Do rats sleep with their eyes open or closed? - briefly
Rats close their eyelids fully during true sleep. In brief dozing states they can keep the eyes partially open.
Do rats sleep with their eyes open or closed? - in detail
Rats possess a thin, translucent nictitating membrane that covers the eye, but the primary eyelids are capable of full closure. During the majority of their sleep cycles, the lids are shut, especially in non‑rapid eye movement (NREM) stages when muscle tone is reduced. In rapid eye movement (REM) sleep, brief micro‑openings can occur, allowing the membrane to become partially visible, yet the eyes are not fully exposed.
Physiological observations support this pattern:
- NREM sleep – eyelids fully closed; brain waves slow, body temperature drops.
- REM sleep – occasional partial opening of the nictitating membrane; brain activity resembles wakefulness, but muscle tone remains low.
- Alertness periods – eyes open, pupils dilate to assess surroundings; rapid eye movements can be seen even when the animal is not sleeping.
Laboratory studies using infrared video and electroencephalography confirm that rats close their primary lids for most of the sleep episode. The nictitating membrane may give the impression of an “open” eye, but it functions primarily as a protective layer rather than a sign of wakefulness.
Evolutionarily, closing the lids reduces visual stimulation and conserves moisture, which is advantageous for a nocturnal, prey‑driven species. The occasional membrane exposure during REM does not compromise safety, as the animal remains largely immobile and insulated by its nest.
In summary, rats predominantly sleep with their main eyelids shut, while the translucent third lid may appear partially open during REM phases, providing limited visual protection without indicating full wakefulness.