Circadian Rhythms Flashcards
What is a biological rhythm?
Distinct patterns of changes in body activity that conform to cyclical time period
What are circadian rhythms driven by?
- By our body clocks, found in all cells of the body, and synchronised by the master circadian pacemaker, suprachiasmatic nucleus (SCN), found in the hypothalamus.
- This pacemaker must be reset so our bodies are in synchrony with the outside world.
How does light affect our circadian rhythms?
- Light provides the primary input to this system, setting the body clock to the correct time in a processes called photoentrainment.
- In mammals, light-sensitive cells within the eye act as brightness detectors, sending messages about environmental light levels direct to the SCN
- The SCN uses this information to coordinate the activity of the entire circadian rhythm system.
What’s a circadian rhythm that is subject to photoentrainment
The sleep-wake cycle
What is the sleep-wake cycle?
This circadian rhythm dictates when we should sleep, and when we should be awake
What are the dips and rises in the sleep-wake cycle?
- The rhythm dips and rises at different times of the day
- Our strongest sleep drives occurs in 2 ‘dips’, 2-4am and 1-3pm (post-lunch dip)
- Sleepiness in these dips are less intense if we’ve had a good sleep
What is homeostatic control in the sleep-wake cycle?
- Sleep and wakefulness are also under homeostatic control.
- When we’ve been awake for long time, homeostasis tell us the need for sleep is increasing as the amount of energy used decreases.
- The homeostatic drive for sleep increase throughout the day, reaching its maximum in the late evening when most people fall asleep.
- The homeostatic system tends to make us sleepier as time goes on, regardless of if it’s night or day
Describe the internal circadian ‘clock’ in the sleep wake cycle
It’s described as ‘free-running’, it will maintain a cycle of about 24-25 hours, even in the absence of external cues
How do the major alternations in sleep affect the sleep-wake cycle?
Jet lag and shift work cause the biological clock (and internal physiological system that are dependent on this) to become out of balance
What are the exogenous zeitgebers in the sleep-wake cycle?
- Light and darkness
- Caffeine
- Social cues
What are the endogenous pacemakers?
- Body temperature
- Hormones
- SCN
Describe a study into circadian rhythms
- Michel Siffre subjected himself to long period of time living underground to study his own circadian rhythms
- While underground, he has no external cues. He simply woke and slept when he felt it was appropriate. The only thing influencing his behaviour was his internal body clock
- After his first underground stay, he resurfaced on the 17th of September believing it was the 20th of August.
- On his 2nd stay, his natural circadian rhythm settled down to just over 24 hours, but with some dramatic variation
- On his final stay, he was interested in the effects of aging on circadian rhythm (as he was now 60). He found his body clock ticked slower, sometimes stretching his circadian rhythm to 48 hours
What are the 2 other circadian rhythms?
- Core body temperature
- Hormone production
Describe the core body temperature circadian rhythm
- It’s at its lowest (36°C) at 4:30am and its highest (38°C) at 6pm.
- During the normal circadian rhythm, sleep occurs when the core temp beings to drop, and body temp starts to rise during the last hours of sleep, prompting a feeling of alertness in the morning.
- Core body temp can affect mental abilities: the warmer we are (internally), the better our cognitive performance
Give evidence that core body temp affects mental abilities
Folkard et al found children who had stories read to them at 3pm (warmer) showed superior recall after a week compared to children who heard the same stories at 9am (colder)
