Bio - Circadian rhythms and Ultradian & infradian rhythms

Question 1

What is a circadian rhythm?

Answer 1

A pattern of behaviour that occurs or recurs approximately every 24 hours, and which is set and reset by environmental light levels.

Question 2

Sleep-wake cycle

Answer 2

Refers to alternating states of sleep and waking that are dependent on the 24-hour circadian cycle.

Question 3

What are examples of circadian rhythms?

Answer 3

Sleep-wake cycle
Body temperature
Blood pressure
Hormone production

Question 4

How long is a circadian rhythm?

Answer 4

24h

Question 5

What type of rhythm is the sleep-wake cycle?

Answer 5

Circadian.

Question 6

What type of rhythm is body temperature?

Answer 6

Circadian.

Question 7

What type of rhythm is blood pressure?

Answer 7

Circadian.

Question 8

What type of rhythm is hormone production?

Answer 8

Circadian.

Question 9

How do circadian rhythms operate?

Answer 9

They are driven by our body clocks, found in all of the cells of the body, and synchronised by the master circadian pacemaker, the suprachiasmatic nuclei (SCN), found in the hypothalamus.

Question 10

Explain how the sleep-wake cycle works

Answer 10

Light and darkness are the external signals that determine when we feel the need to sleep and when to wake up. The circadian rhythm also idps and rises at different times of the day, so our strongest sleep drive usually occurs in two ‘dips’, between 2-4am and between 1-3pm (the ‘post-lunch dip’).

The sleepiness we experience during the circadian dips is less intense if we have had sufficient sleep, and more intense when we are sleep deprived.

Not only is this cycle determined by the circadian rhythm alone, but is also under homeostatic control. So when we’ve 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 and is highest in the late evening when we fall asleep.

Circadian rhythm keeps us awake as long as there is daylight and prompts us to sleep when it gets dark, and the homeostatic system tends to make us sleepier as time goes on throughout the waking period regardless of the time of day.

The internal circadian ‘clock’ is ‘free-running’ so will maintain a cycle of about 24-25h even in the absence of external cues. The circadian system is, however, intolerant of any major alterations in sleep and wake schedules (e.g. shift work or jet lag), because this causes the biological clock (and the internal physiological systems that are dependent on this) to become completely out of balance.

Question 11

What is the difference between the effects of the circadian rhythm and the homeostatic system in the sleep-wake cycle?

Answer 11

Circadian rhythm keeps us awake as long as there is daylight and prompts us to sleep when it gets dark, and the homeostatic system tends to make us sleepier as time goes on throughout the waking period regardless of the time of day.

Question 12

Why is core body temperature a circadian rhythm?

Answer 12

It is at its lowest (about 36 degrees) at about 4:30 am and at its highest (about 38 degrees) at about 6pm.

Normally, sleep occurs when the core temperature begins to drop, and body temperature starts to rise during the last hours of sleep, promoting a feeling of alertness in the morning. A small drop in body temperature also occurs in most people between 2pm and 4pm, which may explain why many people feel sleepy in the early afternoon.

Question 13

Why is hormone production a circadian rhythm?

Answer 13

For example, the production and release of melatonin from the pineal gland in the brain follows a circadian rhythm, with peak levels occurring during the hours of darkness. By activating chemical receptors in the brain, melatonin encourages feelings of sleep. When it is dark, more melatonin is produced, and when it is light again, the production of melatonin drops and the person wakes.

Question 14

Describe the case study for circadian rhythms

Answer 14

Series of studies conducted by the French cave explorer Michel Siffre. On several occasions Siffre has subjected himself to long periods of time living underground in order to study his own circadian rhythms.

While living underground he had no external cues to guide his rhythms - no daylight, no clocks or radio. He just simply woke, ate and slept when he felt it was appropriate to do so. The only thing influencing his behaviour was his internal body clock (i.e. ‘his free-running’ circadian rhythms).

After his first underground stay of 61 days in the southern Alps in 1962, he resurfaced on 17 September believing the date was really 20 August. On the second occasion, he spent 6 months in a cave in Texas. His natural circadian rhythm settled down to just over 24h but with some dramatic variations.

On his final underground stay in 1999, he was interested in the effects of ageing on circadian rhythms (he was 60 years old now). He found that his body clock ticked more slowly compared to when he was a young man, sometimes stretching his circadian rhythm to 48h.

Question 15

Briefly show the pathway that the sleep-wake cycle takes

Answer 15

Sunlight–>Photoreceptor (retina)–>SCN (hypothalamus)

and then to either:

1. (Early morning) Intense/blue light–>Cortisol (pituitary)–>Wake.
2. (Sunset) Faint/red light–>melatonin (pineal)–>Sleep.

Question 16

Where is the control centre for circadian rhythms?

Answer 16

Hypothalamus.

Question 17

Infradian rhythms

Answer 17

Rhythms that have a duration of over 24h, and may be weekly, monthly or even annually.

Question 18

Ultradian rhythms

Answer 18

Cycles that last less than 24h, such as the cycle of sleep stages that occur throughout the night.

Question 19

How long do ultradian rhythms last?

Answer 19

Less than 1 day/24h.

Question 20

What are examples of ultradian rhythms?

Answer 20

The sleep cycle/dream cycle/sleep stages.
The Basic Rest Activity Cycle (BRAC).
The glymphatic system.

Question 21

Why/how are the sleep stages an example of an ultradian rhythm?

Answer 21

Because they follow a pattern of alternating REM (rapid eye movement) and NREM (non-rapid eye movement) sleep, which consists of stages one through to four.

