The biorhythm is a new feature added in Path or Radiance.
Unfortunatly it doesn't affect the game too much and you can go through the game without using it at all.
» What does it do?
Neither the game or the manual explains what the Biorythm actually does,
however it only affects a units hit %.
Depending on were the icon is on the graph you can gain +/- 5% in hit. If the icon is on the top line you gain +5%,
if it's on the very bottom you get -5% anywhere inbetween gets you 0%.
» How do i change it?
You can influence the biorythm by winning battles and taking
as little damage as possible. Likewise the biorythm will drop if you perform badly during battles; losing and taking alot of damage.
» Do skills affect biorythm?
Yes. The Serenity skill halves the effect therefore giving you a +/-2.5% instead of +/-5%. not very useful then.
Tempest however doubles the effect giving you +/-10% hit.