Have seen that article before. This is one of the most chilling bits:
What does 'crying it out' actually do to the baby and to the dyad?
Neuronal interconnections are damaged. When the baby is greatly distressed,it creates conditions for damge to synapses, network construction which occur very rapidly in the infant brain. The hormone cortisol is released. In excess, it's a neuron killer which many not be apparent immediately (Thomas et al. 2007). A full-term baby (40-42 weeks), with only 25% of its brain developed, is undergoing rapid brain growth. The brain grows on average three times as large by the end of the first year (and head size growth in the first year is a sign of intelligence, e.g., Gale et al., 2006). Who knows what neurons are not being connected or being wiped out during times of extreme stress? What deficits might show up years later from such regular distressful experience? (See my addendum below.)
Disordered stress reactivity can be established as a pattern for life not only in the brain with the stress response system (Bremmer et al, 1998), but also in the body through the vagus nerve, a nerve that affects functioning in multiple systems (e.g., digestion). For example, prolonged distress in early life, resulting in a poorly functioning vagus nerve, is related disorders as irritable bowel syndrome (Stam et al, 1997). See more about how early stress is toxic for lifelong health from the recent Harvard report, The Foundations of Lifelong Health are Built in Early Childhood).
Self-regulation is undermined. The baby is absolutely dependent on caregivers for learning how to self-regulate. Responsive care-meeting the baby's needs before he gets distressed-tunes the body and brain up for calmness. When a baby gets scared and a parent holds and comforts him, the baby builds expectations for soothing, which get integrated into the ability to self comfort. Babies don't self-comfort in isolation. If they are left to cry alone, they learn to shut down in face of extensive distress--stop growing, stop feeling, stop trusting (Henry & Wang, 1998).
Trust is undermined. As Erik Erikson pointed out, the first year of life is a sensitive period for establishing a sense of trust in the world, the world of caregiver and the world of self. When a baby's needs are met without distress, the child learns that the world is a trustworthy place, that relationships are supportive, and that the self is a positive entity that can get its needs met. When a baby's needs are dismissed or ignored, the child develops a sense of mistrust of relationships and the world. And self-confidence is undermined. The child may spend a lifetime trying to fill the inner emptiness.
Caregiver sensitivity may be harmed. A caregiver who learns to ignore baby crying, will likely learn to ignore the more subtle signaling of the child's needs. Second-guessing intuitions to stop child distress, the adult who ignores baby needs practices and increasingly learns to "harden the heart." The reciprocity between caregiver and baby is broken by the adult, but cannot be repaired by the young child. The baby is helpless.
Caregiver responsiveness to the needs of the baby is related to most if not all positive child outcomes. In our work caregiver responsiveness is related to intelligence, empathy, lack of aggression or depression, self-regulation, social competence. Because responsiveness is so powerful, we have to control for it in our studies of other parenting practices and child outcomes. The importance of caregiver responsivness is common knowledge in developmental psychology Lack of responsiveness, which "crying it out" represents. can result in the opposite of the afrementioned positive outcomes.
The 'cry it out' approach seems to have arisen as a solution to the dissolution of extended family life in the 20th century. The vast wisdom of grandmothers was lost in the distance between households with children and those with the experience and expertise about how to raise them well. The wisdom of keeping babies happy was lost between generations."
Basically - whilst you may think they should be left to cry because they need to sleep, babies don't know that. They don't know that's why they're being ignored, they just know that the person they trust the most isn't responding to them. So they learn not to trust people.
Heartbreaking, really.