Comment by oralty

Nothing in the generic spec for a "sleep" function specifies you can't drop to a low power state if there is nothing to do. To varying degrees that is how sleep functions work on general purpose OSes. The CPU doesn't just spin for extended periods of time when there is nothing to do (it may spin for short waits). A "schedule a task to be called in 'n' hundreds of microseconds" is effectively a "sleep" function, the use of an explicit callback (PC) vs implicitly saving the PC of the thread is a mere implementation detail.