Tasks are assumed to run concurrently, potentially by sharing a thread of execution. This means that operations blocking an operating system thread, such as std::thread::sleep or io function from Rust's std library will stop execution of all tasks sharing this thread. Other libraries (such as database drivers) have similar behaviour. Note that blocking the current thread is not in and of itself bad behaviour, just something that does not mix well with the concurrent execution model of async-std. Essentially, never do this: