Really that is something you should work out with your group and make it a house rule. However, I use a system that seems to even up the spell casting classes. Arcane Spellcasters need only rest for 8 hours prior to regaining their spells. The magic they cast is channelled directly through the characters and it is taxing so they need that rest period. After that then they are good to go for another round of spell casting. Divine Spellcasters on the other hand are a special case. Most Good Deities grant their followers spells with the rising sun while most Evil Deities grant spells at onset of night (there are exceptions). Since it is the Deity that is granting the spells (the Cleric, Druid, Ranger or Paly isn't really casting anything) I rule that they must stick to the 24 hour period (every daybreak or nightfall and none in between).