The cycle repeats itself about every 90-100 minutes throughout the night, with different stages having different durations.

Question 22

How do the sleep stages progress as an ultradian rhythm?

Answer 22

As a person enters deep sleep, their brainwaves slow and their breathing and HR decreases. During the 5ht stage (REM sleep), the EEG pattern resembles that of an awake person, and it is in this stage that most dreaming occurs.

Stage 1: 4-5% (light sleep, muscle activity slows down, occasional muscle twitching).
Stage 2: 45-55% (breathing pattern and HR slows, slight decrease in body temperature).
Stage 3: 4-6% (deep sleep begins, brain begins to generate slow delta waves).
Stage 4: 12-15% (very deep sleep, rhythmic breathing, limited muscle activity, brain produces delta waves.
Stage 5: 20-25% (rapid eye movement, brainwaves speed up and dreaming occurs, muscles relax and HR increases, breathing is rapid and shallow).

Question 23

Why/how is the Basic Rest Activity Cycle (BRAC) an example of an ultradian rhythm?

Answer 23

Kleitman (1969) - 90 minute cycle during sleep (but also suggested it continues during the day, even when we are awake).

When we are asleep we move progressively through the sleep stages, and when we are awake we move progressively from a state of alertness into a state of physiological fatigue approximately every 90 mins.

Question 24

Who came up with the Basic Rest Activity Cycle (BRAC)?

Answer 24

Kleitman (1969).

Question 25

What is the role of the BRAC?

Answer 25

Research suggests that the human mind can focus for about 90 mins, and towards the end of the 90 min period the body begins to run out of resources, resulting in loss of concentration, fatigue and hunger.

The operation of the BRAC in wakefulness is not as obvious as it is in sleep but Kleitman argued that everyday observations provide evidence of its existence…

… E.g. the familiar 10:30am coffee break allows workers to divide the 9am to noon morning session into two 90 min phases. This pattern is repeated in the afternoon, with cat-naps more likely in mid-afternoon.

Question 26

Why/how is the glymphatic system an example of an ultradian rhythm?

Answer 26

Every 90 mins - happens much more at night (as more parasympathetic) than in the day (2x as fast).

Question 27

Explain what happens in the glymphatic system to make it an ultradian rhythm

Answer 27

Occurs every 90 mins (twice as fast during the night compared to in the day).

• Cerebrospinal fluid (CSF) helps remove waste products from the brain in a convection process.
• This happens in pulses when we’re awake but it’s much faster during sleep.
• Can be applied to understand Alzheimer’s development, and may lead to treatments.

Question 28

What can the glymphatic system be applied to?

Answer 28

Understand Alzheimer’s development, and may lead to treatments.

Question 29

How long are infradian rhythms?

Answer 29

Greater than 24h (can be days, weeks, months or may even be annual).

Question 30

What are examples of infradian rhythms?

Answer 30

The female menstrual cycle (monthly).
The organisation of human activities into weeks (weekly cycles).
Seasonal cycles (seasonal affective disorder).

Question 31

Explain weekly rhythms that are examples of infradian rhythms

Answer 31

There are differences in human behaviour that conform to the weekly cycle of 7 days.

E.g. Although male testosterone levels are elevated at weekends and young couples report more sexual activity at weekends than on weekdays, the frequency of births at weekends is lower than on weekdays.

Halberg et al. (2002) reported seven-day rhythms of blood pressure and HR in humans, but the evidence for weekly infradian rhythms in humans remains sketchy at best.

Question 32

Why/how is the female menstrual cycle an example of an infradian rhythm?

Answer 32

It lasts around 1 month.

Refinetti (2006) - There are considerable variations in the length of this cycle with some women experiencing a relatively short 23-day cycle whereas others have a cycle as long as 36 days. The average appears to be around 28 days.

Question 33

Explain the menstrual cycle as an infradian rhythm

Answer 33

Regulated by hormones which either promote ovulation or stimulate the uterus for fertilisation.

Ovulation occurs roughly halfway through the menstrual cycle, when oestrogen levels peak, and usually lasts for 16 to 36 hours. After the ovulatory phase, progesterone levels increase in preparation for the possible implantation of an embryo in the uterus.

Question 34

What changes do hormones in the menstrual cycle cause to happen in the body?

Answer 34

Oestrogen levels increase serotonin levels and the number of serotonin receptors. It changes the action of endorphins (endogenous opioids) and protects nerves from damage. It can even stimulate nerve growth.

Progesterone increases amygdala activation and lowers its activation threshold. When it is processed, one of its by-products binds to the GABA-a receptor (just like BZ drugs).

Question 35

WHat annual rhythms are associated with infradian rhythms?

Answer 35

In most animals, annual rhythms are related to the seasons (e.g. migration as a response to lower temperatures and decreased food sources in winter), but in humans the calendar year appears to influence behaviour regardless of changes in temperature.

Research suggests a seasonal variation in mood in humans, especially in women (Magnusson, 2000), with some people becoming severely depressed during the winter months (seasonal affective disorder).

The winter is also associated with an increase in heart attacks, which varies seasonally and peaks in winter. In fact, there is a robust annual rhythm in human deaths, with most deaths occurring in January (Trudeau, 1997).

Question 36

What are some symptoms associated with winter seasonal affective disorder?

Answer 36

Overeating, unexplained weight gain, more sleeping than normal, depression.

Question 37

What are some symptoms associated with summer seasonal affective disorder?

Answer 37

Loss of appetite, unexplained weight loss, insomnia, irritability.