Actually, i heard that cats actually can still react to noises when asleep for real. Only time my cats don’t react to the opening of the snack box is if they are obviously dreaming.

While awake, a cat’s brain activity is displayed as clusters of sharp, irregular peaks on an EEG graph. As a cat slips into a light, slow-wave, nonREM sleep, the waves become long and irregular. This sleep phase lasts an average of 25 minutes. You may see your cat sitting upright while snoozing. His muscles are active and so are his senses; he’s ready to bolt out of the Buddha-like meditative state to chase prey or escape danger.

Moving into a deeper sleep phase, his muscles below the neck relax and he tends to roll over onto his side. During REM sleep the EEG shows rapid, erratic brain wave activity similar to the waking pattern. REM activity includes fast, irregular breathing and fluctuating heart rates. Eyes make bursts of jerky movements under the lids both horizontally and vertically. There are between eight and 30 movements per burst. This is when you may notice twitching whiskers, tail flicks, paw movement, and perhaps snoring. REM sleep is when humans dream and the same is theorized for cats, although we can only guess about their dream content. This phase lasts only about five to seven minutes and cats are slow to wake from this deep sleep. Cats flow back to a light sleep and cycle through the two phases until they’re ready to get up. For the first month of life, kittens fall straight into the deep REM sleep state.

https://www.fearfreehappyhomes.com/catnap-connoisseurs-how-cats-sleep/