It's already commonly thought that a warm bath before going to bed helps improve your sleep. A new meta-study has apparently confirmed this to be the case, however, and has even come up with exact figures regarding the best times and temperatures.
Working with colleagues at the University of Texas Health Science Center and the University of Southern California, researchers from the The University of Texas at Austin sifted through data from a total of 5,322 previous studies on the effects of pre-bedtime bathing on sleep quality.
More specifically, they were looking for consistencies between the different studies, regarding the manner in which "water-based passive body heating" affected a variety of factors. These factors included sleep onset latency (how long it takes to transition from full wakefulness to sleep), total sleep time, sleep efficiency (the amount of time spent actually sleeping as opposed to being in bed trying to sleep), and subjective sleep quality.
What they found was that a water temperature range of 104 to 109 ºF (40 to 43 ºC) worked best for improving overall sleep quality. Additionally, in order to lessen the amount of time that it takes to fall asleep (by an average of 10 minutes), that bath should be taken one to two hours before going to bed.
The science behind these findings involves the body's circadian cycle, in which a person's core body temperature normally drops by about 0.5 to 1 ºF around an hour before their usual sleep time. Although it may seem counterintuitive, taking a warm bath reportedly helps facilitate such a drop, as it causes blood to circulate out from the core to extremities such as the hands and feet.
The University of Texas team is now looking into the development of a commercially-viable bed system which would adjust users' body temperature throughout the night, keeping temperature changes in line with those that should be occurring naturally as part of the circadian cycle.
A paper on the research was recently published in the journal Sleep Medicine Review.