Cold plunging in the morning, or activities that move blood to the periphery like sauna or hot showers, can increase REM sleep by dropping core body temperature.

“Another way to increase the amount of rapid eye movement sleep that you get is to get a bigger surge of epinephrine of adrenaline in the early day prior to that sleep. So this is a great reason to do deliberate cold exposure in your shower in the morning. You could also get it through exercise, so exercising early in the morning.”

“if you want to increase your rapid ey movement sleep non-pharmacologically um yeah the cold Plunge in the morning early part of the day for evening um anything that moves uh blood out to your periphery so sauna uh hot shower that sort of thing is going to drop your core body temperature when you get out”

“I also know based on my reading of my sleep on my eight sleeper Whoop That doing C plune in the morning dramatically increases the amount of rapid eye movement sleep I get at night”