Biological Rhythms
Circadian Rhythm:
An example is the sleep-wake cycle
Examples:
Sleep-Wake cycle: It has a periodicity of 24 hours which is synchronised with the day-night cycle
It is controlled by endogenous pacemakers which release hormones (such as melatonin), but is also influenced by exogenous zeitgebers such as light and temperature
Body temperature: Has a periodicity of 24 hours. It peaks at mid-afternoon (4-6 pm) and is the lowest at 4 am
It is regulated by endogenous pacemakers which release of hormones (such as thyroxine), in addition to the exogenous zeitgeber of temperature
Pacemakers and Zeitgebers:
Our Endogenous Pacemaker (Internal) regulates the human circadian sleep-wake rhythm. The endogenous pacemaker for the sleep-wake cycle is a brain area called the suprachiasmatic nucleus (SCN) which is located in the hypothalamus
However, we use Exogenous Zeitgebers, such as light to synchronise our internal body clock with the outside world
In this way, the endogenous pacemaker is alert during the day and sleeps at night
The suprachiasmatic nucleus (SCN):
The SCN is a tiny cluster of cells in the hypothalamus just above the optic chiasm that obtains light information via the optic nerve. The tiny cluster of cells is virtually important for keeping biological time
Endogenous pacemakers:
Sleep-wake studies pointed to the existence of an internal body clock or pacemaker for circadian rhythms
In 1972 two studies demonstrated the SCN roles by damaging the suprachiasmatic nucleus (SCN) in rats
When the SCN was damaged it abolished circadian rhythms in rats and in later studies in hamster
Suprachiasmatic Nucleus (SCN):
The master clock of the body
The clinching evidence for the role of the SCN neurons removed and kept alive outside the brain still shows a circadian rhythm in their electrical activity
This demonstrates that the circadian activity of SCN neurons is intrinsic, or inbuilt
Retinohypothalamic pathway:
There is a direct pathway from the retina of the eye to the SCN, a pathway called the Retinohypothalamic tract
This pathway alerts the SCN to the status of light in the outside world, especially the onset of light in the morning
Pacemakers and Zeitgebers:
The SCN exerts its regulatory role in the sleep-wake cycle via the retinohypothalamic pathway
In response to its input (light rays onto the retina) the SCN signals the pineal gland to suppress the hormone melatonin, which is known to control sleep-wake behaviour
Through this pathway, daylight suppresses the release of melatonin, while darkness leads to an increase
Pineal gland and melatonin:
During the day the pineal gland is inactive, however, when the sun goes and it becomes dark it begins to produce melatonin at 9 pm
As the levels of melatonin rise in the blood you begin to feel less alert and sleep becomes more inviting
The levels in the blood stay elevated throughout the night and drop back down in the day at 9 am
Free-running studies:
Does our sleep-wake cycle remain the same if external influences (i.e light) are kept constant:
The common finding is that the sleep-wake cycle is mostly maintained under these conditions
Therefore implying that the sleep-wake cycle is primarily controlled by endogenous pacemakers, which can maintain an approximate rhythm even when exogenous factors (e.g light) are absent
Circadian rhythm (24 hours):
Aschoff and Weber (1965): Placed participants in a bunker with no natural light. They settled into a sleep/wake cycle of between 25-27 hours, suggesting that (internal) endogenous pacemakers control the sleep/wake cycle in the absence of light cues
Evaluation:
Some participants maintain normal cycles, while others strongly differ (individual differences)
An issue with isolation studies is that they have few participants, making generalisation problematic and studies tend to suffer from poor external validity due to this. Overall this impacts negatively in the ability to generalise the findings to wider society
Circadian rhythm:
Michel Siffre (1972) spent 6 months in a cave with no time cues (Artificial light came on when he was awake)
He settled into a sleep/wake cycle of 25 to 30 hours
After 179 days he thought 151 days had passed, supporting Ashoff and Weber’s findings that Endogenous Pacemakers exert an influence on circadian rhythms
Evaluation:
Artificial light and temperature in the cave could have been confounding variables perhaps leading to inaccuracies which were never considered
The setting itself could be argued to have very low ecological validity
Stage 1:
Lightest (1-7 mins)
Heartbeat slows down
Eye movements slow down
Muscles relax, and might occasionally twitch
Brain waves begin to slow down
Stage 2:
Light (10-25 mins)
Heartbeat and breathing slow down even more
Muscles relax even more
Body temperature drops
Eye movements stop
Brain wave activity slows
Stage 3:
Deep sleep (20-40 mins)
Heartbeat and breathing slow to the lowest levels they will reach during sleep
Muscles stay relaxed
Brain waves slow down even more
Stage 4:
REM (20-40)
Behind your eyelids, your eyes move rapidly from side to side
Breathing speeds up and can become irregular
Heart rate increases
Blood pressure increases
The sleep cycle has a periodicity of 90 minutes (six cycles per night)
In the sleep cycle you begin in light sleep, and pass through several stages of non-REM slow-wave sleep before entering REM sleep
The sleep cycle is controlled by endogenous pacemakers which release or inhibit different hormones (e.g prolactin or cortisol) and neurotransmitters (such as inhibiting norepinephrine)
NREM Stage 1: Changeover from wakefulnesses to sleep
A short period of light sleep in which your HR, breathing, eye movements and brain waves slow & relax your muscles
NREM Stage 2:
A period of light sleep before you enter deeper sleep. Your HR and breathing continue to slow and your muscles further relax. Your body temperature drops and your eye movements stop. Brain wave activity slows but is marked by brief bursts of electrical activity
NREM Stage 3:
A period of deep sleep. It occurs in longer periods during the first half of the night. Your heartbeat and breathing slow to the night. Your heartbeat and breathing slow to their lowest levels. Your muscles are relaxed and it may be difficult to awaken you. Brain waves become even slower
REM sleep:
Occurs 90 minutes after falling asleep
Your eyes move rapidly from side to side behind closed eyelids. Mixed-frequency brain wave activity becomes closer to that seen in wakefulness. Your breathing becomes faster and more irregular, & your HR/BP increases to near-waking levels
Most of your dreaming occurs during REM sleep, although some can also occur in non-REM sleep. Your arm and leg muscles become temporarily paralyzed, which prevents you from acting out your dreams