how old is your baby. If for new born it is but don't delay the milk time. Wake your baby up and then let the baby sleep again. Be careful sleeping too much during the day time not sleep in the night.
NB BB sleep around 20 hrs a day, when the BB is 3 months, it's aroung 17 hrs a day.You can ask the nurse at 健康院, they have some information about the sleep time of BB.
My BB also like to sleep very much, as she not need to drink milk at night when 3 weeks. She always sleep from 9:00 pm and get up at 6:00 am, and haven't wake up at night. It's ok, don't be worry.