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It is quite normal for a cat to sometimes leave their eyes open or partially open during sleep. For cats, their sleep consists of two basic phases: Rapid Eye Movement (REM) sleep, when dreams occur, and Non-REM (NREM) or deep sleep.
The NREM sleep phase is very important as that is when their body repairs and regenerates itself. This is the smaller portion though of a cat's sleeping phase.Up to 60 percent of a cat's sleep is REM sleep. During REM sleep, the cat's eyes move behind the eyelids, limbs twitch, and whiskers move around. This is not as deep a sleep and since cats are very sensetive and aware of their surrondings, in this phase of sleep, they may not fully close their eyes. This better prepares them to react if they perceive any sense of danger around.
I hope this helps and please let me know if you have any further questions.