lights disrupted measurements of the free running circadian rhythm by acting as an EZ. In a study controlling for artificial lights, Czeisler (1999) found the circadian rhythm was closer to 24 hours, with little drift

AO3 - Vetter (2011)

When 27 office workers in an experimental group were exposed to strong blue light, they would shift their circadian rhythms to match the office lighting. A control group of 27 office workers with normal office lighting rhythms matched the shifting time of the natural light of dawn. Strongly suggesting both that light is an EZ and artificial lights can disrupt the endogenous pacemaker by over riding natural light as the primary EZ

AO3 - Ralph (1990)

Golden hamsters with the Tau mutation have a circadian rhythm of 20 hours compared with 24 hours for normal golden hamsters. When the SCN from normal hamsters was transplanted into the brain of Tau mutant hamsters, these hamsters gained a sleep wake cycle of 24 hours. Normal hamsters gained a 20 hour cycle when given the SCN from Tau mutant hamsters. This suggests the SCN is the main EP in the sleep wake cycle

AO3 - Studies have benefitted the economy

Due to sleep pattern disruption leading to anxiety and decreased alertness/vigilance, a psychological understanding will lead to solutions to the negative effects of jet lag and shift work resulting in psychology having a positive effects on the wider economy. Understanding the circadian blood pressure rhythm, helps with timing drug treatments and health treatments

AO3 - Enabled health advice

Knowing the psychological effects of exposure to light, particularly blue light, has implications as technology changes and we are exposed to more blue light late at night from personal devices and new LED road lighting. This provides people with advice on how to maintain a healthy sleep pattern

AO3 - Miles (1977)

It seems that light is the main exogenous zeitgeber and social cues are less effective. Research on a man blind from birth showed that even with social cues, the blind man had a 24.9 hour circadian rhythm, meaning his sleep was out of synchronisation with the rest of society

AO3 - Evolutionary explanation

The evolutionary explanation for the day night cycle of humans and other animals is specialised for day time activity as we are not specialised for night time activity, we have a sleep period at night to protect us from accidental harm and predation