4-5 / 0-1 / 4-5 / 0-1, repeated for at least 1 minute, better 5 minutes or more. (not too deep or light-headedness can happen)
This is resonance frequency breathing (at round about 0,1 Hz) which maximizes heart rate variability and activates the baroreflex (blood pressure). It’s usually used with a HRV biofeedback device to find the best individual breathing frequency, but it’s not strictly necessary. Just find the speed around these marks that feels good (without feeling like you’re out of breath or getting dizzy). Breath into your belly.
There’s tons of studies for its effects, but mostly it activates your vagal control, and thus tones down anxiety, can help with depression and PTSD symptoms when done regularly, and also helps with high blood pressure, can make recovery after heart surgery easier, is used in professional sports, may help with IBS etc. Look for papers on “HRV biofeedback effects”, if you’re interested.
The most immediate effect I personally have is feeling both more relaxed and more alert, similar to after meditation.