How do rats sleep: with their eyes open or closed? - briefly
Rats close their eyelids during sleep. Their thin lids can give the impression of partially open eyes, but the visual system is effectively shut down.
How do rats sleep: with their eyes open or closed? - in detail
Rats possess fully functional eyelids that close during the majority of their sleep cycles. Throughout non‑rapid eye movement (NREM) sleep, the eyelids are shut, and the animal remains motionless, heart rate and respiration decrease, and brain activity shows high‑amplitude, low‑frequency waves. During rapid eye movement (REM) sleep, the eyes move rapidly beneath the closed lids, a pattern visible in electro‑oculogram recordings. The nictitating membrane, a translucent third eyelid, may be partially visible but does not remain fully open.
Occasionally, rats exhibit brief periods of “open‑eye” drowsiness while remaining alert to predators. In these micro‑arousals, the eyelids are partially raised, allowing visual monitoring without fully awakening. Laboratory observations note that:
- NREM: eyelids fully closed, muscle tone low, body posture relaxed.
- REM: eyelids closed, rapid eye movements detected, muscle atonia present.
- Micro‑arousal: eyelids partially open, increased cortical activity, readiness to flee.
The closed‑eye state is essential for restorative processes such as protein synthesis and memory consolidation. Experimental studies using polysomnography confirm that complete lid closure occurs in over 95 % of recorded sleep bouts, with only transient partial openings during brief awakenings. Therefore, rats primarily sleep with their eyes shut, opening them only during brief, vigilant states or when transitioning between sleep stages.