It Is OK Once the hospital phoned and put back my appointment to 3 pm instead of 10 am I had not eaten or drunk anything since 7 pm previous day so I continued to fast until Blood test at 2pm some 27hours My instructions were nil by mouth, after I complained they said I could drink water when fasting Made me feel stupid. but they should say Except water to be clear'.
In your case if they don't open until 12pm you can have a snack at 11 30 previous night or if they open at 11 am snack at 10.30 etc