Pivot points are all about highs and lows. I believe one should pick the time period for highs and lows not randomly, but having in mind some human cycle (because humans are those who trade).
Therefore 24H period is perfect - a human day. 8H and 4H makes sense as it is the lenght of a shift (or half shif from morning till lunch).
Any shorter period than 1 hour does not make sense for me.
Another idea is to consider European open time as the beginning of the forex day. Because main events happen in Europe and USA, so Japanese are usually the last who react to some news/event.
Take care and have fun
