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Summary of Psychology of light and time (0HM200)
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Summary Psychology of light and time (0HM200)Lecture 1 – Sleep-wake regulation, circadian rhythms, circadian effects of light
Circadian rhythms: various metabolic chemical endocrine & behavioral processes alternately through
high and low activity phases with a periodicity of about 24h. Have to be persistent, robust, and
entrainable
- Notation LD 16:8 or LD12:12, light schedule how many hours light and dark
- Free running period in a chronobiological graph: waking up a little later every day (no
ipRGC’s or in a dark cave without time cues), the endogenous rhythm is not 24h
- Loban study forced desynchronization: 27 hr day 8, but 8 subjective days. Urinary flow
followed sleep-wake rhythm (8 peaks, influence of exogenous cue), Potassium secretion
follows 24h rhythm (9 peaks, endogenous cues). In desynchronization some physiological
behavioral rhythms run at a different pace than others, here the two are desynchronized.
- Internal/circadian time is subjective time, CT0 is subjective dawn, CT12 subjective dusk
Sleep-wake regulation:
- Early observations: there are two principles guiding sleep, circadian principle: the longer we
are active, the shorter we sleep. Homeostatic principle: the longer we are active, the deeper
your sleep (not longer due to circadian) to recover
- Studies:
o Sleep deprivation: awake for 3 days, still a clear circadian rhythm
o SWS (slow wave sleep indicator)/EEG: power decreases across the
night, upper line recovery after long wakefulnessthey sleep deeper, more
power/short wave activity
o Duration of sleep after sleep deprivation: first sleep shorter, after 16 h the
circadian rhythm will let you sleep longer
o Internal desynchronization: 46 days in the lab, again free-running since it is
shifting in waking up time, at day 27 person had a 48h schedulealso
explained by above graph
o Actogram: plot of activity vs rest depicted in 24 hours, every new line is new 24
hours. Double plotted actogram: : twice 24 hours, so 48 hours next to each other,
you can clearly see delays or advances
- Two processes are driving sleep, the circadian process C (alerting signal) has
a sinusoidal repeating shape with a period close to 24 h and is driven by the
SCN, and the homeostatic process S (sleep drive) which increases
exponentially while awake and decreases exponentially while asleep, origin
of this is still a bit unclear.
- Sleep propensity: homeostatic sleep pressure-circadian wake-drive, in a well-entrained
system: in the first part of the day they are counteracting each other, later C is making you
sleepy and work together with S to make you sleepy
, - Borbely two-process model/somnostat: the distance between the
homeostat and the lowest circadian line is the sleep pressure. The
circadian process works like a thermostat that switches off at a higher
threshold than it switches on. Sleep pressure increase in morning, at
threshold H (high) sleep is initiated, at L (low) sleep is terminated. The
distance between two circadian rhythm is the threshold between
waking up and falling asleep. High (H) threshold moves up when you are in bright noisy
conditions, lowers when you are in warm conditions or stay in bed.
- Under constant wakefulness: some factors influenced by circadian, some by homeostat,
some by both (do not return to baseline, but you see a circadian rhythm)
- Interaction between process C and S: when you stay awake you are moving diagonal
on the two axis, the circadian component will be extra the longer you are awake. Y
axis is tiredness
Biological clock/SCN suparchiasmatic nuclei:
- Masters circadian pacemaker, it is not driven by sunlight but are synchronized by the 24h
patterns of light (most important) and temperature. It runs approximately 24h, is persistent
in all conditions, it can free-run (run longer e.g. 25h in dark conditions), and it can entrained
back to 24h (amplitude increases). SCN is a synchronizer for every organ clock (every organ
has its own clock)
- Intrinsic photosensitive retinal ganglion cells (iPRGC’s): contain a photopigment melanopsin
which is sensitive to the blue part of the spectrum (480 nm peak). BUT the IPRGC is also
firing when it receives input from the rods and cones, the blue signal (photon) received by
the IPRGC’s directly goes to the internal clock/SCN, the red and green and blue signal from
cone go to visual part in brain.
Two pathways through which light influences humans: image forming (rod & cone to visual cortex)
influences comfort, experience, and performance through visibility of tasks, non image forming
(ipRGC to SCN) affects well-being (mood), and performance via acute alerting effects or sleep quality
via circadian effects
Entrainment: optimal synchronization of length and period of circadian clock with the environment
(LD-cycle), needed since our clock does not exactly run 24h
- Entrainment effect of light are (circadian) time dependent, phase
response curve: predicts the phase delay/advance you will experience
from a light pulse as a function of the internal timing/circadian phase of
the pulse. Middle is mid sleep point and minimum core body
temperature (4:00), closer to this point means stronger effect. If you
receive light on retina before mid sleep point you will delay your clock
(clock will run slower, shifts sleepiness later). If you are exposed to light in very late night or
early morning the clock will be advanced (run faster, shifts sleepiness earlier). The shift is
forever (e.g. 1 hour advance), until you get a new signal.
- Two types of resetting
o Type 1: small or big delay/advance depending on time of light stimulus
o Type 0: if you have a light stimulus at the critical time (lowest core body
temperature) you can shift the clock 12 hours (completely turn the clock)
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