Biological Rhythms, Sleep, and Hormonal Regulation in Psychology

Endogenous Rhythms

Internal biological clocks that anticipate environmental changes; include circadian (daily) and circannual (annual) rhythms.

Suprachiasmatic Nucleus (SCN)

Hypothalamic region generating circadian rhythms.

Retinohypothalamic Pathway

Specialized melanopsin-containing retinal ganglion cells respond to light mainly in the blue spectrum, sending signals to the SCN to reset the clock.

PER and TIM genes

Clock genes producing proteins that regulate circadian rhythms; light breaks down TIM to reset rhythms.

Zeitgebers

Environmental cues such as light, meals, temperature that set/reset the biological clock; light is the main zeitgeber for land animals.

Melatonin

Hormone released by the pineal gland, controlled by SCN; production rises 2-3 hours before bedtime and helps regulate sleep timing.

Stage 1 Sleep

Theta waves; light sleep, muscle relaxation begins.

Stage 2 Sleep

Theta-like waves plus K complexes and sleep spindles; deeper relaxation; combined Stage 1 and 2 form ~50-60% of sleep.

Stages 3 & 4 Sleep

Delta waves; deep sleep, hard to awaken, non-REM dreaming.

REM Sleep

Rapid eye movements, muscle atonia, vivid dreaming; occupies 20-25% of sleep.

Sleep Cycles

About 90 minutes per cycle; 4-5 cycles per night; REM periods lengthen later in the night.

Pontomesencephalon

Releases acetylcholine, glutamate, or dopamine; maintains arousal and responses to new tasks.

Locus Coeruleus

Releases norepinephrine; enhances attention and memory, especially to emotionally meaningful stimuli.

Tuberomammillary nucleus

Releases histamine to promote arousal.

Lateral and posterior nuclei

Release orexin, critical for maintaining wakefulness.

Basal Forebrain

Releases acetylcholine projecting to cortex and thalamus.

Functions of Sleep

Energy conservation (lowered body temperature and muscle activity during sleep), restoration of brain and body (e.g., dendritic branching and synapse modification), memory consolidation (especially during REM and SWS), brain development (more REM sleep in infancy).

Activation-Synthesis Hypothesis

Dreams arise as the cortex interprets random brainstem activity (PGO waves).

Neurocognitive Hypothesis

Dreams are a form of thinking under unusual conditions with suppressed prefrontal cortex activity.

Defensive Activation Theory

Visual cortex activation during REM prevents cortical reorganization from sensory deprivation.

Homeostasis

Active physiological regulation maintaining variables within set ranges (set points) via negative feedback.

Allostasis

Predictive adjustments the body makes proactively depending on situational needs.

Hypothalamic POA/AH

Central area integrating signals to regulate body temperature.

Compensatory Responses When Too Hot

Vasodilation via inhibiting norepinephrine, increased acetylcholine activity for sweating, reducing basal metabolism.

Compensatory Responses When Too Cold

Shivering, vasoconstriction via norepinephrine.

Fever Mechanism

Cytokines from white blood cells stimulate vagus nerve, increase prostaglandins, raising hypothalamic set point.

Osmotic Thirst

Triggered by high extracellular solute concentration causing water to leave cells; detected by receptors near third ventricle (OVLT and SFO) and digestive tract; leads to vasopressin release and drinking behavior.

Hypovolemic Thirst

Caused by low blood volume/pressure detected by baroreceptors and kidneys; involves renin-angiotensin system and vasopressin; triggers vasoconstriction and drinking.

Best Fluids to Relieve Thirst

Water for osmotic thirst; for hypovolemic thirst, fluids containing electrolytes to restore volume.

Insulin

Enables glucose uptake into cells, rises after meals, signals satiety.

Glucagon

Signals hunger, released when blood glucose falls.

GLP-1

Short-term satiety hormone from intestines, promotes insulin release, receptor agonists used in treatments like semaglutide.

Leptin

Produced by fat cells, signals long-term satiety and increases energy expenditure.

Ghrelin

Secreted by stomach, stimulates hunger via hypothalamus.

Arcuate nucleus

Contains hunger cells (release NPY, AgRP, GABA - inhibitory) and satiety cells (release POMC, CART, glutamate - excitatory).

Paraventricular nucleus (PVN)

Excited by satiety cells; inhibits lateral hypothalamus (LH).

Lateral hypothalamus (LH)

Releases orexin to facilitate feeding; orexin release is high during hunger, low during satiety.

Ventromedial Hypothalamus (VMH)

Output inhibits feeding; damage leads to overeating and weight gain.

Diabetes

Type I (lack insulin production); Type II (insulin resistance or insufficient production).

Hypoglycemia

Low blood sugar, can occur due to various causes including medication.

Prader-Willi Syndrome

Genetic disorder with hypothalamic dysfunction causing hyperphagia and increased orexin and ghrelin.

Binge Eating Disorder

Characterized by recurrent uncontrollable binges with increased ghrelin early on and genetic risks.

Bulimia Nervosa

Alternating bingeing and purging or excessive exercising; associated with low serotonin and elevated ghrelin.

