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Sleep architecture: non-REM sleep has three stages, and stage 3 is deep sleep; REM sleep follows. The sequence stage 1 → 2 → 3 → REM repeats several times a night. Early in the night deep-sleep periods are long and REM periods short; later, REM lengthens and deep sleep shortens. The first REM period begins about 90 minutes after falling asleep.

Sleep pressure (the homeostatic drive): adenosine builds up while you are awake, and the drive to sleep gets stronger every hour you stay awake. That is why people sleep longer and more deeply after going without sleep. Caffeine works by blocking adenosine.

The circadian clock: a roughly 24-hour biological clock is coordinated by the suprachiasmatic nucleus in the hypothalamus, which receives light information directly from the eyes. As it gets dark, the pineal gland releases melatonin, which helps make you sleepy. Evening exposure to room or device light is linked to less sleep.

Experiment: Sleep and reaction time: a correlational study

1. Each pupil takes a numbered card and writes, privately, roughly how many hours they slept last night next to their number on the record sheet (optional: anyone may skip).
2. Ruler-drop test in pairs: the catcher holds thumb and finger open at the 0 cm mark; the dropper releases the ruler without warning; read the catch distance in cm.
3. Do 5 drops per person after one practice drop. Record the mean catch distance (shorter = faster reaction).
4. Plot catch distance (y) against hours of sleep (x) for the class as a scatter graph.
5. Describe any pattern. Is there a correlation? How strong?
6. Evaluate: name confounding variables (practice, time of day, caffeine, attention, honesty of reported sleep) and explain why this study cannot prove cause and effect.

Pupil number	Hours slept (self-reported)	Mean catch distance (cm)	Notes (practice, caffeine…)
1
2
3
4
5
6

Questions

1. Sketch how deep sleep and REM sleep change across one night, and explain the pattern.

2. Explain the difference between the homeostatic sleep drive and the circadian clock, with one example of each.

3. A pupil needs 8.5 h and sleeps 6.5 h on 5 school nights and 9.5 h on 2 weekend nights. Calculate the weekly shortfall.

4. Describe your class scatter graph and give two reasons why it cannot prove that less sleep causes slower reactions.

Sources for every fact on this sheet: echoesof.science/school/sleep/sources/

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