Calmer than
sleeping.
By one stress measure, humming scored below sleep.
What they found
Using continuous Holter heart monitoring, this pilot study tracked the same 23 people across four states: humming, physical activity, emotional stress, and sleep. Humming produced the lowest stress index of all four, with other heart-rate-variability changes consistent with reduced stress.
Why it matters
It puts a number on something humming practitioners describe by feel. By this stress measure, the humming state scored lower than the sleeping state in the same people.