Controlling for age and sex, the diagnostic groups did not differ on any sleep parameter except for total sleep time; the
autism group had significantly lower total sleep time than both comparison groups (Table). Overall, 33% (34/102) of
the children had an abnormally elevated PLMI of 5 or more. This did not differ across diagnostic groups (chisquared
¼ 3.1, P ¼ 0.21 [35% autism, 44% developmentaldelay, 17% typically developing]). Given that the groups did not differ on serum ferritin levels, and that analysis of covariance tests showed no significant interaction between diagnosis and serum ferritin inpredicting the measured sleep parameters (all P > 0.05), the three groups were combined for the correlational analyses.