Hi, my best friend lets her 5month old daughter fall asleep at 6pm and doesn't wake her up to eat until 5am, she how thinks it is OK to allow her to sleep that long without feeding her in between those hours. My question is how long is too long for a 5month to sleep without eating?

If a baby does not wake up to eat, unless there are medical reasons you've been told, let the baby sleep. She will wake when she is hungry. After newborn stage, 11 hours isn't unreasonable.