10 - Circadian Rhythmns Flashcards
What are biological rhythms? Give three examples
Cyclical changes in physiological systems e.g. circadian, ultradian and infradian.
Define circadian rhythm
Any cycle that lasts 24 hours
These optimise an organism’s physiology and behaviour to best meet the varying demands of the day/night cycle.
Describe the mechanism behind circadian rhythms
Circadian rhythms are driven by the suprachiasmatic nuclei (SCN) in the hypothalamus.
This pacemaker (controls the rate at which something occurs) must constantly be reset so that our bodies are in synchrony with the outside world.
Natural light provides the input to this system, setting the body clock to the correct time in a process called photoentrainment. In mammals light-sensitive cells within the eye act as brightness detectors, sending messages about the environmental light to the SCN. The SCN then uses this information to coordinate activity of circadian rhythms.
How are the circadian rhythms coordinated?
photoentrainment - when natural light provides the input to the system, setting the body clock to the correct time
In mammals light-sensitive cells within the eye act as brightness detectors, sending messages about the environmental light to the SCN.
SCN uses this information to coordinate activity of circadian rhythms.
Describe homeostatic control
Sleep and wakefulness are also under homeostatic control.
When we have been awake for a long time homeostasis tells us that the need for sleep is increasing because of the amount of energy used up during wakefulness.
This homeostatic drive for sleep increases gradually throughout the day, reaching its maximum in the late evening.
What controls our sleep-wake cycles?
Light and darkness
When are the strongest sleep drives?
2: 00-4:00am
1: 00-3:00pm
When is core body temperature highest and lowest?
lowest (36°C) - 4:30am
highest (38°C) - 6:00pm
How is body temperature related to the sleep wake cycle?
Sleep occurs when the core temperature begins to drop and body temperature starts to rise in the last few hours of sleep, prompting a feeling of alertness in the morning.
A small body temperature drop happens between 2:00 and 4:00pm, when most people feel sleepy.
How are hormones related to the sleep wake cycle?
Hormone release follows a circadian rhythm.
Release of melatonin from pineal gland is at its peak during hours of darkness.
Melatonin induces sleep by inhibiting the neural mechanisms that promote wakefulness. When it is light again the production of melatonin drops.
What is the advantage of circadian rhythms?
Practical application - chronotherapeutics. The time that patients take medication is very important for treatment success. Essential that right concentration of drug is released in the target area of the body at the time the drug is most needed. For example, the risk of heart attack is greatest during the early morning hours after waking. Medications have been developed that are taken before the person goes to sleep but are not released until the vulnerable time of 6:00 am.
What are the disadvantages of circadian rhythms?
- Research on circadian rhythms has not isolated people from artificial light, because it was believed only natural light affected circadian rhythms. However, more recent research suggests this might not be true. Participant’s circadian rhythms were altered down to 22 hours and up to 28 hours by using artificial light alone.
- There are individual differences in the length of circadian rhythms. One research study found that cycles can vary from 13 to 165 hours
- Another individual difference in circadian rhythms is when they reach their peak. ‘Morning people’ prefer to rise early and go to bed early whereas ‘evening people’ prefer to rise late.
- Possibly temperature controls our body clock, not light. The SCN transforms information about light levels into neural messages that set the body’s temperature. These fluctuations in temperature set the timing of the cells in the body.
