Other illnesses including heart disease, Alzheimer's disease and hormonal problems can also disturb the normal pattern of sleep.

BBC: High sleeping pill use 'worrying'

Dr. Amy Evans, a pediatrician and medical director of the Center for Breastfeeding Medicine in Fresno, California, says that as many as 5% of all women have underlying medical conditions that prevent or seriously hinder lactation: hypoplasia, thyroid problems, hormonal imbalances and insufficient glandular tissue, among others.

CNN: Is the medical community failing breast-feeding moms?