Two good tools to cool down your Self and your Nervous system are triangle or box breathing. Here's how you do it.
To cool down your nervous system, going from a stressful or sympathetic (fight or flight) mode to a relaxing parasympathetic (rest and digest) mode, you can use triangle or box breathing exercises.
This could be done by a nasal inhale 5 s - hold 5-10 s - nasal exhale 5-10 s (hold 5- 10 s for a "box") times 10 or for 5 minutes (see videos below).
Triangle breathing exercise
Box breathing exercise
The key is to exhale longer than you inhale, such as using Ujjayi breathing technique, since you discard excessive Carbon Dioxide and lower your heart rate. Also, make sure to inhale through your nose since that helps in releasing nitric oxide which dilates your blood vessels.
Ujjayi breathing technique
Summary
Triangle and Box breathing are good breathing exercises that will increase and enable your recovery from both physical and psychological stress. Use these exercises in combination with the Ujjayi breathing technique for the best recovering and relaxing effect after training/stress, during your work hours and/or before going to bed.