Chronobiology of sleep Flashcards
Homeostatic drive for sleep:
1- why does sleep tendency increase?
2- what happens if the time we are awake increases?
3- what happens if sleep is delayed?
4- what accounts for the homeostatic drive for sleep?
1- due to being awake a long time
2- the length of time we are awake increases, sleep pressure mounts and we accumulate sleep debt
3- If we delay sleep and stay awake for longer than usual, we will sleep for a longer period once we get the chance to sleep, known as sleep rebound
4- adenosine build up
Circadian pressure for sleep:
- what is another factor affecting our tendency to go to sleep or be awake?
- according to Borbély (1982) what two processes is sleep controlled by?
- what is this known as?
- Also, the time of day (light and darkness) affects our tendency to go to sleep or to be awake
- Borbély, 1982: Sleep is controlled mainly by two processes:
— Homeostatic (S) and Circadian (C) pressures govern a predictable pattern of maximum sleepiness and maximum alertness throughout the 24h cycle
— This is widely known as the two-process model for sleep
Two-process model of sleep (Borbély, AA)
S drive is telling us we’ve been awake for a long time and we should go to sleep. Then this becomes quiet until the next time we wake up.
The C is regular- daytime and nighttime
Biological rhythms
1- what are endogenous cycles?
2- how have endogenous rhythms evolved?
3- Circadian “circa” and “diem” meaning
4- what is the most prominent circadian rhythm?
1- Endogenous cycles (“generated from within”): Living organisms display regularity in certain behaviours, that are generated within the organism. They match light and dark cycles.
2- Endogenous rhythms have evolved through evolution on planet earth and can be of various durations, ultradian or seasonal (breeding, hibernating, migrating, etc)
- offers the advantage to plan ahead i.e store food
3- Circadian “circa” and “diem” - rhythms or regular patterns of activity associated with a 24h-cycle associated with the earth’s rotation
4- The most prominent circadian rhythm is that of sleep and wakefulness but there are many more
Average Body Temperature
This also has a circadian rhythms
Body temp fluctuates based on the 24 hour circle
When we go to sleep its 0 and then after 2/3 hours after we go to sleep out body temperature takes a dive. Then before we wake up it starts to climb.
Circadian Rhythms across functions
- melatonin- starts to get secreted in the evening and then it goes down the next day
- temperature
- alertness- drops in the evening
- task performance- gets bad in the evening / early morning hours
- triacyglycerol
Early discoveries:
- what are biorhythms not unique to?
- 1729 what was discovered?
- Biorhythms are not unique to humans and animals
— Plants and flowers may open during the day and close during the night - 1729: French geologist, Jean Jacque d’Ortous de Mairan experimented with the mimosa plant
— Even if isolated from light, dark or temperature cues the leaves continued their rhythmic behavior
Suspecting its Existence: Curt Paul Richter, Prof. of Psychobiology at Johns Hopkins (1927)
1- what concept did he introduce?
2- what did he perform?
3- what happened to rats?
4- book name and hypothesis?
1- Introduced the concept that the brain generates its own rhythms, so it must have a biological clock which he attempted to locate in the brain of wild rats
2- He performed electrical lesions in various parts of their brain in order to locate the biological clock
3- The rats lost their rhythmic behaviour after damage to the hypothalamus (he thought if this rhythmicity is lost it will cause probelms with the biological clock and be detrimental to our health)
4- In his book “Biological clocks in Medicine and Psychiatry” (1965) he hypothesized that many disorders may result from disruption of the biological clock
What is the biological clock? (disruptions causing)
(R.Y. Moore & V.B. Eichler (U Chicago) and F.K. Stephan & I. Zucker (UC Berkeley), 1972)
The suprachiasmatic nucleus (SCN) of the hypothalamus
Lesions of this nucleus disrupted circadian rhythms of wheel running, drinking, and hormonal secretion – thus named the “master clock”
The suprachiasmatic nucleus (SCN):
1- what does it consist and location?
2- what does it generate?
3- lesion implications?
1- The SCN consists of a group of cells in the hypothalamus, right above the optic chiasm (“supra” and “chiasm”)
2- The SCN generates circadian rhythms in a genetically controlled manner – even if raised in constant light or dark conditions the animals develop circadian rhythms
3- If the SCN is lesioned the animals still engage in behaviors but in a haphazard way
The SCN:
1- activity of neurons? (time of day)
2- function?
3- impact of transplantation of SCN?
1- Recording electrodes in the SCN confirm that neurons are more active during the light period than during the dark period
2- Autonomous function - a single cell extracted from the SCN and raised in tissue culture continues to function in a rhythmic pattern
3- Transplantation of an SCN into a donor organism results in the recipient following the donor’s rhythm (the donor will dictate the activity of the recipient)
SCN firing in light vs dark
(Pattron and Hastings, 2018)
firing only during day time and then quiet during night- showing a clear distinction of when its day and night. (this persists so if you put them in a dish they will continue to do this- the clock is still ticking)
lights on - they stop exercising. When the lights are off they start exercising.
