So a lot of times I get asked if a warm bath close to bedtime will help you with sleep onset, and the answer is yes. Just like everything in your body, your body temperature is set on a circadian rhythm. It's at its lowest point between 4:00 am and 6:00 am. Now, it reaches its highest point between 4:00 pm and 6:00 pm, and it starts to drop again. This signals the onset of sleep.
And to help expedite that process, if you take a warm bath or shower at night and you step out into the cooler environment, that will help decrease your body temperature quicker.
Now for the same reason, I suggest people sleep in a cool, dark environment, because the cooler you are, the better sleep that you get. And research has shown that sleeping in a warm environment does increase sleep fragmentation.
So taking a warm bath about 90 minutes before your bedtime should help with expediting the drop in your body temperature.