The Pool & Spa News magazine (July 1998) reported that a Gallup poll found bathing to induce sleep common among the 1,000 adults surveyed, and a Consumer Reports survey reported that readers with mild sleep disorders listed a warm bath as one of the most popular remedies.
Studies suggest that immersion in hot water (such as a spa which has a constant temperature) before bedtime can ease the transition into a deeper, more restful sleep. This may be due to a temperature shift, since the body's core internal thermostat drops after leaving the water, which signals the body that it's time to sleep. Or, the sleep improvement may be related to hot water's relaxing properties - the buoyancy of water reduces body weight by approximately 90%, relieving pressure on joints and muscles, creating the sensation of weightlessness. The hot, swirling water leaves you feeling both mentally and emotionally relaxed.
In addition, spa-induced sleep is a natural remedy, unlike alternative sleeping aids such as prescription drugs, over-the-counter remedies and alcohol - all of which can make you feel groggy and have other adverse side effects.