Use of sleep() in multithreaded programming is a bad code smell. There are other safer, more accurate, ways of suspending execution of the current thread until a particular event has occurred, though they may take a little more work to integrate into a program than sprinkling sleep's.
Spinlock implementations sometimes use sleep() calls in them to yield the current thread to others that may be ready to run but are currently being blocked. It's still a bad code smell though because spinlocks, at least in user space code, are used to avoid the overhead of yielding to other threads. There are also other locking mechanisms, like mutexes/futexes, that will accomplish the same yielding behavior if it's really needed, like if it's anticipated that the code waiting for the lock may be held up for a while.
Spinlock implementations sometimes use sleep() calls in them to yield the current thread to others that may be ready to run but are currently being blocked. It's still a bad code smell though because spinlocks, at least in user space code, are used to avoid the overhead of yielding to other threads. There are also other locking mechanisms, like mutexes/futexes, that will accomplish the same yielding behavior if it's really needed, like if it's anticipated that the code waiting for the lock may be held up for a while.