[Haskell-beginners] "sleep" in Haskell

I can't find a "sleep" function in Haskell. This relates to my wish to use PortMidi and control the timing of actions. Maybe there is another way to do this sort of thing in Haskell. Can anyone help, either with PortMidi or just a general explanation of how IO operations could be placed at certain points in time? Thanks, Mike