Keeping an internal sense of time Flashcards
Rhythms of life
Biological rhythms associated with a solar day - circadian
* Ultradian: < 24 h (NREM-REM sleep cycle = 90-120mins in adults, infants = 50mins, another e.g. is breathing cycle (time taken to breath in + out = 3-4secs in, 2-3secs out))
* Circadian: 24-28 h (sleep-wake cycle, also body temp, HR, hormones: melatonin + cortisol)
* Infradian > 28h
○ Circaseptan: 1 week (the common cold) = 1 week to recover
○ Circatrigintan: 1 month (menstruation) = not exactly 30 days, may be 28 days, some 21 but roughly a month
○ Circaannual: 1 year (migration of birds), also reproduction (sperms at healthiest in winter and early spring = increased chance of survival + therefore greater chance of conception)
Characteristics of circadian rhythms
- Exhibit peaks + troughs
○ Peak known as the acrophase
○ Amplitude of a curve starts from the midline = known as mesor
○ In circadian rhythms we are interested in the amplitude of a curve as it tells us about the strength of the rhythm
○ Orange line on graph = typical of shift workers = sleep is disrupted = generally sleep is not consolidated as light is out
§ Therefore if measure their core body temp, particularly after a night shift or after a person has been sleep deprived chronically = the strength of their cycle is reduced
§ Amplitude can also be measured by angle of degree
§ Usually our temp rhythm is under entrained condition = means our rhythm is time-locked to the light-dark cycle = time locked to the external env
UNDER ENTRAINED CONDITION
- Under entrained condition:
○ the rhythm keeps a distinct phase-relationship to the zeitgeber (light) + to one another, e.g., sleepwake rhythm + core temp rhythm
○ *zeitgeber - time cue = light source is a very powerful zeitgeber for our circadian rhythms
Sleep propensity
Sleep propensity ties to core body temperature
Propensity = inclination/or how much you want to do that
- If go to sleep at acrophase strong bias to not let you fall asleep = is at 6:00pm on average = will take place in afternoon but can be at 5:00pm or 8:00pm for the night owls = differs slightly because our genetic make-up is different
core body temp + sleep propensity
- Sleep begun when core body temp starts to rise = past nadir = as temp climbs up = sleep propensity goes down
- If wake up at this time in night = can have difficulty going back to sleep
- sleep begun at the start of the falling slope will be long = because this is around your bedtime = as the temp falls your propensity for sleep is very high
Phase of a rhythm
- defined w/ respect to an easily identifiable reference point of the endogenous (within/naturally occurring) circadian oscillation, e.g.,
○ the nadir (lowest)/ peak of the melatonin rhythm
○ the nadir/ peak of the cortisol rhythm - E.g. of endogenous rhythms = HR, breathing, body temp
MELATONIN = PEAK MIDNIGHT
CORTISOL = PEAK MORNING I.E. 6:00-7:00 AM
Phase relationships between circadian rhythms
- Phase shift
- Relative dif b/w the timing of the two rhythms
○ here an illustration - Phase looking at here is peak body temp
- Top curve = normal rhythm
- Bottom curve = when you have been sleep deprived for 36hrs or more
- Alertness and temp = dropped, core body temp effected due to sleep deprived = strength of the rhythm has been dampened, temp has been dampened, alertness dampened
Chronotype
Chronotype:
* the person’s natural circadian rhythmicity
* Distribution of MSFsc varies
○ (MSFsc - mid-sleep time on free days corrected for sleep debt on workdays)
* has a genetic component
○ not a fixed trait
* depends on:
○ light exposure (zeitgeber strength)
§ the phase (Ψ) of the sleep-wake cycle controlled by the sun + not local time
- Can change chronotype = will take approx 2 weeks but can do so if want to change from late chronotype to early because chronotype is powerfully influenced by light exposure + also depends on strength of the exposure
- First thing in the morning = the light intensity is not so high/blue light not as high but around mid-morning if exposed to morning sun = it can shift you to become an early type if a late one
chronotype continued
Chronotype:
* depends on:
○ Sex
§ males + females are different chronotypes - depending on age
○ Age
§ Childhood – Early chronotype
§ Adolescence – Late chronotype
§ Late adulthood – Early chronotype
- Young people tend to be early chronotype = go to bed early, wake up early
- Older adults 45ish to 65 + onwards = tend to be early chronotype
- On other hand adolescence + young adults = late chronotype = not your doing = is a natural phenomenon = all teenagers they have a specific preference to go to bed late = modulated if early chronotype
- Females late chronotype but even later than males
Questionnaire for assessing chronotype:
- Morningness and Eveningness Questionnaire, MEQ
- Munich Chronotype Questionnaire, MCTQ √
Assessing sleep timing (morningness–eveningness)
- The instrument: the morningness–eveningness questionnaire (MEQ) (Horne and Östberg, 1976):
○ preferred bedtime
○ get-up time
○ best time for physical exercise