Anorexia Nervosa

Severe food intake reduction, fear of gaining weight; linked to increased amygdala activation and cortical changes.

Sex Chromosomes

Male = XY, Female = XX.

SRY gene

Located on Y chromosome, initiates male gonadal differentiation.

Females

Absence of SRY and androgens leads to ovary development; Mullerian ducts form female structures; Wolffian ducts degenerate.

Males

Androgens promote Wolffian duct development; testes release Mullerian inhibiting hormone (MIH) causing degeneration of Mullerian ducts.

Sex Hormones

Androgens, estrogens, and progestogens produced by gonads and adrenal glands.

Hormone Effects on Cells

Binding membrane receptors, acting intracellularly, or affecting gene expression.

Organizing Effects

Long-lasting, structural (often prenatal) effects of hormones.

Activating Effects

Temporary effects based on current hormone levels (e.g., puberty, pregnancy).

Brain Structure in Males

Males have larger total brain volume and more white matter.

Brain Structure in Females

Females have more gray matter at birth.

Alpha-fetoprotein

In females, it binds estradiol preventing brain masculinization.

Testosterone in Males

Crosses blood-brain barrier and is converted to estradiol for masculinization.

Play Behavior Differences

Males prefer rough, active play; females prefer calmer, cooperative play; influenced by hormones and brain immune signaling.

GnRH in Males

Stimulates LH from anterior pituitary leading to testosterone production.

GnRH in Females

Stimulates both FSH and LH affecting ovarian estrogen and progesterone cycles.

Male Hormone Cycle

Follows daily circadian rhythm.

Female Menstrual Cycle

Involves fluctuating FSH, LH, estradiol, and progesterone.

Prolactin

Milk production in females, increases caregiving behaviors in males and females, inhibits leptin to increase feeding post-birth.

Oxytocin

Promotes uterine contractions, milk letdown, social bonding, affectionate and protective parenting behaviors.

Estradiol

Modulates neural circuits for parental care, influences pup retrieval and nesting in rodents; fluctuating levels may relate to postpartum depression.

MPOA/AH

Brain area crucial for maternal behavior; activity increases with maternal responsiveness.

Vasopressin and Oxytocin

Key neuropeptides for pair bonding; prairie voles show high receptor densities for pair bonding.

Vasopressin Receptor Gene Polymorphisms

Linked with male pair bonding behavior in human studies.

SRY Mutations

Can cause varying degrees of masculinization or lack thereof.

Turner Syndrome (X)

Females with underdeveloped ovaries, short stature, cognitive deficits.

Klinefelter Syndrome (XXY)

Males with underdeveloped testes, hormone imbalance, cognitive and mood disorders.

Triple X Syndrome (XXX)

Females with variable presentations.

Congenital Adrenal Hyperplasia (CAH)

Adrenal overdevelopment causing androgen excess; females exhibit varying masculinization of brain and genitalia; treated with glucocorticoids and androgen antagonists.

Androgen Insensitivity Syndrome (AIS)

Males lacking androgen receptors, leading to feminized or ambiguous genitalia and brain; treatment tailored by gender identity.

Sex Bias in Neuroscience Research

Ignoring or favoring one sex may skew understanding of neurobiology.

Actions to Reduce Bias

Include balanced sampling, sex-specific analyses, and reporting.

Endogenous rhythms

Internal biological cycles that occur without external cues.

Circadian rhythm

A 24-hour cycle regulating sleep, wakefulness, hormones, and temperature.

Main zeitgeber for humans

Light.

Master circadian clock

The suprachiasmatic nucleus (SCN).

Retinohypothalamic pathway

Special melanopsin retinal ganglion cells send light signals to the SCN.

PER and TIM proteins

They regulate the timing of circadian rhythms.

Melatonin secretion increase

2-3 hours before sleep.

Sleep stage with sleep spindles and K-complexes

Stage 2.

Waves dominating slow-wave sleep (Stages 3 & 4)

Delta waves.

Stage featuring vivid dreaming and muscle paralysis

REM sleep.

Duration of one sleep cycle

About 90 minutes.

Pontomesencephalon function

Promotes general arousal and alertness.

Neurotransmitter released by the locus coeruleus

Norepinephrine.

Hypothalamus release for wakefulness

Orexin.

Basal forebrain role

Releases acetylcholine for cortical activation.

Functions of sleep

Energy conservation, restoration, memory consolidation.

Activation-synthesis hypothesis

Cortex interprets random brainstem signals during REM.

Neurocognitive hypothesis

Dreams are a form of thought under altered brain conditions.

Homeostasis

Maintaining internal stability via negative feedback.

Allostasis

Predictive adjustment based on anticipated needs.

Brain area regulating body temperature

POA/AH (preoptic area of hypothalamus).

Cause of osmotic thirst

Increased solutes outside cells → cells lose water.

Hormone released during osmotic thirst

Vasopressin.

Best remedy for osmotic thirst

Water.

Cause of hypovolemic thirst

Low blood volume or pressure.