The experimenters then kept it dark- the mice had an idea of when the lights used to be on so they maintained their rhythmicity.
What makes this clock tick?
Findings of…
- Konopka and Benzer 1971
- Jeffrey Hall & Michael Rosbash
- Michael Young
Konopka and Benzer 1971: the 24h cycle was changed in mutant flies
- (Drosophila Melanogaster) and suggested a gene on the X chromosome
- Then the 3 researchers below discovered one by one the genes and it took some time to discover how these come together to create the function of the clock.
Jeffrey Hall & Michael Rosbash
- Studied the SCN in Drosophila
- 1984: Discovered the “per” gene and the “PER” protein (period)
— The PER protein builds up and goes down within a 24-hour cycle
Michael Young
- 1994: Discovered the gene “tim” that produces the “TIM” protein (timeless)
- When TIM meets PER they combine and shut the period gene down
What about in mammals?
1- what did work from fruit flies show?
2- what was found by chance?
3- What did Joseph Takahashi at Northwestern University do?
1- Work from fruit flies to mammals showed that the system is conserved across species
2- Mutant golden hamster found by chance, to have a rather short period of ~20-22h long – (Ralph and Menaker, 1988, Science)
3- Joseph Takahashi at Northwestern University actively searched for mutations in circadian rhythms in mice (forward genetics- trying cosmetations and then looking at changes in behvaiour)
— Identified a mutant mouse with particularly long cycle of ~25-28h (Vitaterna et al., 1994; Takahashi, Pinto and Vitaterna, 1994)
— The mutation was in the clock gene
The Molecular Mechanism:
In Drosophilla vs Mammals
Drosophilla:
2 proteins that act as transcription factors. They come together into the enhancer box and they initiate the transcription of certain genes (per, tim). They start to be transcribed and translated into the per protein and tim protein. These are in the cytoplasm. They start to accumulate and as they become more and more they start to bump into each other and become dimers. They both walk together now. When they are dimerised they go back into the nucleus and acts as repressors. They act to inhibit the transcription that really produced them in the first place. Inhibit until there is few of them left and the production is stopped. Takes place in 24h time period.
Mammals:
- Similar situation
- Changes in proteins
- The activators are the clock and the BMAL 1. These initiate the transcription of the Per 1-3 or Cry1,2. These translate into proteins in a similar way, they dimerise, come back into the nucleus and inhibit their own production.
Transcription-Translation-Inhibition- Feedback loop
1- what are involved in this clock
2- what process
3- what happens next
4- what happens again
1- A few genes and their protein products are involved in this clock
2- Transcription from DNA to mRNA, to translation into proteins which form dimers
3- These dimers enter the nucleus in order to inhibit transcription and then they decay
4- The cycle begins again in a daily rhythm
(When we are awake there is the mRNA that is rising for the per and time as they get transcribed and translated. Then they go down at night time. The opposite as true because the proteins take time to form but when they form they inhibit production and also decrease. )
The SCN in mammals
Clock and BMAL1 as ‘activators’
Period and Cryptochrome as ‘repressors’
What happens without external cues?
Aschoff (1965)
- Humans were placed in an underground banker where no cues were available
- Allowed to select their light-dark cycle and turn the lights on and off at will
- The participants continued to show daily sleep-activity rhythms of slightly greater than 24h cycles, even though they drifted. But they maintained their rhythmicity.
- Demonstrated that humans have an endogenous biological clock which governs sleep-wake behavior
Biological Clock and “Time Givers”
1- what is the biological clock influenced by?
2- what is the most potent zeitgeber for humans?
3- what are zeitgebers often tied to?
4- how does entraining occur?
1- The biological clock is influenced by external cues called Zeitgebers (“time givers”)
2- The most potent zeitgeber for humans is light although there are others (temperature, noise, tides, feeding, etc)
3- Zeitgebers are often tied to our social environment such as school start time, family routines and mealtimes
4- When a zeitgeber resets a biorhythm, that rhythm is said to be entrained (maintaining constant rhythms due to zeitgebers)
Entrainment vs free-running
entrained because they are adjusted/ in sync with the presence and the absence of light. Daytime- awake, lights off- asleep.
free-running (not a fixed pattern)- dim light that is on constantly. There is no variability. There is some drifting.
How does light reach the SCN?
Retinohypothalamic tract
Formed by a special population of ganglion cells (photosensitive retinal ganglion cells-PRGCs- all they do is respond to the intensity of light which make up) ~1-3% of ganglion cells (Berson et al 2002)
Photosensitive retinal ganglion cells (PRGCs)
1- what do PRGCs have?
