BBC reports:

Leaving young babies to cry themselves to sleep can harm their developing brains, a parenting expert claims.

Dr Penelope Leach says recent scientific tests show high levels of the stress hormone cortisol develop in babies when no one answers their cries.

If this happens over long periods and repeatedly, it can be "toxic" to their brains, she says in a new book.

Dr Leach suggested unattended extreme crying bouts of 30 minutes or more could be damaging to babies.

What is bad for the baby must not only be bad for the baby alone. In adults, crying affects brain processing. It disrupts thinking, brings reason to a standstill, and stalls or breaks problem-solving mechanisms. Crying should only signal that all options have been exhausted. Crying is for the cornered.