www.ncbi.nlm.nih.gov/pmc/articles/PMC5440010/#!po=0.500000
Ok so I've had s look at this now and it's interesting! However not as clear cut as "more sleep = better".
In the studies on memory and post-learning sleep the metric used was children who had no nap vs children who had s nap "of 30 mins or more", with no indication that a longer nap equalled a better performance.
In fact a lot of the studies are based on the benefits of naps (with length unspecified in the systematic review), rather than overall amount of sleep or duration of sleep at night.
The studies looking at night sleep largely consider night sleep as a proportion of total sleep, rather than as a simple value of total amount of sleep or Vs number of night wakings or length of sleep interval.
The study also notes "Despite the fact that all the reviewed cross-sectional studies showed sleep-associated benefits, there were no consistent findings amongst the studies regarding the component of sleep (i.e. sleep duration, sleep efficiency, night awakenings, etc.) that benefitted cognition."
Which indicates that as long as the child is getting enough sleep over all (for them) whether they get that in big chunks or little chunks may not be significant.
The longitudinal studies looking at infant sleep disturbance versus IQ at school age found no significant difference after adjusting for psychosocial adversity, nor did it find any correlation between IQ and sleep patterns recorded at school age.
The conclusions also say "From the reviewed literature, we conclude that sleep plays a key role in those domains with its maturation paralleling, preceding, as well as resulting from interactions with cognitive and physical maturation. Exact mechanisms have not been the focus of this review and still remain to be understood; however, the maturation of central nervous system structures like the hypothalamus or the neurotransmitter system underlies both cognitive development and the regulation of sleep/wake cycles." Which I think is broadly science speak for "we don't know how this works yet" and while sleep may affect development development also affects sleep."
Thanks for this though, I did find it really interesting to read and it did give me some thoughts. My daughter has always been very very smart and physically ahead of her age, but she does also struggle with shyness and high levels of emotional response to things not going how she expects I've always just put that under the heading "sensitive" (or possibly Asperger's, which runs undiagnosed in her father's family). But I am open to the idea that her sleep patterns play a part in her trouble emotionally self-regulating. She does sleep through now at 3.5 and has got about a year, but only for about 10 hours a night (no nap). An earlier bed time does seem to result in a longer night's sleep, and a better mood generally; so might be good for me to read more into the studies reviewed in this SR!