○ Appetite = late don’t like eating immediately in morning, alertness, tiredness + sleep inertia (wake up groggy)
○ mental + physical perf - Produces a score –> “morningness” (high value) “eveningness” (low value), “intermediate”
Assessing chronotype (phenotype)
- The instrument: the Munich ChronoType Questionnaire (MCTQ) (Roenneberg et al., 2003)
○ subjective sleep times on both work + free days
○ calculate the midpoint of sleep on work-free days (MSF) as phase marker
○ correct for potential oversleep on free days due to sleep-loss during the work week –> MSFsc (a time in hours) - Assessing the phase of entrainment, Ψ (later)
○ –> by measuring the rhythm of melatonin production or core body temp rhythm
Chronotype influences:
1. Physical activity and sports performance
○ the circadian rhythm of activity levels (Vitale et al., 2015)
○ ratings of perceived exertion + fatigue scores in submaximal + self-paced physical tasks performed in the morning:
○ M types seem to have more of an advantage
§ less fatigued in the first part of the day than N types (Neither), + E types
§ M-types have better athletic performances, as measured by race times, in the morning than Ntypes + E-types (Vitale & Weydahl, 2017)
Chronotype influences:
2. Sleep duration
- Late: during work days have less sleep, on work-free days = looks like they are catching up on sleep so getting more sleep on work free days
- Early: more sleep on work days, less sleep on work-free days
- Intermediate: have similar amount of sleep on work free + work days
- early chronotype, they tend to have better sleep, more sleep during the workday. And then, of course, on weekends they can delay their bed time due to social activities, et cetera, so the sleep duration is reduced
- for intermediate chronotype, probably in the middle. We don’t have to worry too much about that.
- The late chronotype during work days, they tend to have less sleep, the reason being that they are late chronotype. They prefer to go to bed late, but because of work, school or uni situations to attend classes, they need to get up early, + consequently they are sleep deprived. However, they catch up their sleep on weekends
- so this study was done in 2014.
Differences in the circadian pacemaker’s program
- Variability of sleep duration in the general population
- If consistently get less than 6h sleep a night and feel refreshed = short sleeper, if not then you are sleep depriving yourself = same for long sleepers
- short-sleepers: <6hrs (shorter biological night)
- long-sleepers: > 9hrs sleep (longer biological night)
Variability of sleep duration between long and short sleepers - first need to look at melatonin + body temp to explain why you may be a short or long sleeper
- If look at area under the curve = long sleeper area is a lot higher = melatonin concentration is higher for the long sleepers compared to short sleepers
- Temp is lower for long sleeper compared to the short sleepers
Secondly = the time spent in this lower core body temp, your nadir of the temp trough, is longer for long sleepers compared to the short sleepers - These physiological mechanisms explain the differences b/w the short and long sleepers and is due to genetic differences
SUMMARY: - the duration of the nocturnal interval of high melatonin levels is longer in long than short sleepers
- the nocturnal interval of low body temperature is longer in long than short sleepers
Variability of sleep duration between long and short sleepers - cortisol
- The nocturnal interval of increasing plasma cortisol levels was longer in long sleepers than in short sleepers
- For the long sleeper = cortisol level was longer before they peak = means they have a longer time spent during sleep
- Whereas for the short sleepers = sleep pressure dissipates very quickly + also as soon as they wake up their cortisol also peaks
The three clocks
- The Sun clock – set by the rotation of Earth = positioning of sun explains timing
- The circadian clock – set by our individual body clock
- The social clock – set by human societies (about our behaviour + determined by our behaviour)
The Sun clock
Sun
* Prime source of life –> light
Light
* the source of energy through photosynthesis = w/o the sun we cannot produce energy
○ Requires sun + CO2 to make carbohydrate molecules = + these carbohydrate molecules eventually produce energy = when carbohydrates is oxidised it produces energy
○ Carb molecules transferred from organism to organism = carbs form a food chain for us/all organisms on earth
* the most important source of info about time of day
* the prominent zeitgeber (external timing signal/cue) for the body clock
○ Resets the master clock
Light intensities:
* In direct sunlight - can reach illuminances up to 100,000 lux (measurement unit for light intensity)
* In full daylight - 25,000 lux
* In closed rooms (e.g., standard office) ~500 lux, often lower
* Daylight availability depends on geographical location + season
The physical clock vs the body clock
- What are the similarities + differences b/w the physical + body clocks?
