Circadian Rhythm

Circadian Rhythm

sleep-wake cycle/rhythm that repeats every 24 hours

How are daily periods of sleeping and waking controlled?

By your 24-hour clock and environmental cues

Endogenous 24-hour clock

biological mechanism that regulates circadian rhythms internally.

Zeitgebers

external cues that synchronize circadian rhythms, such as light and temperature.

How is our biological clock reset?

It is reset each morning by environmental cues (zeitgebers)

Entrained sleep and wakefulness

is the alignment of sleep patterns with the external environment, influenced by zeitgebers. (periods of sleep and wakefulness become entrained so we wake up at about the same time each day)

What happens without environmental cues? (ex: if you’re in constant dim light) 

You depend only on your biological clock (same # of hours/day but no longer tied to environmental cues)

Suprachiasmatic nucleus (SCN)

controls circadian rhythms by synchronizing daily biological and behavior rhythms; master clock of the brain

Where is the SCN located?

In the hypothalamus; just above the optic chiasm

How does the SCN work?

coordinates all the biological clocks of the body using light to synchronize each clock (every cell in the body has a biological clock!)

Melatonin

helps control the body’s sleep cycle; levels change with age

How does melatonin change with age?

production/release of melatonin decreases with age; (By teen years, nightly melatonin release is delayed, leading to later sleeping and waking)

What happens when we get less melatonin production/release?

we get less sleep

What happens when the eyes receive light? (ex: from sun)

Pineal gland’s production of melatonin is inhibited by SCN > helps keep us awake

What happens when the eyes do not receive light?

Melatonin is produced in the pineal gland and we become tired.

What are the 2 factors that influence the sleep-wake cycle?

Circadian factor and homeostatic factor

Circadian factor 

biological clock that cycles about every 24 hours (gets help of zeitgebers); our urge to fall asleep depends on this

Homeostatic factor

how long you’ve been without sleep; the urge to fall asleep also depends on how long we go without sleep

It’s 3:00am. You’re awake but feel dead tired for 2 reasons:

1. Circadian factor: biological clock tells brain that this is supposed to be the ‘sleeping’ phase of your sleep-wake cycle

2. Homeostatic factor: tells your brain that you have been many hours without sleep

If you remain to stay awake from 3am to 9am, you begin to feel less tired. Why?

1. Homeostatic factor: need for sleep has increased

2. Circadian factor: signaling that you have entered the ‘waking’ phase of your circadian cycle 

3. In this case: your biological clock overrides the homeostatic drive to sleep > thus you feel more awake as the new day begins

What are the 5 stages of sleep?

wake, NonREM1, 2, 3, and REM

Wake

state of being conscious and not asleep

Non-REM stage 1 (N1)

transition from wakefulness to sleep; only lasts a few minutes, easily awakened during this stage

Non-REM stage 2 (N2)

period of sleep deeper than N1 and takes up majority of sleep time; heartbeat and breathing slow further and body temperature drops

Non-REM stage 3 (N3)

deepest stage of non-REM sleep (aka slow-wave sleep) and is the most restorative stage for physical recovery and growth; difficult to wake someone up from this stage

REM

where most dreaming occurs; essential for brain functions like learning and memory

What does REM stand for?

Rapid eye movement

What happens to your body during the REM stage?

Brain activity increases and your body becomes temporarily paralyzed to prevent you from acting out of dreams; eyes dart back and forth rapidly behind closed eyelids

Electroencephalogram (EEG)

electrodes on scalp record brain’s electrical activity.

Synchronized EEG

neurons fire in synchrony with one another and generate EEG of high amplitude and low frequency (when we are drowsy)

What does a synchronized EEG mean?

sleepy

Desynchronized EEG

Neurons fire out of synch with one another and generate EEG with low amplitude and high frequency (when we are alert)

What does a desynchronized EEG mean?

alert

What does an EEG show when we are awake?

low amplitude and high frequency waves

What is the name for the EEG waves when we are awake?

Beta and Alpha waves

How does Alpha waves change?

Alpha waves increase in amplitude as you relax

What does an EEG show when we become drowsy and enter progressively deeper stages of sleep?

EEG waves become higher in amplitude and lower in frequency

What is the name for the EEG waves when we are in Non-REM sleep?

Delta waves and “slow wave sleep”

What happens to EEG during REM sleep?

EEG has a low amplitude and high frequency (resembles EEG while awake and alert)

Electrooculogram (EOG)

records eye movement

What happens to the EOG throughout sleep?

when your eyes aren’t moving, amplitude of EOG is flat — but during REM, EOG increases

Electromyogram (EMG)

records action potentials on muscle fibers that lead to muscle contraction

What happens to EMG through sleep?

progressively decreasing from wakefulness through stages 1 to 4 of NREM sleep

When is EMG flat?

During REM sleep

When do REM episode recur?

About every 90 minutes; grows longer as the night progresses

When is someone most likely to sleep walk?

During nonREM sleep

What’s sleep like for a fetus? (early months after conception)

A fetus will spend all its time in REM sleep > allows significant metabolic activity with brain development

How long does a newborn (age 0) sleep?

