The milky white is fresh THC(a), THC(a) alone is not what makes it relaxing or sedative. The decomposing THC(a), as evident by the yellowing or amber coloring to the milky trichomes is what makes it more sedative, Decomposing degraded THC(a) is what contributes to the relaxing sedative effect. Some people do say this increases the pain killing and muscle relaxing effects. THC(a) becomes THC and CBN when THC(a) is exposed to light, oxygen and heat. Most of THC(a) is directly converted to THC when heat, especially extreme heat is applied such as in smoking or cooking. The process of turning THC(a) into THC is called decarboxylation. THC and THC(v) are the more psychoactive canabinoids in cannabis. CBN and CBD are considered more relaxing or sedating.
Here is a pie chart listing the numerous forms of canabinoids in cannabis and what they do:
Happy growing,
~MacG