Your question is interesting but hard to answer. The noun "sleep" is a commonly used word. (EX-1) Most people need 8 hours' sleep a day. (EX-2) I didn't get much sleep last night. (EX-3) Do you talk in your sleep? (EX-4) Have a good sleep! "Sleeping" is a verb-derived or gerundive noun and it means an action of sleeping as the previous poster A
"Most people need 8 hour's sleep a day". Here,is it proper to say..'hour's sleep'? Instead , shall we say 'Most people need eight hours of sleep a day".
I made a mistake. I should have written "8 hours' sleep". I'm not sure whether "eight hours of sleep" is wrong or not. But "eight hours' sleep" is idiomatic. Two hours' drive. Ten minutes' walk, two weeks' holiday, etc..
Yes, you are right. Saxon possessives are usually used to express the possession by persons or animals. But somehow they are used also to modify a noun with a phrase connoting time or duration.