2- What do they not rely on?
3- What had no effect on circadian rhythms?
4- What happens to part of this tract?
1- their own photopigment called melanopsin and can respond directly to light (especially blue light) to release glutamate as well as pituitary adenylyl-cyclase activating peptide (PACAP)
2- They do not rely on rods and cones which explains the fact that blind people remain entrained
3- Experiments where rods and cones were inactivated had no effect on circadian rhythms
4- Part of this tract terminates in the midbrain to control the size of the pupil in response to light and to suppress melatonin
The retino-hypothalamic tract
- zooming into the retina (membrane at the back of your eye)
- rods and cones at the back and the cells in between
- Eventually reach ganglion cells which normally their axons form together their optic nerve and they go back into the brain
- Where as in this case we’re not really dealing with information about the objects and what we see and the colour they are dealing with responding to the presence/ absence of light and they feed the info back to the SCN and inform it or set it/ reset it about the aspect of light.
Melatonin
- Facilitates sleep and has been shown to act as a mild zeitgeber
- It is released 14 hours after awakening and about 2hrs before sleep onset by the pineal gland
- Reaches its peak near the middle of the sleep period, and returns to its low waking levels by the end of sleep
- Strongly controlled by C, however, light striking the eyes inhibits its release. If there is an advance or delay in the schedule of light and dark, it will take time for the melatonin release to follow
- Melatonin has been shown to cause sleepiness, speed sleep onset and facilitate sleep maintenance (available in pill form)
- secreted in the evening usually, a couple of hours before we go to sleep
- Feeds back to the SCN to help it adjust its activity
- goes into sleep mode
What does the SCN drive/ responsible for?
The SCN drives a number of slave oscillators, each responsible for the timing of a different type of behaviour ie. drinking, sleeping, body temp, activity
SCN gives info to start or inhibit activities.
It affects a lot of brain areas and the outcome of that. It has a lot of slave oscillators because its giving input into how other areas are working- not just in the brain but for other areas such as the heart and liver.
It should affect the other systems in sync (but this is not always the case)
Variability across humans
1- phase of?
2- what is a chronotype?
3- what cab differences in chronotype result in?
4- what factors contribute to chronotypes?
1- Our phase of entrainment or chronotype varies
2- Chronotype is sometimes referred to as morningness (‘larks’) or eveningness (‘owls’)
3- Differences in chronotype may result in “social jet lag”
4- Factors that contribute to our chronotype are:
- Genetics (variability in the clock genes)
- Age
- Environment (presence, strength and structure of appropriate Zeitgebers)
Phase of entrainment and Zeitgebers
1- in some people, what is the phase of entrainment?
2- what is the main zeitgeber?
1- In some people, the phase of entrainment is extreme and leads to syndromes known as delayed sleep phase syndrome (go to sleep later and wake up later) and advanced sleep phase syndrome (go to bed early, wake up early) **
(cannot fall asleep until 3am- delayed or regularly wake up at 4am- advanced)
2- The main zeitgeber is exogenous while the entraining signals for cellular clocks are endogenous, such as transmitters or hormones
- i.e. the liver may receive input from the SCN but it also receives cues from feeding and metabolism
Delayed Sleep Phase Disorder (DSPD)
1- What is Familial DSPD associated with?
2- characterised by?
3- commonality?
1- Familial DSPD is associated with a variation in the human circadian clock geneCRY1
- it provides a genetic underpinning for “night owls”
2- Characterized by a persistent delay of sleep onset and offset times relative to the societal norm
3- The most commonly diagnosed type of circadian rhythm sleep disorder with an estimated prevalence of 0.2–10% in the general population
Age:
larks vs owls?
morningness vs eveningness?
We all start off (infancy and childhood) and finish off (adulthood and old age) as morning people “larks” (wake up early and go to bed early)
During adolescence there is an increasing shift towards “eveningness” – teenagers become “owls”, which is particularly difficult given that school starts early in the morning (A. Carskadon, USA)
Morningness associated with more positive outcomes and reports of being happier compared to evening people vs Eveningness has been associated with lower grades, increased likelihood for alcohol use, smoking, overeating, and engaging in risky behaviors
Age and Gender:
Timings of lateness for women and men
60+?
- Women reach their maximum in lateness at 19.5 years of age while men continue to delay their sleep until 21 years of age.
- Men have later chronotypes for most of their adulthood, and this sex difference disappears around age 50
- Those 60+ on average become earlier chronotypes than they were as children
sum: women tend to be morning people compared to men
Environment: The strength of the zeitgeber:
- what is important?
- what is a typical feature of a circadian rhythm that does not receive a strong enough zeitgeber?
