Do rats close their eyes when they sleep? - briefly
Rats do close their eyelids during sleep, entering a state of reduced visual activity. They can also exhibit brief periods of REM sleep with their eyes partially open.
Do rats close their eyes when they sleep? - in detail
Rats lack a true upper eyelid; instead they possess a thin, translucent nictitating membrane that covers the eye only when the animal is in a deep, immobile state. During typical sleep cycles, especially rapid eye movement (REM) phases, the membrane remains partially open, allowing limited visual input. In non‑REM sleep, the membrane may be drawn more tightly over the cornea, reducing exposure but not achieving full closure as seen in species with complete eyelids.
Physiological observations support this pattern:
- Anatomical constraint – the absence of a movable upper lid prevents full occlusion.
- Nictitating membrane function – provides protection and moisture while permitting some light transmission.
- Sleep stage variation – deeper, non‑REM periods show tighter membrane positioning; REM periods exhibit more frequent micro‑movements of the membrane.
- Laboratory evidence – video recordings of laboratory rats show the membrane partially covering the eye during bouts of quiet rest, while during active wakefulness the membrane is fully retracted.
Consequently, rats do not completely shut their eyes while sleeping; they rely on the nictitating membrane to partially shield the eye, with the degree of coverage modulated by the depth of the sleep stage.