Blood sugar level does not go down in response to the surgery, but to the contrary, they go up. In your dad’s case the insulin dose needs to be adjusted. Also make sure that after taking evening dose of insulin he does not miss out on the diet. In fact, if the sugars are as low as 35 – 65mg/dl, then the evening dose of insulin can be totally omitted and keep a tab on blood sugar levels. You may also like to consult your physician in this regard and to consider changing insulin to oral medications.