Your vet should give you post-op instructions when you pick her up. Is there any special reason why she is staying overnight? That is not usually done, since most cats recover more quickly in their own homes, they are sent home as soon as the anesthesia begins to wear off unless they need supportive care, such as IV's or round-the-clock monitoring, which shouldn't be needed for a spay. Regardless, she will be very drowsy and will probably not be hungry for a day or two. She will probably appreciate being left alone, too, although it is very hard to do when you love her and are worried about her. If she can go to a dark but safe secluded place like a closet or under a bed she will probably do that, and it's best to let her. But also keep close enough tabs on her so that if she starts running a fever, bleeding, or vomiting you will know. These are rare but if they occur you will need to call your vet immediately. After about 2 days she will probably be close to her normal self again, but she will not be fully recovered for about 10 days to 2 weeks, so try to keep her inside (if she is an inside or inside/outside cat) and don't let her play too hard for that time. Since she is getting neutered early you will not likely notice any change in her personality. If she were older you might notice that she gets more sedate and friendly.

Good for you for doing the right thing to help control the pet overpopulation problem.