Every fact in the three sleep lessons is listed here with where it comes from. The numbers in [brackets] on the lesson pages link to this list.
How we rate sources
High: space agencies (NASA), peer-reviewed open textbooks (OpenStax, Rice University) and professional scientific societies (American Physical Society).
Medium-high: reputable science news reporting directly on a primary document.
Medium: single-author essays and popular history sites. We only use these for history details, and never on their own: each one is paired with a second source.
Wikipedia was not used as a source. Numbers are rounded (for example 9.8 m/s² for Earth's gravity). The text was drafted with help from AI (Claude) and then checked against these sources. Corrections: hello@echoesof.science.
Where the facts are simplified or less certain: the recommended hours (9–12 for ages 6–12, 8–10 for ages 13–18) are ranges from the American Academy of Sleep Medicine, quoted by CDC [3]; individuals differ. NINDS gives a single typical figure of about 9.5 hours for school-age children and teens [1]. The survey figures (57.8% of middle-school and 72.7% of high-school students not getting enough sleep in 2015, and 75% of students in 2025 data) come from self-reported surveys of different groups [3, 4], so they are estimates and are not directly comparable. The links between short sleep and health are associations from studies; they do not prove that short sleep alone causes them. Scientists are still studying exactly why we sleep. These lessons are general education, not medical advice: anyone with ongoing sleep problems should talk to a doctor or the school nurse.
All sources
[1] Brain Basics: Understanding Sleep
National Institute of Neurological Disorders and Stroke (NINDS, NIH) · Reliability: High
Type: National health research institute
Sleep seems to clear toxins that build up in the brain while we are awake and supports the brain pathways for learning and memory. Two types: REM and non-REM (three stages; stage 3 is deep sleep). In REM, arm and leg muscles are temporarily paralysed so we don't act out dreams; first REM about 90 minutes after falling asleep. Most people dream about two hours a night. The suprachiasmatic nucleus in the hypothalamus gets light information from the eyes and runs the roughly 24-hour body clock; melatonin from the pineal gland makes us sleepy as it gets dark. Adenosine builds up while awake and caffeine blocks it; the sleep drive gets stronger every hour awake. Babies need 16–18 h, school-age children and teens about 9.5 h, most adults 7–9 h. Tips: regular times, exercise but not just before bed, avoid caffeine late, relaxing routine, no TV or phone in the bedroom.
Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD, NIH) · Reliability: High
Type: National health research institute
The cycle runs non-REM stage 1, 2, 3, then REM, and repeats several times a night; early in the night deep sleep is long and REM short, later REM gets longer. Stage 1: heartbeat and breathing slow, muscles relax. Stage 2: brain waves slow, body temperature drops. Stage 3: heartbeat and breathing at their lowest, tissue growth and repair. REM: brain active, breathing fast and irregular, eyes dart back and forth, dreaming, body immobile. Studies linked REM deprivation with difficulty remembering what was learned before sleep.
Centers for Disease Control and Prevention (CDC) · Reliability: High
Type: National public health agency
Recommended (American Academy of Sleep Medicine): ages 6–12, 9 to 12 hours per 24 hours; ages 13–18, 8 to 10 hours. 2025 data: 75% of students are not getting adequate sleep. Too little sleep is linked to higher risk of obesity, type 2 diabetes, poor mental health and injuries, and to attention and behaviour problems. Tips: regular bedtime and wake-up time including weekends, media curfew or devices out of the bedroom, dim evening lighting.
[4] Sleep in Middle and High School Students (archived feature)
Centers for Disease Control and Prevention (CDC) · Reliability: High
Type: National public health agency (archived page)
2015 Youth Risk Behavior Surveys: about 6 in 10 middle-school students (57.8%, 9 states) and about 7 in 10 high-school students (72.7%, national sample) did not get enough sleep on school nights. Children and teens who don't get enough sleep have a higher risk of obesity, diabetes, injuries, poor mental health and problems with attention and behaviour.