Purpose of sleep: evolutionary and restorative theories (WACE Year 12 Psychology Unit 4)
“Purpose of sleep: evolutionary and restorative”
The evolutionary explanation says sleep evolved because it helped animals survive: it keeps them inactive and safe when activity would be dangerous or wasteful, and conserves energy, which is why sleep patterns differ between species. The restorative explanation says sleep allows the body and brain to repair and recover, for example through the release of growth hormone in deep NREM sleep and the recovery of mental functions such as attention and mood.
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What this dot point is asking
The syllabus names two purposes of sleep. You should explain each, give evidence, evaluate them, and apply them to examples.
The evolutionary explanation
- Sleep is an adaptation that increased survival.
- Sleeping keeps an animal still, quiet and hidden during the part of the day when it is least able to find food or avoid predators (for humans, who rely on vision, the night).
- Sleep conserves energy when activity would be unproductive.
- Evidence: sleep patterns vary with each species' ecology. Large grazing animals that must spend many hours feeding and watch for predators sleep for relatively short periods, while predators with safe resting places sleep for longer.
- Limitation: it does not explain why sleep is still needed when an animal is completely safe, or why being asleep, which reduces awareness of danger, would be safer than resting quietly while awake.
The restorative explanation
- Sleep allows the body to repair tissue, build muscle and restore energy. Growth hormone is released mainly during deep NREM sleep.
- Sleep allows the brain to recover: attention, concentration, mood and memory are restored, and REM sleep is associated with brain development and processing of memories and emotions.
- Evidence: deep NREM sleep is linked to growth hormone release, and people who are deprived of sleep show impaired attention, reaction time and mood that recover after sleep.
- Limitation: some restoration can happen during quiet rest while awake, and the amount of sleep does not always increase after a physically demanding day.
The evolutionary explanation explains why sleep evolved; the restorative explanation explains what sleep does. Most psychologists see them as working together.
A Year 12 student after a cross-country race
After running a school cross-country race, Chloe sleeps deeply that night.
- Restorative: her body uses deep NREM sleep to repair muscle tissue, with growth hormone released, and her brain recovers so she can concentrate at school the next day.
- Evolutionary: humans sleep at night because we see poorly in the dark; sleeping then kept our ancestors safe and conserved energy during hours when finding food was difficult.
Marker's note: name the explanation, give its mechanism and link it to the scenario.
- Describing restoration only for the body
- Include brain restoration: attention, mood and memory.
- Giving evidence without linking it
- Say which explanation the evidence supports and why.
- Treating the theories as rivals
- They answer different questions and can both be true.
Exam-style questions
Questions in the style of SCSA exam questions on this dot point, each with a worked answer. They are written by ExamExplained unless tagged "Past paper"; the year shows the paper a question is modelled on.
Original6 marksCompare the evolutionary and restorative explanations of the purpose of sleep, giving one piece of supporting evidence for each.Show worked answer →
Two marks for each explanation with evidence and two marks for the comparison.
- Evolutionary
- Sleep evolved because it increased survival: it keeps an animal inactive and hidden when activity would be unproductive or dangerous, and conserves energy. Evidence: sleep time varies with a species' ecology; for example, large grazing prey animals that need to feed for long periods and stay alert to predators tend to sleep only a few hours, while predators such as lions sleep much longer.
- Restorative
- Sleep allows the body and brain to recover and repair from the day's activity. Evidence: growth hormone, which supports tissue repair and growth, is released mainly during deep NREM sleep, and sleep deprivation impairs attention, mood and physical functioning.
- Comparison
- The evolutionary explanation focuses on why sleep was selected for (survival and safety); the restorative explanation focuses on what sleep does for the body and brain (repair and recovery). They are complementary rather than contradictory.
Original3 marksExplain one limitation of the evolutionary explanation of sleep.Show worked answer →
Up to three marks for a limitation that is identified, explained and linked to sleep.
For example: it does not explain why we still need sleep when we are completely safe. If sleep only kept animals out of danger, a person in a safe home should be able to stay awake without harm, yet sleep deprivation still impairs attention, mood and health. This suggests sleep also serves a restorative function.
Practise this
Sources & how we know this
- Psychology ATAR course Year 12 syllabus (effective from 1 January 2024) — School Curriculum and Standards Authority (SCSA)
- Psychology ATAR course: syllabus and support materials — SCSA
- Past ATAR course examinations — SCSA