One of the well known benefits of massage is that it can reduce stress.
During a massage endorphins are released by the pituitary gland, these are the body’s natural feel good hormones. The release of endorphins suppresses stress hormones, such as cortisol and adrenaline. Endorphins can also have an analgesic effect, reducing the sensation of pain. The reduction of pain leads to a more relaxed state and can attribute to the effect you are seeing.
Another effect of endorphin release is a reduction in blood pressure. Reduced blood pressure suggests a more efficient circulation that is putting less pressure on the cardiovascular system. You are likely to notice longer, deeper breaths and a reduced heart rate. Both of which can make a horse sleepy and more relaxed. Heart rate is an easy parameter to measure to help understand how a horse is reacting to any situation.