I use sauna; 10-15 minutes in hot, followed by a short break (1-2 minutes), then up to 2 minutes immersed in cold water. Repeat 5-6x. I feel like a new person afterwards, total mental clarity and many ideas immediately. Also removes stress. It might take you a few months to get adapted to that cycle though (2-minute cold immersion is not trivial at first). If you start feeling bad, abort, you can try next time.