There are many popular science books to take a hot bath before going to bed as a good way to improve their sleep quality.
It's supposed to be good for sleep, it's scientific.
Sleep itself is not a passive process, but the proactive process, because people need sleep, sleep is one of the life's needs, and as important as diet, drinking water, air, the sunshine. Sleep comes from the relaxation of all parts of the body, including muscle relaxation, brain relaxation, cardiovascular relaxation, and so on. If you make a conscious effort to relax before you go to sleep, it's certainly good for sleep as a preparation. Taking a hot bath is one of the ways to relax.
According to the opinion of the physical therapist, soak in hot water, can make blood vessels around, the whole body most blood would flow into the expansion of blood vessels, reduce blood relative of internal organs. Because of the relative decrease in blood flow to the brain, the brain will feel tired, yawning and drowsy, and therefore conducive to sleep.