How Long should I make my niece stay awoke after shes bumped her head?

My 8month old niece is learning to walk and she pulls up on everything, she pulled up on the entertainment system while I was looking for her pacifier... I didnt leav her unattended we were in the same room I had my back turned for maybe a se, but anyway she fell and bumped her head on the floor, Im really worried, because shes sleepy and Im afraid to let her go to sleep, when is it safe for me to allow her to sleep?