During which sleep stage is body temperature poikilothermic?

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Poikilothermy means body temperature varies with the surrounding environment rather than being strictly kept at a constant level. In REM sleep, the brain largely suppresses the autonomic processes that regulate core temperature, so the body loses the ability to actively adjust heat production and loss. As a result, the core temperature tends to drift toward ambient temperature. In contrast, thermoregulation remains more functional during non-REM sleep (and is fully active during wakefulness), so temperature is better maintained. Hence, REM sleep is the stage where body temperature behaves most like poikilothermy.