16 hours sleeping; over half of this time is spent in REM sleep

How does our time in REM sleep change as we age?

As we age, we spend more hours awake and less time in REM sleep

How do sleep patterns vary across species?

large animals generally sleep less than small ones; small animals rapidly cycle between non-REM and REM sleep (8 min)

Why do large animals sleep less than small ones?

Large animals (ex: horse, cow) have lower metabolic activity and expend less energy so they have less need for sleep (compared to bats or cats)

How does a dolphin sleep?

A dolphin sleeps one hemisphere at a time (one enters slow-wave sleep and the other stays awake)

Why does the dolphin (and other aquatic animals) sleep one hemisphere at a time?

allows the animal to navigate while resting half the brain; no evidence of REM in aquatic mammals

What role does the thalamus play in sleep?

The thalamus plays a key role in waking. It has excitatory projections to the entire cerebral cortex

Intralaminar nucleus

located in the thalamus; electrical stimulation of this area generates wakefulness and cortical arousal

What are neurochemicals important for?

Arousal

Waking-on neurons

neurons that are on during waking and release neurotransmitters that generate alertness/arousal

Locus coeruleus + Raphe + Posterior hypothalamus

send projections to the cortex; release neurochemicals

What does the Locus coeruleus release?

Norepinephrine

What does the Raphe release?

serotonin

What does the posterior hypothalamus release?

histamine and orexin

PPT & LDT - What do they release?

They release acetylcholine to the thalamus (which then sends axons to the cortex)

Stimulants

enhance activity of neurotransmitters

Sleep-on neurons

to initiate sleep; releases GABA in the preoptic area of the hypothalamus

What effect does sleep-on neurons have when released?

They turn off the waking-on neurons in the brainstem and hypothalamus

What are the 2 critical factors that activate sleep-on neurons?

1. circadian clock in the SCN

2. accumulation of adenosine, a chemical in the brain that fuels sleep after long bouts of wakefulness

Caffeine

blocks adenosine receptors to prevent sleep inducing effects

What happens between the cortex and thalamus when waking?

The thalamus relay neurons transmit sensory information to the cortex

What happens between the cortex and thalamus when sleeping?

Reticular neurons in the thalamus inhibit the relay neurons

What controls REM sleep?

Surgical separation of cat brainstem from forebrain prevents REM sleep

1. if cut is anterior to pons, then no REM sleep

2. if cut is posterior to pons, then see REM sleep

3. BUT medulla neurons inhibit motor neurons in torso and limbs via reticulospinal tract > the cat can now move during REM sleep

Reticulospinal Inhibition

When medulla neurons inhibit motor neurons during REM sleep

What happens when there is lesioning posterior to the pons?

enter REM without motor “paralysis” and moves about acting out dream

can dreams occur in non-REM sleep?

Yes, you can dream in all sleep stages but dreams that are vivid and emotional mostly occur during REM sleep

What are replayed during nonREM sleep?

Memories — so if you wake up during this stage, you are likely to recall “dreams” that sound like events from earlier in the day more than dreams during REM sleep

What happens when we dream?

1. posterior regions of occipital and parietal cortex are typically active (visual nature)

2. Prefrontal regions are mostly inactive but can be activated if recall having thoughts or lucid dreams

Which regions are active during dreams?

Posterior regions of occipital and parietal cortex

What regions are inactive during dreams?

Prefrontal regions

Are emotions in dreams negative or positive?

75% of emotions are negative — thought to be because of high levels of sympathetic nervous activity (flight/fight) during REM 

Insomnia

inability to sleep (most common sleep disorder); common causes are stress and anxiety

Hypnotic drugs

medications used for insomnia

Besides drugs, how else can insomnia be alleviated?

Through cognitive-behavioral therapy and exercise

Narcolepsy

intense sleepiness during daytime

1. cataplexy occurs (sudden loss of muscle tone for a few minutes while awake)

2. sleep paralysis (after waking or just before sleep for a few minutes)

3. hypnagogic hallucination (dream-like experiences while awake, 70% of us experience at least once)

What is a major cause factor of narcolepsy?

Loss of orexin (produced in hypothalamus) - impacts sleep and appetite

Sleep apnea

loss of oxygen while sleeping bc/ of blockade of airway passages 

Continuous positive air pressure (CPAP)

goes into the respiratory system to treat sleep apnea; keeps the airways open during sleep by providing air at a pressure just high enough to prevent collapse of airway

REM sleep behavior disorder (RBD)

sleeper does not undergo muscle paralysis that normally keeps us from acting out dreams (sleep walking, kicking, hitting)

1. due to lack of reticulospinal inhibition (medulla input to spinal cord to muscle)

2. 10:1 male to female ratio, most common in older age

3. no effective treatment

Benefits of sleep

1. sleep and immune system function (sleep deprivation weakens immune system)

2. removal of brain toxins (clears out waste products)

3. memory consolidation

4. restorative effects (on mood and cognition, helps with weight maintenance)

Circadian Entrainment

periods of sleep and wakefulness become entrained (synchronized to light cues)

How can you flip a hamster’s sleep schedule?

By having lights off during the day and on at night.