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What are the three processes that mediate sleep regulation?
HOMEOSTATIC PROCESS:
Homeostatic debt, sleep pressure
Sleep pressure starts upon awakening and diminishes with sleep
CIRCADIAN PROCESS
Circadian phase: highs/lows of sleep propensity, largely independent of preceding sleep period
ULTRADIAN PROCESS:
Sleep architecture within a period of sleep, alernating structure of REM/NREM sleep
How long is the ultradian process?
Less than 24 hours
What is the ultradian process modulated by?
Circadian + homeostatic process
How are sleep stages distributed during early vs. late night by ultradian process?
Slow wave sleep is predominant early in the night
REM sleep becomes more predominant later in the night
What are circadian rhythms?
Rhythms of living organisms that last ≈ day
Dynamic balance btwn stability of system + adaptability to demands of the environment
Balance between modulation of internal organization (i.e. physiological functioning, temperature, hormones, heart rhythm, hormonal secretion..) & external (synchronization with day-night, seasonal changes, social demands..)
What does the circadian system look like physiologically?
Suprachiasmatic nucleus (SCN): Main oscillator, circadian master clock
Other organs + peripheral tissues have their own circadian clocks -> subordinate to SCN
What happens if the SCN is lesioned?
Body's oscillators become desynchronized
Modulates body during sleep so that you don't get hungry; thirty; need to use bathroom; etc --> without SCN, body gets out of control
Animals who had SCN lesioned would usually die
What are zeitgebers?
Environmental cues or triggers that affect Circadian Rhythms
What are some examples of zeitgebers?
Light
Food
Activity-rest patterns
Social cues
What is melatonin?
Sleep inducing hormone released by pineal gland (neighbor of SCN)
Stimulated by darkness & suppressed by light
Acts on the SC
Opposite function of cortisol
Involved in immune function
Fun fact: melatonin production increases during winter -> why you feel more tired overall
When do cortisol + melatonin levels rise?
Melatonin first prior to onset of sleep, cortisol starts rising during REM sleep so that you are more arousable
Theory for why nightmares happen in the morning - cortisol is high so you can be more stimulated
What is melatonin secreted by?
Pineal gland
What role do photosensitive retinal ganglion cells play in sleep?
Play a role in the production of melatonin + contain melanopsin (light-sensitive protein)
Project to SCN and VLPO
Control pupil dilation
Need higher intensity light than cones
Keep firing after light exposure for some time
What is the role of blue light in sleep?
Blocks melatonin, making it harder to fall asleep
Does so by activating retinal ganglion cells, which project to SCN
How can one peripherally measure phases of the circadian rhythm? (endogenous rhythms)
Constant routine protocols
Measure: -temperature -melatonin concentration in blood -blood pressure -growth hormone -sleep diary -actigraphy
What is the pathway via which information about light gets conveyed by retinal ganglion cells?
Via retino-hypothalamic tract
Has direct connections from reintal ganglion cells to SCN
Why do retinal ganglion cells have a slow response to light?
So that you do not feel immediately alert after being exposed to light (ex: waking up in the middle of the night and using a light, but being able to fall asleep after)
What is the pathway of the circadian system?
Light received by eye
Retinal ganglion cells sense light and send it to SCN
SCN sends signals about which phase it is, neurotransmitters excreted
behavior is altered based on signals (metabolite hormones; feeding-fasting cycles' rest-activity cycles)
What is the hierarchal organization of the circadian system?
Principal oscillators (in brain)
Internal synchronizers/local tissue oscillators
Local molecular loops
Local clock output genes
Clock-dependent physiology
Clock-dependent pathology
What is the average circadian rhythm (length of cycle) in adults?
24.15 hours
What are 'free-running' circadian rhythms?
When circadian rhythm changes/shifts constantly despite constant environment
What does sleep look like in the blind?
Non-24 hour cycles, desynchronization, week effect of zeitgeibers
Receive most cues socially (when other people eat/go to sleep)
What happened to Scarasson, who spent months in a cave without light?
Circadian rhythm shifted/operated chaotically
24 hours per day first month, then ranged from 18-52 hours
What is a chronotype?
A person's pattern of sleep and wakefulness throughout a typical day
Morningness to eveningness
Subject measure
Varies with age + gender
What are early chronotypes associated with?
Migraines
Conscientiousness and stability personality traots
What is a late chronotype associated with?
Nightmare frequency
Increased risk of depression
Enhanced amygdala response (emotional regulation)
Lower test grades (mismatch between late chronotype + early school start)
Smoking
What are some issues with the concept of a chronotype?
Doesn't account for mixed chronotype + extreme phase delay
Doesn't account for social factors -> some people were able to reduce social jet lag, went to sleep earlier/later
Polyphasic sleep, sleep fragmentation among WEIRD (white, educated, rich, etc) population
What is a circadian period?
The circadian period refers to the rhythms’ overall time
Ex: 24 hours
What is a circadian phase?
The circadian phase refers to the specific point in the 24-hour cycle when an organism's biological processes, such as sleep-wake patterns and hormone secretion, are at their peak or lowest. It is regulated by the body's internal circadian clock and can be influenced by external factors like light exposure and social cues.
