This NYT's Motherlode blog post from 10/16 details what parts of the sleep cycle best support different types of retention - great advice when your child is finishing that last bit of homework (and simultaneously texting her BFF's) way after her regular bedtime.
http://parenting.blogs.nytimes.com/2014/10/16/want-to-ace-that-test-get-the-right-kind-of-sleep/?emc=edit_ml_20141016&nl=style&nlid=67805423&_r=0