1, it's so comfortable. Healthy cats sometimes do this. It feels like yawning. It's a cat's breathing.
It is not good for a cat to sigh like this. It may be poor breathing or even difficulty, and it is generally a precursor to respiratory diseases such as colds and tracheitis.
Take a good look at this cat. If it only sighs like this occasionally, don't worry.
If it often does this, it also has sneezing, runny nose, trembling and other performances, so be sure to be vigilant.