When someone sleeps during the day and is awake during the night. Not necessarily a hooker but originating from a hooker in the strange hours they keep i.e. 6am go to bed, 6pm time to wake up.
Anything that is not 9 to 5 i.e. shift worker, insomniac, sex maniac, prowler, hooker, street hustler, prostitute, drug dealer, addict, paranoid schizophrenic, stalker, peeping tom, voyeur, exhibitionist etc
"Why doesn't Jo-Beth wake up like the rest of us folk?" "Oh she's keeping hooking hours now after she got involved with the booze and dope scene of Hollywood." See Hollywood Slut Syndrome.
"Lindy-Lou is either a vampire, or just keeping hooking hours...either way something's up!"