He smiled gently as she drifted into slumber, despite how she protested along the way. She worked too hard on too little rest. Hypnos could tell. He could always tell. Of course, he thought that everybody worked too hard on too little rest. Himself excluded, obviously.
It would be tempting to allow her to sleep her fill, until she was truly rested, but that would likely take hours. Unfortunately he could not let Iris sleep for that length of time. It would do little to improve her ability to work by being rested if she were subjected to censure for being lax on the job. Hera would disapprove should her messenger be gone that long.
She needed to sleep more herself; she'd be far less cranky.
No, it would not be a brief sleep for Iris. He would assign some of his sons to the task that was asked of him, and by the time that was finished he would wake Iris and send her on her way. It would only be a nap, but Hypnos would make certain it was the best nap of her life.