Physical Development

Cephalocaudal Pattern: growth sequence that gradually works from top to bottom of the body, meaning the head and upper parts of the body develop faster than the lower parts.
Proximal Distal Pattern: growth sequence in which growth starts at the center of the body and moves toward the extremities. For example, an infant learns to control their torso before gaining control over their fingers.

Body Proportions: Proportions change dramatically over the lifespan. In a fetus at $2$ or $5$ months, the head is disproportionately massive compared to the body. By age $25$ , the head's proportion of the total body size has significantly decreased.

Puberty is defined as a brain-neuroendocrine process that stimulates rapid physical changes and sexual maturation, primarily in early adolescence
The Biological Trigger: The process is initiated by the brain when the hypothalamus sends signals to the endocrine glands.
Timing and Onset:
- Boys: Usually begins between age $10$ and $13.5$ and typically concludes by age $17$ .
- Girls: The onset of menstruation (menarche) occurs between ages $9$ and $15$ .
- Growth Spurt: in both weight and height. The average age for the growth spurt is $11$ for girls and $13$ for boys, but the full developmental range spans from $9$ to $16$ years old.
Hormones: Hormones are powerful chemical substances secreted by endocrine glands and carried by the bloodstream
- Testosterone: a hormone associated in boys, including voice deepening, height increase, and deepening of the voice.
- Estradiol: A type of estrogen associated in girls with breast, uterine, and skeletal development

While puberty is biological, it also impacts one’s psychological and socioemotional development

Early Adulthood: physical peak in terms of strength, speed, and resilience.
Middle Adulthood: physical slowdown:
- Individuals may lose $1$ to $2$ inches in height and gain weight.
- Sarcopenia: The specific age-related loss of lean muscle mass and strength
- The Climacteric: The general period of fertility decline in both men and women.
- Menopause: For women, this is the end of menstruation, typically occurring around age $47$ or $48$ . Symptoms include hot flashes, headaches, dizziness, heart palpitations, and aching joints.
Late Adulthood:
- Facial wrinkles and age spots.
- Weight often drops due to further muscle loss. Height decreases due to posture changes and osteoporosis.
- Ageism: U.S. culture often judges women more harshly than men for these natural biological changes.

The brain consists of two hemispheres and four main lobes: frontal, parietal, temporal, and occipital, containing billions of neurons.
Infancy and toddlerhood: increase in connectedness between neurons over the course of the first two years of life
Early childhood: brain grows rapidly, by age 5, weighs 90% of adult brain
Myelination: The process of adding insulation to the neural wiring, which increases the speed of electrical signals and processing.
The Adolescent Brain:
- Prefrontal Cortex: The "CEO" responsible for planning and impulse control; it is not yet fully developed.
Early adulthood: peak functioning
Late Adulthood Brain: The brain becomes smaller and lighter, and processing speeds decline.

Sleep is physically and mentally restorative, crucial for executive function, attention, behavior, and good health. REM sleep promotes brain development and memory consolidation.
Circadian rhythm: one’s natural sleep-wake cycle.
Sleep Needs by Age:
- Newborns: Require $16$ to $17$ hours per day, with about half of that being REM sleep.
- SIDS (Sudden Infant Death Syndrome): infants stop breathing. To prevent SIDS, infants must sleep on their backs without blankets or objects in the crib.
- Children: Require $10$ to $13$ hours.
- Adolescents: Biological circadian rhythms shift, making them stay up and wake up later. They need $9.5$ hours, but only about $25\%$ of U.S. teens get $8$ or more hours.
- Adulthood: The ideal range is $7$ to $9$ hours.
- Late Adulthood: Insomnia becomes a common issue.

Lifespan: the maximum numbers of years an individual can live human life, estimated at $120$ to $125$ years.
Life Expectancy: The average number of years a person is expected to live. In the U.S., the current average is $76.1$ years.
- Disparities: Females ( $79.1$ years) live longer than males ( $73.2$ years). Asian Americans have the highest expectancy, Native Americans the lowest, and Black Americans average $70.8$ years.
Types of Aging:
- Gerontologists: specialists who study aging
- Primary Aging (Senescence): Universal, irreversible decline due to time.
- Secondary Aging: Decline due to illness, environmental factors, and lifestyle choices.
Theories of Aging:
- Evolutionary Theory: Suggests that after childbearing years, natural selection stops screening out harmful conditions, letting aging processes continue.
- Cellular Clock Theory: Cells have a limit on how many times they can divide, eventually losing the ability to repair tissue.
- MTOR Pathway Theory: Involves a pathway that senses food energy; too much MOTR activity (from constant eating) may speed up aging, while calorie moderation may slow it.
- Hormonal Stress Theory: Long-term exposure to stress hormones like cortisol wears down repair systems and increases vulnerability to disease.

aging is multifactorial, and it isn’t caused by just one thing