Physical clock - Keeps time
- Ticks at a constant rate
- Has a clock period that’s almost exactly 24h
Body clock - Keeps time
- Has an internal network of clocks = SCN in brain + peripheral clocks
- Has a clock period that is NOT exactly ~24h
- Needs to be reset daily
The body clock
Why is there a need to temporally (time-based) align the internal clocks w/ the natural cycles of the environment?
Circadian timing
* Controls melatonin production
* Allows bodily functions to be coordinated + be in synchrony w/ external env
○ sleep, alertness, temp, muscle strength, metabolism, hormonal output
○ During daytime your body works differently to when its night = a lot of the enzyme activities are very active during the daytime (a lot of the proteins are being synthesised to cater for the type of activity that you do such as exercise, eating, digestion etc) so therefore the proteins that are produced to target these activities have to be different for night time = as night time is designed for you to sleep + rest
* Confers good health, perf + athleticism
The Body Clock
- Located in the suprachiasmatic nucleus (SCN)
○ Master clock/ pace-maker, the circadian clock, oscillator, or temporal program - Endogenous clock
○ Slightly longer than the 24h day
○ Under genetic control
○ Adjustable in response to light but weakly to social cues - The SCN clock is reset primarily by the external light-dark cycle but also
○ non-photic input (from the pineal gland) - Peripheral tissue clocks are influenced by other zeitgebers e.g., the timing of food intake, exercise
The social clock
- Social cues are non-photic (not to do with light)
○ social interactions, behavioural activity, sleeping behaviour - Does the human clock entrain to social cues?
○ not directly - Blind people (lacking physiological light responses due to suppression of melatonin/ pupillary reaction):
○ In the absence of light (endogenous system works thoroughly) –> clocks free-running - So how do these blind people live a 24-h day?
○ Endogenous rhythms exist but clock period lengthens (e.g., by a few minutes) each day
○ Need apply strong 24h social cues to entrain the body clocks
The circadian time-keeping system
- Muscle clock = being the largest peripheral clock actually can feedback directly to the SCN = has some effect directly on the SCN
- The SCN also direct the pineal gland to inhibit or release melatonin based on the light/dark cycle
- Light will supress melatonin during the day = keep you alert
- But at night with dark onset melatonin is released = will promote sleep
- Social clock can not entrain master clock but can disrupt the clocks
Circadian terms
- Zeitgeber - a temporal/ time cue
○ Photic: light
○ Non-photic: temp, eating or drinking patterns etc - Light (zeitgeber) entrains the body clock
Entrainment
- a temporal or time-locking process
- E.G. the sun creates a time-locking process w/ the master clock, aligns the master clock, resets the master clock
Why is entrainment required?
- Because our solar day + our body clock length = big discrepancy in terms of the length of these 2 clocks
- Solar day (natural cycle): Zeitgeber period (T, tau) = 24h
- Endogenous Circadian period (τ, tau) = Average 24.2h
○ ∴ needs daily resetting! - If not reset, say, the endogenous phase is shifted by 0.1 h (6 min) every day:
○ in 10 days the circadian system of the animal will be 1 h ahead w/ respect to a natural cycle (T)
Entrainment - benefits
Adaptive value:
* If τ of endogenous rhythms not matched to T of natural cycles (∴ left to own free-running properties)
○ –> endogenous rhythms will dissociate from the natural cycles
* Temporal cues (light/dark, temp, humidity, tides, social activity, + several other cycles):
○ –> help rhythms to adapt + anticipate to natural periodic changes
* By entraining to the light/dark cycle = we know that during the daytime we are active organisms carrying out a lot of different kinds of activities + your body systems are adapted to these activities to allow you to perform at your best, on other hand during dark period want to shut down many of these activities including cellular activity so that we can rest, sleep, retain memory, consolidate memory + recover from all the damages/injuries that occur + also build up molecules i.e. increase muscle mass
How does the human clock entrain (synced) to the 24-h day?
