There are two very important reasons why they should stay overnight. One-it keeps them completely confined and immobile for the critical period after surgery, two-it allows the vet. staff to re-evaluate the animal in the morning. You know, check temp, check incision, give meds, clean them up, etc. At the clinic I worked at, everything stayed overnight, but some clinics are different. It depends on your vet's opinion. Hope it helps!!