My cat leaves her kittens alone for hours at a time.?

My cat gave birth three days ago, and for the first day she was very attentive to their needs and never moved from them at all. Yesterday though, she left around noon and wasn't back until dark. We dismissed it as a one time thing because she came back for them. Today, she stayed with them all morning, but disappeared and hasn't been back yet. I'm certain she'll be home around morning, but I can't understand why she's leaving her offspring without food for so long. Don't kittens need to nurse every two hours?