Being a parent myself, I remember feeling desperate to have more sleep, and confused about baby sleep training methods.There is now a body of emerging neuro-development research which may help new parents to know the facts so they do not put these pressures on themselves.
This research explains that, newborns have quiet sleep with 1 hour sleep cycles which are important for wiring the brain, REM sleep is very important for emotional health and its only at 3 months of age that a baby starts to know day and night. At 3-5 months old slow wave sleep appears and they continue to have 1 hour sleep cycles. Its only at 9 months of age does a baby start to sleep through the night! this is due to physiological changes related to glycogen storage capacity and the orexin hormone which controls sleep (Nils Bergman Skin to Skin Workshop 2016 , Thomas 2014, Schaal 2004).
There has been growing concerns of the effects of sleep training methods in early infancy. In 2013 The Australian Association for Infant Mental Health Inc (AAIMH) published a position paper on 'controlled crying' and its unintended negative impact on an infants psychological and emotional wellbeing. If you would like to read this position paper go to: https://aaimhi.sslsvc.com/key-issues/position-statements-and-guidelines/AAIMHI-Position-paper-1-Controlled-crying.pdf