= need 3 elements
* Primary zeitgeber: light
* Mediator: the eye
* Photoreceptor: melanopsin = found in the eyes
BLUE LIGHT
- Blue light is a very important form of light that signals the photoreceptors melanopsin = these receptors are highly sensitive to blue light
- Research that shows if you are exposed to the near infrared light it improves your health tremendously = penetrates through hair, skull, skin down to the mito level and actually energises the mito = which are the organelles that produce energy
PROCESS:
- Blue light that enters the eye
- At back of eye is retina + in retina is retinal ganglion cells + that is where the melanopsin receptors are located within the retinal ganglion cells
- And from here the signal travels to the SCN where it is transferred to the PVN + it then travels down the spinal cord to superior cervial ganglia before it takes the signal back up to the pineal gland
- It is this pineal gland that releases melatonin
- What happens when you wake up = light enters retina + suppresses pineal gland melatonin secretion
internal sense of time from revision lecture
- Internal sense of time = has to do with body clock
- For majority of people = body clock = more than 24hrs = to be aligned to light/dark cycle = need to be exposed to light in the morning to advance your physiological rhythms a little bit = so can align with L/D cycle
- Physiological rhythms = most one that affects us is your sleep/wake cycle
PRIMARY ZEITGEBER = light = especially blue light = info received by the melanopsin receptors on the retina of the eyes = then received by the SCN = then signals downstream, to the pineal gland = where the pineal gland in response to light will stop the secretion of melatonin
On waking want to be alert and ready for the day = want to increase cortisol + suppress melatonin
Melanopsin
Melanopsin
* the non-visual photosensitive retinal ganglion cells
* Rods (rhodopsin) for night vision
* Cones for daytime colour vision
* Melanopsin (<1% of retinal ganglion cells)
○ Discovered in 2002
- a blue-light sensitive photopigment at ~460nm
- sends info about env illumination to nonvisual centers including the SCN (suprachiasmatic nucleus)
- mediates non-image forming light responses:
○ suppression of melatonin secretion
○ an increase in alertness + perf
○ phase-shifting of sleep timing
Retinal illumination
- Photoentraining the SCN to light-dark cycles depends on:
○ Light intensity, duration, spectrum, timing relative to the phase of the circadian rhythm
§ short-wavelength-sensitive
§ most sensitive to light stimuli during the biological night, + far less sensitive to light in the middle of the biological day
§ the circadian system of indivs who get little bright light exposure may become more sensitive to moderate levels of light = e.g. spend a lot of time indoors, nurses etc.
(Duffy et al., 2009)
Blue light varies across season and day
- During the winter period, the amount of blue light exposure is a lot less concentrated from after 9:00am to about 2:30ishpm
- Whereas in the summer time - even from 7:00am where you are exposed to light = find the amount of blue light is still in abundance and especially during midday there is a lot of blue light
- Not much difference in summer and winter from about 10:00am to 3:30pm = fairly similar, however after 3:00pm in the arvo, find that in the winter time the amount of blue light is vastly reduced compared to the summer = means that if you want to reset your clock make sure you do so when blue light is at its highest level, means around mid-morning is perfect
Light and melatonin suppression
- Daily alternation of light and darkness regulates pineal melatonin levels
- Even at night when you are exposed to very low light intensity = it can still suppress melatonin release except that at a lower level
- If you want a consolidated good nights sleep = do not be exposed to blue light for at least 30mins before bed = sleep a lot better
Retinal illumination summary?
- Morning exposure to sunlight:
- Resets the body clock
- ↑ alertness
- ↑ mood & vitality (due to ↑ in brain serotonin level )
- ↑ core body temp
- ↑ cognition
- Inadequate env light and/or ganglion photoreception can cause:
- circadian disruption
- ↑ risk of insomnia, depression
- Systemic disorders –> early mortality
Ageing
* Due to pupillary miosis (pupil constriction) + ↓ crystalline lens light transmittance
= ↓ circadian photoreception
