Circadian variation of potency of placebo as analgesic

The pain threshold in healthy teeth changes significantly depending on the label of the placebo. Systematic observations during the whole day indicate that the effect of the placebo shows significant circadian variations. When a placebo is labelled an analgesic, there is a greater and quicker increase of the pain threshold in teeth during the day-time than during the night hours when the pain threshold ascends less or may even descend. A placebo can produce up to 40% of the pain threshold elevation produced by a normal analgesic.