- what becomes more extreme as the strength of the zeitgeber is decreased?
- The amplitude of day-night light intensity is important
- ‘Relative coordination’a typical feature of a circadian rhythm that does not receive a strong enough zeitgeber
- i.e. constantly in a dimly lit room so that there is lack of strong day-night difference. This may be reversible if a person is placed in a tight schedule of appropriate zeitgebers
- Extreme chronotypes at both ends will become even more extreme when the strength of the zeitgeber is decreased
- Those with shorter than 24h free-running period will move towards an earlier time
- Those with longer than 24h free-running period (typical for most humans) the clock will move towards a later phase
Having a contrast between light and day can help to set and maintain clock - relative coordination
If we have this it can improve several things such as digestion, hormones, alertness
Seasonal Affective Disorder (SAD):
1- what is it?
2- what do people experience?
3- what can reset the circadian clock and ameliorate depression, name?
1- A type of depression where low levels of light in the winter are not sufficient to entrain the circadian rhythms
2- People experience phase-delayed sleep and temperature rhythms
3- Exposure to bright light can reset the circadian clock and ameliorate depression – phototherapy
Jet lag:
1- what is it?
2- stems from?
3- characterised by?
4- travelling west vs travelling east?
1- A disruption of the circadian rhythms due to crossing time zones
2- Stems from a mismatch or misalignment of the internal circadian clock and external time
3- Sleepiness during the day, sleeplessness at night, and impaired concentration
4- Travelling west “phase-delays”** (much later) **our circadian rhythms whereas travelling east “phase-advances” (people here will be getting ready for bed) our circadian rhythms – people find it more difficult
Alignment with other clocks in the body:
- drosophilla?
- brain/ heart/ muscle/ lung?
- rats?
- Isolated body parts in drosophila are able to produce circadian rhythms and similar evidence is found in mammals
- In the brain, the heart, muscle, lung there are clock genes that undergo oscillatory changes in gene expression
- When rats are subjected to ‘jetlag’ experiments in which the light and dark cycle is shifted by several hours, the rhythms shift with different speeds in different organs (some adjusted quickly and others took a while)
- The SCN adjusts within one cycle, but the liver can take up to six days to synchronize with the new cycle
- Thus, there can be misalignment of clocks within the body in the absence of regular exposure to zeitgebers
- Feeding restriction experiments have shown that the liver may function based on feeding timing rather than the SCN’s instructions.
Circadian clock and disease
Clock dictating sleep and wakefulness and arousal. Depending on how we affect our clock through these environmental zeitegbers we may give positive or negative information. This may contribute to us being healthy, having energy to go about our day or we could start feeling bad, developing disease.
To some extent we can control things.
Shift workers
- Shift workers often need to make major adjustments especially those working late at night until early in the morning
- Night-shift workers often fail to adjust completely even though they try to sleep when not working
- Shift work (chronic jet lag) suppresses or delays the expression of core clock genes in the SCN as well as cell cycle-related genes
- Health risks are high and lead to increased mortality risk - WHO listed shiftwork as probable carcinogen
Shiftwork sleep disorder:
- defined by?
- prevalence?
- symptom?
- side effect?
- Defined by excessive sleepiness or insomnia accompanied by total sleep time reduction
- 10-38% prevalence in shift workers
- Reduction in alertness and performance
- Depression, ulcers and other GI problems, and sleepiness-related accidents
Effects on shift-workers
Reproductive effects: spontaneous abortion, low birth weight, prematurity
Cardiovascular effects: 40% increased risk for angina pectoris, hypertension, myocardial infarction
Mental health: stress, anxiety, depression, neuroticism, reduced vigilance, ‘burnout syndrome’
Circadian rhythm disruptions: body temp, respiratory rate, hormonal production, menstrual cycle, urinary excretion, cell division
Brain effects: sleep loss, REM sleep reduction, stage 2 sleep reduction, fatigue, reduced brain volume
Gastrointestinal disorders: dyspepsia, heartburn, abdominal pains, flatulence
Increased cancer: breast cancer, colorectal cancer
Shift-work schedules tried in Europe
reviewed different schedules to identify if some people are in a better position than others
results: no one schedule was okay. People never really managed to adjust and have a normal rhythm
What else is true?
- Time of day affects performance in human subjects on a wide range of cognitive tasks measuring attention, executive functions and memory
– Fluctuations in performance are contingent upon one’s chornotype
- Treatment of disease can be influenced by circadian rhythms i.e. surgery outcome or pharmacotherapy
– Toxicity of a drug varies from 20-80% depending on the time of day (not necessarily considered in drug studies) - chronotherapy
– Risk for illness also changes depending on the time of day, i.e. there is a higher likelihood of a stroke or heart attack in the morning compared to the afternoon
