What does the phrase safe to sleep actually mean? To answer this, think of blocking calls (APIs): a blocking call is one where the calling process (or thread) is put into a sleep state because it is waiting on something, an event, and the event it is waiting on has not occurred yet. Thus, it waits – it "sleeps." When, at some future point in time, the event it is waiting on occurs or arrives, it is woken up by the kernel and proceeds forward.
One example of a user space blocking API includes sleep(3). Here, the event it is waiting on is the elapse of a certain amount of time. Another example is read(2) and its variants, where the event being waited on is storage or network data becoming available. With wait4(2), the event being waited on is the death or stoppage/continuing of a child process, and so on.
So, any function that might possibly block...